QC 
 896 
 
 M3 
 1912 
 

472. 
 
 . S. DEPARTMENT OF AGRICULTURE, 
 
 WEATHER BUREAU. 
 
 UIIOMETERS AND THE MEASUREMENT 
 OF ATMOSPHERIC PRESSURE. 
 
 A PAMPHLET OF INFORMATION RESPECTING THE THEORY AND 
 
 CONSTRUCTION OF BAROMETERS IN GENERAL, WITH 
 
 SUMMARY OF INSTRUCTIONS FOR THE CARE 
 
 AND USE OF THE STANDARD WEATHER 
 
 BUREAU INSTRUMENTS. 
 
 IIKITLAK F. IXSTRVMKNT DIVISION. 
 
 KlUTHiN, 
 
 BY 
 C. F. MAKVIN, 
 
 PROFESSOR OP METEOROLOGY. 
 Prepared under direction of WILLIS L. MOORE, Chief U. S. Weather Bureau. 
 
 WASHINGTON: 
 
 GOVERNMENT PRINTING OFFIOKL 
 1912. 
 
 I 
 
W. B. No. 472. 
 
 U. S. DEPARTMENT OF AGRICULTURE, 
 
 WEATHER BUREAU. 
 
 BAROMETERS AND THE MEASUREMENT 
 OF ATMOSPHERIC PRESSURE. 
 
 A PAMPHLET OF INFORMATION RESPECTING THE THEORY AND 
 
 CONSTRUCTION OF BAROMETERS IN GENERAL, WITH 
 
 SUMMARY OF INSTRUCTIONS FOR THE CARE 
 
 AND USE OF THE STANDARD WEATHER 
 
 BUREAU INSTRUMENTS. 
 
 rillCULAR F, INSTRUMENT DIVISION. 
 
 FOURTH EDITION, 
 
 BY 
 
 C. F. MARVIN, 
 
 i 
 
 PROFESSOR OF METEOROLOGY. 
 
 Prepared under direction of WILLIS L. MOORE, Chief U. S. Weather Bureau. 
 
 WASHINGTON: 
 
 GOVERNMENT PRINTING OFFICK. 
 1912. 
 
8CIENCW 
 
 PUBLICATION may be pro- 
 -*- cured from the Superintendent of 
 Documents, Government Printing Office 
 Washington, D. C., at 15 cents per copy 
 
EARTH 
 
 SCIENCES 
 LIBRARY 
 
 LIST OF ILLUSTRATIONS. 
 
 I'age. 
 
 1,2,3. Mecurial barometer, with Fortin cistern 11 
 
 4, 5, 6, 7, 8. Verniers 13 
 
 9. Tuch barometer cistern 16 
 
 10. Fixed cistern barometer 17 
 
 11. Tube and cistern of marine barometer 20 
 
 12. Marine barometer and box 20 
 
 13 . Funnel method for filling barometers 22 
 
 14. Boiling method of filling barometers 22 
 
 15. Air-pump method of filling barometers 24 
 
 16. Aneroid barometer 32 
 
 17. Goldschmidt's aneroid 33 
 
 18. Dial barometer 37 
 
 19. Sympiesometer 38 
 
 20. Howson's barometer 38 
 
 21. Three-liquid barometer 38 
 
 22. Foreman's barograph 41 
 
 23. Marvin's normal barograph 44 
 
 24. Electric motor mechanisms 45 
 
 25 . Compensated siphon barograph, Marvin system 47 
 
 26. Three-piece siphon tube 47 
 
 27. Time marker 55 
 
 28. Richard's barograph 56 
 
 29. Improved barometer box 59 
 
 30. Barometer, cistern, and ring support 60 
 
 31. Old style barometer box 61 
 
 3 
 
 171 
 
LETTER OE TRANSMITTAL 
 
 U. S. DEPARTMENT OF AGRICULTURE, 
 
 WEATHER BUREAU, 
 Washington, D. C., January 3, 1912. 
 
 SIR: I have the honor to transmit herewith the revised copy of a 
 pamphlet prepared by Prof. C. F. Marvin, containing information 
 and instructions to the observers of the Weather Bureau for the care 
 and use of barometric apparatus, and to request that it be published 
 as Circular F, Instrument Division, fourth edition. 
 Very respectfully, 
 
 WILLIS L. MOORE, 
 Chief U. S. Weather Bureau. 
 Hon. JAMES WILSON, 
 
 Secretary of Agriculture. 
 
 5 
 
NOTE TO OBSERVERS. 
 
 U. S. DEPARTMENT OF AGRICULTURE, 
 
 WEATHER BUREAU, 
 Washington, D. C., January 8, 1912. 
 
 The details of information respecting barometers contained in the 
 accompanying pages are furnished for the instruction and guidance 
 of observers, who will keep themselves informed upon the subjects 
 treated, and, in particular, will observe the technical duties relating 
 to the care and use of barometric apparatus summarized for their 
 convenience in Section VII. The provisions of this section, as more 
 fully set forth in the body of the pamphlet, will replace instructions 
 on barometers heretofore issued. 
 
 WILLIS L. MOORE, 
 Chief U. S. Weather Bureau. 
 
COXTKXTS. 
 
 Page. 
 
 Introductory The barometer 9 
 
 I. Mercurial barometer 10 
 
 II. Aneroid barometers 31 
 
 III . Miscellaneous barometers 36 
 
 IV. Barographs 39 
 
 V. General instructions 57 
 
 VI. Concerning the elevation of stations 81 
 
 VII. Summary of special instructions for observers of the Weather Bureau. . 85 
 
 VIII. Numerical tables 88 
 
 Index 105 
 
 7 
 
BAROMETERS AND THE MEASUREMENT OF ATMOSPHERIC 
 
 PRESSURE. 
 
 INTEODUCTOEY. 
 HIE BAROMETER. 
 
 1. The hydrostatic principle, by virtue of which the pressure of the 
 air is measured by the ordinary barometer, was first formulated at 
 Florence in 1643 by Torricelli, whose famous experiments demon- 
 strated, not only that the air exerted a very great pressure, but that 
 this pressure changed slightly from day to day. 
 
 12. Torricelli' s barometer. To repeat Torricelli's experiment, fill 
 a clean, dry, preferably warm, glass tube, closed at one end, with pure, 
 dry mercury, using care to exclude all air. The length of the tube 
 must, in general, exceed 30 inches. Close the open end of the tube 
 firmly with the finger tip, and submerge it in an open cup of mer- 
 cury. Upon removing the finger and causing the tube to stand 
 vertically, a portion of the mercury will pass from the tube into the 
 cup, leaving a vacuous space, known as Torricelli's vacuum, in the 
 top of the tube. The column of mercury remaining in the tube will, 
 at sea-level stations, be about 30 inches high. The weight of this 
 mercury is sustained by and exactly balances the downward pressure 
 of the air upon the surface of the mercury in the cup. The height of 
 such a mercurial column, therefore, becomes a measure of the pressure 
 of the air, and Torricelli seems to have been the first to discover that 
 the height of such a column varied from day to day. 
 
 3. Siphon barometer. Instead of constructing the barometer in the 
 manner just described, where the cistern and tube are in separate 
 parts, the tube may be made longer and turned up at the bottom so 
 as to resemble the letter J, forming what is commonly called a siphon 
 barometer, the long arm of which is closed at the top. 
 
 4. Pressure of one atmosphere. Suppose the area of the inside of the 
 barometer tube to be just 1 square inch, then a 30-inch barometric 
 column will contain just 30 cubic inches of mercury. Now, 1 cubic 
 inch of mercury weighs 0.4906 pound, which, multiplied by 30, gives 
 the ordinary sea-level pressure of the air to be 14.718 pounds per 
 square inch. This quantity is frequently used by engineers, and is 
 called a pressure of one atmosphere. 
 
 9 
 
10 BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 
 
 5. This pressure of 14.7 pounds per square inch is, in the main, 
 nothing more than the weight of an air column having a sectional 
 area of 1 square inch and extending vertically to the upper limits of 
 the atmosphere. In addition to the weight, pure and simple, how- 
 ever, such influences as the wind, the rapid heating and cooling 
 in confined layers of air, and other causes modify by small amounts 
 the elastic pressure of the air. 
 
 6. Other forms of barometers. Within the past 50 or 60 years a 
 form of barometer, made entirely of metal, has been devised, and is 
 widely used at sea and by tourists and others on account of its con- 
 venience and portability. This form is commonly designated the 
 aneroid, a word which signifies " containing no liquid." The instru- 
 ment is also often called the holosteric barometer, meaning ' ' wholly 
 of solids." Aneroids, though often highly sensitive, are, at best, 
 much less accurate than properly constructed mercurial barometers, 
 as will be explained later. 
 
 . . > 
 
 I. MERCURIAL BAROMETERS. 
 
 ., 
 
 7. Fortin barometers, Weather Bureau pattern. In order that the 
 height of the mercurial column may represent accurately the true 
 pressure of the air, and in order to detect the comparatively small 
 changes of pressure from day to day, many refinements are necessary 
 in the construction of the instrument and great precision of measure- 
 ment is required. An excellent form of the mercurial berometer, 
 satisfying the requirements just stated, was devised by Fortin, and is 
 now very widely used the world over. The particular pattern used 
 by the Weather Bureau, sometimes called the Green barometer, is 
 figured on page 11. 
 
 8. The barometer consists of a glass tube, about J-inch inside 
 diameter, closed at the top and inclosed in a thin metal tube, through 
 which large openings are cut on opposite sides, exposing to view the 
 glass tube and mercurial column. The graduated scale is formed 
 at one side of this opening, and a short tube or sleeve, also gradu- 
 ated (shown at C, figs. 1 and 2), encircles the barometer tube and 
 slides smoothly within the metal part, motion being given to it by 
 means of the milled head, D, and a small rack and pinion inside. 
 
 At E, figure 2, is shown what is called the attached thermometer. 
 The bulb of this is entirely concealed within the metal tube, and is 
 between it and the glass barometer tube, so as to show as nearly as 
 possible the mean temperature of both the brass tube and the mer- 
 cury. 
 
 9. Cistern. The special feature of the barometer is a cistern so 
 constructed that the level of the mercury within may be changed 
 greatly and adjusted to a fixed index point. 
 
BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 11 
 
 The topmost portion of the cistern consists of a small boxwood 
 piece, Gj figure 3. The glass tube t, passes through the central portion 
 of this, to which it is secured by a piece of soft kid leather folded in 
 a peculiar manner and securely wrapped to both the glass tube and 
 the boxwood cap, G. The flexible joint thus formed will not allow 
 the mercury to escape, but permits the passage of air to and from 
 the cistern. 
 
 The remaining portions of the cistern are the short glass cylinder, 
 F, figure 3, the two curved boxwood pieces, i and j, and the kid 
 leather bag, N, with adjusting screw, 0, clamps, etc. 
 
 It is plainly seen that on turning the screw, 0, the leather bag may 
 be folded up into or withdrawn from the curved boxwood chamber, y, 
 in a manner to cause any desired change in the level of the mercurial 
 surface. 
 
 10. Ivory point. At h, figure 3, is shown what is technically called 
 the "ivory point/' which projects downward from the top of the cis- 
 tern and forms a fixed and definite point, to which the level of the mer- 
 cury in the cistern can be adjusted in taking readings of the barometer, 
 as will be described hereafter. 
 
 The ivory point is, therefore, the zero end of the scale, from which 
 all the measurements of the height of the column are made. 
 
 1 1 . Scale of barometer. The scale of the barometer is seen on the 
 left of the opening, at the top. It is most conveniently made of a 
 separate strip of metal, although sometimes it is engraved directly 
 on the metal tube itself. The length varies from about 4 inches, for 
 use at stations of only moderate elevation above sea level, to from 10 
 to 15 inches, or more, for barometers intended to be used in balloons 
 or on lofty mountain summits. The graduations on the scale also 
 vary, being only 10 spaces to the inch in many instances and 20 in 
 others; the latter graduation is to be preferred on account of the 
 greater accuracy attainable in readings. 
 
 The scale of the barometer when engraved on a separate strip is 
 attached to the metal tube by small screws in such a manner that it 
 may be adjusted slightly up and down, so that the 30-inch mark, for 
 example, of the graduations can be placed at exactly the right dis- 
 tance from the ivory point. This adjustment being made, the scale 
 should not be moved afterwards. 
 
 12. Vernier. A vernier is a device by which one is able to ascer- 
 tain accurately much smaller fractional subdivisions of a graduated 
 scale than could otherwise be observed by the eye without the aid 
 of a microscope. For example, with a scale having only 20 subdi- 
 visions to the inch a vernier enables us to ascertain accurately the 
 one-thousandth part of an inch. The name of the device is derived 
 from its inventor, Pierre Vernier. This portion of the barometer is 
 the little graduated scale, C, figures 1 and 2. 
 
12 BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 
 
 FIG. l. 
 
 o 
 
 FIG. 2. 
 Mercurial barometer, with Fortin cistern 
 
 FlO.3. 
 
BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 13 
 
 A vernier consists, essentially, of a small graduated scale, the 
 spaces upon which are just a certain amount smaller or larger than 
 those on the main scale. When two such scales are placed together 
 some particular line of the one will always be coincident, or very 
 nearly so, with a line on the other, and from this circumstance the 
 position of the zero line of the vernier in reference to the scale can 
 be very accurately determined, as will be readily understood from a 
 study of the following figures and explanation: 
 
 30.177 
 FIG. 8. 
 
 Figure 4 exhibits the manner of graduating a vernier so as to sub- 
 divide the spaces upon the scale into tenths. In the figure, 6 is the 
 scale and a is the vernier. The lower edge of the vernier, which in 
 this case is also the zero line, is exactly opposite or coincident with 
 30 on the scale. The tenth line on the vernier is coincident with the 
 ninth line above 30 that is, a space of 9 divisions on the scale is 
 divided into 10 spaces on the vernier, so that each space on the latter 
 is one-tenth part shorter than a space on the scale. In the present 
 case the spaces on the scale represent inches and tenths; hence the 
 difference between the length of a space on the vernier and one on 
 the scale is T V of T V = T^(r of an inch. This principle of matching 
 two scales having spaces of slightly different magnitude is always 
 followed hi the construction of verniers, though, of course, the num- 
 ber of spaces embraced by the vernier is varied to suit the circum- 
 stances and the degree of minuteness desired. Moreover, in some 
 instances, the vernier embraces one more space on the scale, instead 
 of one less, than the number of its own subdivisions that is, 10 
 spaces on the vernier may be made to correspond to 11 spaces on 
 the scale. 
 
14 BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 
 
 If, as we have seen, the spaces on the vernier are one-tenth smaller 
 than on the scale, then, in the adjustment shown in figure 4, the first 
 line above the zero on the vernier is one-tenth part of the space, the 
 next line two-tenths, the next three-tenths, etc., distant from the 
 line next above on the scale. When, therefore, we find the vernier in 
 such a position as shown in figure 5, where the fifth line on the ver- 
 nier is coincident with a scale line, it' is very clear that the zero line 
 of the vernier must be just five-tenths above the scale line next below. 
 Now, since we imagine these scales to represent niches and tenths, 
 then figure 5 will read, 30.15 inches. 
 
 13. Estimation of fractions on a vernier. In many cases it will 
 happen that no single line on the vernier will be exactly coincident 
 with a scale line, but that one line will be a little above while the 
 next line on the vernier will be a little below the corresponding scale 
 lines. 
 
 In the case shown in figure 6 the seventh and the eighth lines on 
 the vernier are each nearly in coincidence, but neither one is. exactly 
 so. This indicates that the reading is somewhere between 30.27 and 
 30.28. Moreover, we can clearly see that the eighth line is nearer 
 coincidence than the seventh. We, therefore, estimate that the true 
 reading is about 30.277. We might, probably, with as great accuracy 
 have selected 30.278. 
 
 If the scale and vernier are accurately graduated, such readings 
 by a practiced observer will rarely be in error by more than 0.002 
 inch. It is important in estimating the fractions that the eye be 
 exactly in front of the lines being studied. 
 
 14. In figures 7 and 8 are shown verniers applied to a barometer 
 scale having 20 parts to the inch. In this case 24 parts on the scale 
 are divided into 25 parts on the vernier. By the principle already 
 explained in paragarph 12, the value of the subdivisions effected by 
 such a vernier, or, as it is most frequently expressed, the least count 
 of the vernier, will be fa of fa =5^ of an inch. In reading the 
 vernier, therefore, each line will represent 0.002 inch, so that the 
 fifth, tenth, fifteenth, twentieth, and twenty-fifth lines will repre- 
 sent one, two, three, four, and five hundredths of an inch, respec- 
 tively, and are so numbered. 
 
 As described in paragraph 13, the lines in this kind of vernier also 
 may not be exactly in coincidence; but in such a case, owing to the 
 smaUness of the spaces, it is not of any special advantage in making 
 our estimate to consider whether coincidence is nearer one line than 
 the other. In ordinary practice we simply take midway between. 
 Thus m figure 8 the reading is between 30.176 and 30.178; we there- 
 fore adopt 30.177 as the proper reading. 
 
 15. Caution against error. When the zero line of this style of ver- 
 nier is next above one of the shortest lines on the scale, as was the 
 
BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 15 
 
 case in the example above, some attention is necessary in order to 
 take off the correct reading. For example, in figure 8 we find that 
 coincidence on the vernier is between lines designated 26 and 28, 
 which corresponds to a reading of 0.026 or 0.028, or, taking midway 
 between, 0.027. On the scale itself, however, we see the graduation 
 next below the first line of the vernier is 30.150. The complete read- 
 ing is found by adding the parts thus: 30.150 + 0.027 = 30.177. It 
 frequently happens with beginners that the 0.050 represented by the 
 short line on the scale is overlooked and omitted entirely that is, 
 the above reading might be called 30.127. Whenever readings are 
 made with a scale and vernier of this character, special pains must 
 be taken not to omit adding 0.050 to the vernier reading when the 
 first line below the zero of the vernier is a short one. 
 
 16. Strain on the cistern of a barometer. When the mercury is sent 
 up to the top of the tube of a barometer by screwing up the cistern, 
 an internal hydrostatic pressure is produced proportional to the 
 amount by which the length of the column has been increased. 
 This pressure tends to force the mercury through the joints of the 
 cistern or the joints and pores of the leather bag. This is more par- 
 ticularly the case with a barometer at an elevated station, where, owing 
 to the diminished air pressure, the column may need to be raised 10 
 or 15 inches in filling the tube, greatly endangering the cistern. For 
 this reason the Fortin barometer cistern is not wholly satisfactory, 
 as it is difficult to make and keep the joints so tight that the mercury 
 will not be able to find its way through some very small crevices; 
 such leakage soon impairs the barometer. 
 
 17. Tuch cistern. Many of the barometers of the Weather Bureau 
 are fitted with an improved form of cistern, devised by Mr. Charles 
 B. Tuch, of the Instrument Division. The construction of this is 
 shown in figure 9. 
 
 The chamber for the mercury is formed of the iron cylinders, c, fc, 
 provided with windows at the top, and a small glass cylinder,/. The 
 glass barometer tube is fastened into a metal piece, 6, by means of 
 several thicknesses of leather washers, held and clamped by a screw, 
 e, above. The piston, o, fits the cylinder very snugly and can be 
 moved up and down by means of the milled-head screw, W, thereby 
 adjusting the mercury to any desired level. 
 
 18. Fixed cistern barometers. It is very evident that as the column 
 of mercury rises and falls in a barometer tube there is a corresponding 
 change in the level of the surface in the cistern, and as long as the 
 quantity of mercury in the whole barometer remains the same, it 
 follows, except for slight temperature effects, that the true height of 
 the column of mercury may always be found simply from readings at 
 the top end, a due allowance being made for the slight rise and fall in 
 the cistern. 
 
16 BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 
 
 y- 
 
 m 
 
 Adjustable cistern barometers, such as 
 described in paragraphs 9 and 17, are, in 
 general, the most accurate, as the cor- 
 rection for capillarity, paragraph 26, is 
 usually more constant, and the acci- 
 dental escape of a little mercury from the 
 cistern does not matter. Still, very ac- 
 curate results may be obtained by the 
 use of well-made barometers with fixed 
 cisterns, and this form is often adopted 
 in the construction of barometers for use 
 on shipboard. 
 
 The relation between the true length 
 of the column and the observed position 
 of its top depends upon the relation 
 between the inside areas of the cistern 
 and barometer tube. When this relation 
 has been once worked out it is then nec- 
 essary in reading the barometer to observe 
 only the position of the top of the column 
 and apply a "correction for capacity." 
 (See paragraph 24.) 
 
 19. Contracted barometer scale. As the 
 correction for capacity in barometers with 
 fixed cisterns remains the same so long 
 as the quantity of mercury within the 
 barometer and the inside area of tube 
 and cistern are unchanged, it will not be 
 necessary to apply a capacity correction 
 to every reading made, provided we use 
 on the barometer a scaie having all its 
 divisions shortened by just the proper 
 amount to compensate for the capacity 
 effect. To understand this more clearly, 
 imagine a barometer with the top of the 
 column just 30 inches above the surface 
 of the mercury in the cistern. Suppose 
 the sectional area of the barometer tube 
 at the top is only one-fiftieth as great as 
 that of the cistern (this is about the 
 usual relation) . Now if we imagine the 
 column to rise a distance of 1 inch in the 
 tube, it will then seem to become 31 
 
 inches high, but when the column rises 1 inch, the mercury in tho 
 cistern falls one-fiftieth of an inch, and, therefore, the real height of 
 
 Hr* 
 
 FIG. 9. -Tuch barometer cistern. 
 
BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 17 
 
 the column must be 31^V inches; that is, we may say, that each inch 
 of a scale represents l^V inches of change in the real height of the 
 mercurial column. If, therefore, a special scale be prepared having the 
 spaces representing inches, each one fifty-first part of an inch shorter 
 than a true inch, then readings of our imaginary barometer on such 
 a scale will indicate the true height of the column, presupposing of 
 course that the sectional areas of the tube and cistern are uniform, 
 and that the scale is adjusted|to a proper distance from the cistern. 
 
 20. By methods of calibration man- 
 ufacturers are able to construct scales 
 and barometers of great accuracy 
 in accordance with the above prin- 
 ciples, and they are very convenient 
 to use. 
 
 21. It is obvious that if a barometer 
 tube in such an instrument is broken 
 it will be difficult to find another so 
 nearly the same size that it could be 
 used with the old scale; generally a 
 new scale is also required. 
 
 22. In figure 10 is shown a cut of an 
 excellent barometer of the fixed-cis- 
 tern type, devised by Schneider Bros., 
 of Jersey City, N. J. One of the special 
 features of the barometer is the means 
 provided for filling the cistern and 
 tube with mercury so that the barom- 
 eter can be shipped safely from place 
 to place. 
 
 To fill the cistern with mercury, 
 the barometer is first very carefully 
 and gradually inclined and inverted. 
 When fully inverted the mercury suf- 
 fices to fill up the cistern just to the 
 throat of the contracted portion at G. 
 By screwing up the milled head, H, 
 the plate, G, closes against the bottom 
 of the cistern and completely im- 
 prisons the mercury with only a little free space for expansion. If the 
 barometer is now turned erect the mercurial column can not descend 
 unless the screw, H, is loosened, whereupon the mercury flows into the 
 previously unoccupied space below the plate, G, and permits the 
 column to resume its normal level. 
 
 23. The capacity correction required for a barometer with fixed 
 cistern and true scale graduated in standard units, and not contracted, 
 
 2293712 2 
 
 Fxo. 10. Fixed cistern barometer. 
 
18 BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 
 
 as explained in paragraph 19, can be determined from careful meas- 
 urements of the internal diameters of the tube and cistern made 
 before the barometer is filled, but the accuracy of this correction 
 should always be checked by subsequent comparisons, as indicated 
 below. 
 
 24. Capacity correction, how found. It is evident that by sliding the 
 scale of a fixed-cistern barometer up or down it can be so adjusted 
 that a reading at some one point is just right ; for example, we may 
 place the 30-inch mark so that when the top ot the column is at this 
 mark the surface of the mercury in the cistern is just 30 inches 
 below. If the sectional area of the tube is a, and that of the cistern 
 A, then, if the mercurial column in the tube rises one scale division, 
 
 the fall in the cistern will be only the * part of one division. That 
 is, the correction for a scale reading just one division above the 30- 
 inch mark is : + ^ divisions ; for a reading two divisions above the 
 
 correction is : + 2 -j , etc. This, expressed in a mathematical formula, 
 becomes 
 
 (Correction = C= (h R )-*; 
 
 ^ 
 
 in which R is the reading at which the correction is zero and h is the 
 observed reading, uncorrected for temperature. This may be reduced 
 to the following simpler form 
 
 C=nh m; 
 
 in which m and n are two quantities whose values are best determined 
 from a complete series of readings of the actual height of the mer- 
 curial column, as compared with the reading of the top of the column 
 of the fixed-cistern barometer. As the level of the mercury in the 
 cistern is generally not visible in barometers of this type, the direct 
 measurements of the heights of the column can not be made, and the 
 necessary actual heights must, therefore, be obtained from readings 
 of some standard barometer. 
 
 To determine the values of m and n accurately by comparisons, 
 observations should be made over a greater range of pressures than 
 ordinarily occur from day to day, and the best results will require 
 observations under pressure artificially changed to suit. 
 
 25. Changes of temperature may cause the sectional areas of the 
 tube and cistern to have a different relation than that assumed in 
 the formula above, and may also change the value of R , but these 
 effects are slight and are not considered here. 
 
BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 19 
 
 26. Mercurial barometer for marine use. Mercurial barometers con- 
 structed upon the For tin system and other forms such as described in 
 the preceding paragraphs are almost universally used on land for the 
 measurement of atmospheric pressure, and no other form of barometer 
 affords as great accuracy in the measurement of that pressure. If, 
 however, such an instrument were placed aboard ship at sea the 
 column of mercury would surge up and down the tube more or less 
 violently with every motion of the vessel, and readings would be 
 rendered inaccurate or impossible. 
 
 27. This difficulty has been overcome in a form of instrument 
 known as the Kew, or marine, barometer. Its distinguishing char- 
 acteristics consist in substituting for the simple straight tube 
 of uniform bore commonly employed in land barometers, a tube 
 having a wide bore for 6 or 8 inches of the upper portion only. 
 Below this the tube has thick walls with a small capillary bore only a 
 few hundred ths inch in diameter. Near the bottom end the bore of 
 the tube is again enlarged to form an air trap, all as shown in figure 11. 
 If small quantities of air chance to enter the open end of the tube tlicv 
 arc not likely to enter the small point of the inner tube, but lodge 
 instead in the surrounding space, as indicated, where the air must 
 remain and does not affect the barometric readings. It may even be 
 removed from the trap when the barometer is undergoing repairs. 
 
 28. The flow of mercury through the capillary bore takes place so 
 slowly that the column can not surge up and down the tube seriously 
 with the relatively quick motions of the ship. At the same time the 
 height of the column adjusts itself to the slow changes of atmos- 
 pheric pressure, and thus more or less perfectly answers the desired 
 objects. 
 
 _".). Figure 12 shows a high grade mercurial barometer a<laptc<l to 
 all the requirements of marine use, together with a special gimbal sup- 
 porting bracket and small wooden box, into which the barometer and 
 bracket are folded and thoroughly protected and secured when not 
 in use. 
 
 30. The glass tube and boxwood cistern, all as shown in figure 11, 
 are secured inside the bronze-metal jacket provided at the top with 
 a long, slotted opening through which the top of the glass tube and 
 mercurial column can be seen. A scale of graduations is fixed beside 
 the opening and a vernier of a form already described in paragraph 12 
 i- arranged to slide up and down so as to enable accurate mea-uic- 
 ments of the height of the mercurial column to be made. 
 
 31. For marine use it is necessary that the barometer be live to 
 han.u: in a vertical line despite the rolling and pitching of the vessel. 
 For this purpose the well-known arrangement of irimbal rin<r- i- 
 formed upon the outer extremity of a hinged bracket and >ecmed to 
 the barometer at a point some inches above the middle. 
 
20 BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 
 
 ill 
 
 FIG. 11. Tube and cis- 
 tern of marine ba- 
 rometer. 
 
 FIG. 12. Marine barometer and box. 
 
BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 21 
 
 In the position shown in the picture the barometer is readv for 
 reading, and the tube will swing on the gimbals so as to remain 
 nearly or quite vertical. After a reading has been taken the barome- 
 ter must not be left exposed, as it is very liable to injury by violent 
 oscillations in heavy weather. In the equipment of the standard 
 Weather Bureau design the whole bracket barometer and all are 
 arranged to fold up compactly within the small mahogany case, the 
 lid of which closes with a spring clasp, and not only secures the barom- 
 eter from accidental damage but from undue exposure to atmos- 
 pheric- influences as well. 
 
 32. How barometer tubes may be filled. Processes that may be fol- 
 lowed in filling barometer tubes for high-grade instruments are so 
 rarely described and so little known that a short description of some 
 methods frequently employed at the Weather Bureau with highly 
 satisfactory results will doubtless be of interest to a number of 
 readers. 
 
 The object of any filling process is simply to introduce pure mer- 
 cury and totally exclude all air, moisture, or other foreign matter, 
 especially of a gaseous nature, that may possibly later occupy the 
 barometric vacuum and cause errors by the pressure it exerts upon 
 the top of the mercurial column. 
 
 33. Funnel method. It may seem that the desired result could be 
 obtained by carefully introducing clean mercury through a long 
 slender-stemmed funnel reaching quite to the closed end of the tube. 
 (See fig. 13.) A suitable funnel may easily be made by drawing 
 down the end of a short piece of rather wide glass tubing. Such a 
 method is sometimes used and will, indeed, give approximate results, 
 but it will be found upon investigation that while the mercury seems 
 to drive out all the air, yet a good deal will still be found in the 
 vacuum. Originally, this air mixed with water vapor is strongly 
 adherent to the walls of the glass tube by reason of a peculiar prop- 
 erty this character of glass is found to have. When the barometric 
 vacuum is formed, some of the gaseous matter thus attached to the 
 tube is liberated and by its pressure depresses the mercurial column 
 several hundredths of an inch, as has been shown by careful experi- 
 ment-. 
 
 In all cases it is of great importance that the inside walls of barom- 
 ter tubes be perfectly clean. New tubes are thoroughly cleaned 
 with whiting or other suitable material while open at both ends, and 
 while still warm and dry the top end is closed and the cistern end 
 tapered and finally fused shut. 
 
 Small tubes (one-quarter inch and less) that have become soiled by 
 use, exposure, etc., can not be easily cleaned properly, and such are 
 never used a second time in the Weather Bureau work. The methods 
 
22 BAEOMETEES AND MEASUEEMENT OF ATMOSPHEEIC PEESSUEE. 
 
 given in paragraph 35 for cleaning larger tubes may, however, be 
 used even with these. 
 
 One very simple and excellent method of driving off nearly all the 
 air and moisture condensed on the glass walls is given in the next 
 paragraph. 
 
 34. The boiling method. This is a simple method commonly 
 employed with all small tubes, say, one-fourth inch diameter, more 
 or less; such, for example, as is required in the several types of 
 barometers that have been heretofore described. Much larger tubes 
 are frequently boiled, but these when full of hot mercury are diffi- 
 cult to handle, a strong heat is required, and the danger of serious 
 accidents is considerable. 
 
 It is well at first to warm more or less the whole tube, and the cup 
 of clean filtered mercury r from which the supply is drawn should 
 also be gently warmed. 
 
 Sufficient mercury to fill the tube 3 or 4 inches is introduced by the 
 aid of a funnel such as shown in figure 13, except that the slender stem 
 need be only 2 or 3 inches long. In the absence of such a funnel it is 
 quite as well to employ a small paper cone of the kind commonly 
 used in filtering mercury. The mercury in the tube is then boiled 
 carefully over a good Bunsen burner flame. (See fig. 14.) For this 
 purpose the tube is held easily in the hands and moved continuously 
 through the flame and rotated so as to avoid undue local heating of 
 the tube. As the heating proceeds the air and moisture vapor first 
 form in minute silvery-white bubbles, giving the tube a frosted ap- 
 pearance. These enlarge, and after actually boiling the mercury for 
 a while all evidence of formation of bubbles on the walls disappears, 
 and further boiling of the mercury takes place with sudden bursts 
 and with sharp metallic clicks as the portions of the boiling mercury 
 strike each other or the walls of the tube. When it is apparent that 
 the gases on the walls of the tube have been driven off sufficiently, a 
 fresh quantity 3 or 4 inches of warm mercury is added, and this 
 portion then heated and boiled. The line of separation between the 
 new and the old mercury is rendered plainly conspicuous by the 
 frosted appearance previously mentioned. 
 
 These operations are repeated until the mercury reaches 3 or 4 
 inches from the tip of the tube, the latter portion being filled by the 
 careful use of the funnel without boiling. If the walls of the tube 
 are clean and dry the boiling method is easy to employ and gives 
 very high vacua. 
 
 35. Cleaning large tubes. Tubes that have contained mercury of 
 which oxidized and impure portions may still adhere more or less 
 closely to the wall should first be treated with dilute nitric acid (1 
 part in 20), and then thoroughly rinsed with plenty of water. 
 
 i For notes and remarks on testing and filtering mercury, see pars. 137 and 138. 
 
BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 23 
 
 Ammonia or some other alkali may be added, if desired, after which 
 the operations described below should be followed. 
 
 Introduce several inches of soapy water and whiting with tissue- 
 paper pulp. It is often easiest to put into the tube several small 
 sheets of cheap straw or manila tissue paper, and add the water and 
 whiting afterwards. 
 
 FIG. 13. FIG. 14. 
 
 Funnel tube and boiling method of filling barometer tubes. 
 
 This creamy mass can be strongly shaken about inside the tube 
 and serves to scour and clean the walls in a very satisfactory manner. 
 It is then removed by copious rinsing with clean water and after- 
 war<U with distilled water. After draining some minutes, strong 
 alcohol in moderate quantities is several times introduced and suc- 
 cessively drained out and the tube given a final draining for half an 
 
24 BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 
 
 hour or so, if convenient, after which it is ready for drying and 
 filling. 
 
 36. Air-pump method of filling. This method, with numerous modi- 
 fications, has been employed by the writer in a large number of cases 
 with very satisfactory results. The method requires a good air 
 pump, drying tubes, beakers, burners, stands, etc., such as are gen- 
 erally available in any physical laboratory. 
 
 FIG. 15. Air-pump method of filling. 
 
 The apparatus is arranged as shown in figure 15. The exhaustion 
 and funnel tube Ff will probably require to be made up to suit require- 
 ments by some one a little familiar with simple glass-blowing opera- 
 tions. For most purposes this may be attached to the barometer 
 tube by a short piece of soft pure rubber tubing. The outside end 
 of the funnel is drawn down into a long capillary extension which is 
 
BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 25 
 
 bent several times, as shown, so as to dip into the cup of mercury. M. 
 Too fine a capillary should be avoided, and it is generally necessary 
 and easy to weaken the capillary, as at o, by heating it a little, so that 
 later the tube will break off at this point when a torsinal strain is 
 put upon it by twisting the bent extremity, a b c. The point at c is 
 closed by fusion, to begin with. A stopcock may be employed, as 
 at dj and in this case the breaking of the tube is not required, but if 
 the stopcock leaks even a little, the result may be defective and the 
 arrangement first described is often best. 
 
 37. Drying and filling the tube. In order to dry the tube it is alter- 
 nately nearly exhausted and dry air admitted while the walls are 
 more or less continuously heated by playing over the tube with the 
 ilume from a Bunsen burner. These operations must be repeated 10 
 or more times and the tube kept hot. Throughout these operations 
 the mercury is excluded and the funnel tube partakes at least partly 
 in the drying influences. While the tube is kept quite warm and 
 the vacuum maintained at a high point the capillary is broken at 
 a and the mercury in M, which has been heated in the meantime, 
 is permitted to flow. M need not be large enough to contain all 
 the mercury required, but additions may be frequently made and 
 the whole kept quite warm. The filling will take place slowly, de- 
 pending upon the size of the bore of the inlet tube. The vacuum 
 must be maintained at a high point until the mercury fills the barome- 
 ter tube, when the flow may be stopped by admitting air to the pump. 
 The vessel M must also be removed if there is any tendency of the 
 mercury to flow one way or the other by gra vitiation. 
 
 For the very finest effects the barometer tube can be exhausted 
 by a Sprengel or other high vacuum pump, but in this case the rubber 
 tube connections must be replaced by glass and tused joints. 
 
 38. It may be remarked here that the very high vacua with which 
 we are familiar nowadays, in X ray and other such tubes, are by no 
 means essential, except in the highest grade of "normal" barometers 
 (see paragraph 45), where results depend upon the absolute height 
 of the mercurial column. In the case of instruments in which a cor- 
 rection is found by comparison with a normal, and especially in baro- 
 graphs where the results depend entirely upon differences in the posi- 
 tion of the mercurial column, simple methods of filling give entirely 
 satisfactory results. In these cases the gaseous pressure in the 
 vacuum is so nearly constant that no serious error is involved. 
 
 Suppose, for example, that the residual air in a barograph tube 
 exerts a pressure of 0.1 inch, which would be inexcusably bad fill- 
 ing. Now, since we set the pen of the barograph to agree, from time 
 to time, with a standard barometer, the only effect the air can have 
 is such as results from changes in pressure due either to changes 
 in temperature or changes in the volume of the vacuum chamber. 
 
26 BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 
 
 A 20 change of temperature between settings of the pen is no 
 usual but would introduce an error in this case of only about 0.003 
 inch, whence, with reasonably good filling, the errors from imperfect 
 vacua are entirely insignificant. 
 
 39. Errors of barometers. No matter how carefully a barometer 
 may be made, certain errors due to various causes can hardly be 
 eliminated. In the first place, if any residual air or vapor or any 
 kind of gaseous matter remains in the top of the barometer tube, 
 the column of mercury will not rise as high as it should. We know, 
 likewise, from pnysical laws, that the capillary forces acting between 
 the free surface of mercury and the glass walls at the top of the 
 column also operate to prevent the mercury from rising as high as 
 it should in the tube. Still other errors arise from faults in the 
 graduation of the scale and from failure to place it and the vernier 
 at exactly the positions they should occupy. 
 
 It is not practicable, or necessary, as a rule, to determine these 
 errors separately. When an instrument is completed, its readings 
 are carefully compared with those of a standard barometer. The 
 differences found in this way represent the outstanding effect of the 
 several sources of error mentioned above and are commonly called 
 the "correction for instrumental error and capillarity." 
 
 40. There is still another source of considerable variation in the 
 readings of mercurial barometers, namely, the influence of temper- 
 ature, a rise of temperature expanding both the metal scale and the 
 mercurial column. If both mercury and scale expanded equally no 
 correction would be necessary, but the mercury expands much more 
 than the scale, so that a large correction is required for temperature. 
 
 41. The following detailed discussion of the several errors men- 
 tioned above will make the matter more clear. 
 
 (1) Corrections for capillarity. In all barometers having compara- 
 tively small tubes, that is, of less diameter than from 0.7 to 1 inch, the 
 top of the mercurial column, or the meniscus, as the rounded surface 
 is generally called, will nearly always be quite convex on account of 
 the capillary action between the mercury and the glass. In conse- 
 quence of this the mercury column is actually depressed a slight 
 amount and never indicates the true barometric height. This source 
 of error is one of the most troublesome to which barometers are sub- 
 ject, as the capillarity is never quite constant and there is no prac- 
 ticable method by which its changing value can be accurately deter- 
 mined in the daily use of an instrument. 
 
 The error due to capillarity is nearly always eliminated as far as 
 possible from the scale reading by adjusting the scale so that allow- 
 ance will be made for the average capillary depression. If an ordi- 
 nary barometer be carefully examined it will be found that the 30- 
 inch mark on the scale is appreciably less than 30 inches from the 
 
BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 27 
 
 ivory point. In general, the difference represents the amount the 
 mercurial column is depressed by capillarity. A portion of a barom- 
 eter scale is shown enlarged at v, figure 2 (at the top and at the right) . 
 The index line at v is made accurately 30 inches from the ivory point , 
 but the 30-inch -line on the scale is shown set slightly below to oi 
 the capillary depression. 
 
 (2) Correction for imperfect vacuum. It is generally assumed that 
 the space in a barometer tube above the mercurial column is a perfect 
 vacuum, and that there is no downward pressure upon the top of the 
 column of mercury. This, however, is not strictly the case in any 
 instance, and often an appreciable quantity of air or water vapor i- 
 present. Any vapor that the mercury may give off is, of conr-e. 
 always present. This latter, however, is very small and is never 
 considered except in the most refined investigations. If, therefore. 
 anv such pressures exist upon the top of the column it will be de- 
 pressed, and a correction, which may be properly called correction 
 for imperfect vacuum or reduction to perfect vacuum, should be ap- 
 plied. Such a correction will vary with both the temperature ami 
 the volume of the space. If the trace of air present is slight, * ifl 
 nearlv always the case in any good barometer, the correction for 
 vacuum will be nearly constant, provided the volume is not changed 
 much by great changes of pressure, as the changes corresponding to 
 ordinarv changes in temperature are comparatively small. There- 
 fore, in ordinary observatory barometers this correction, like the one 
 for capillarity, is included in the correction for instrumental error-. 
 When, however, a barometer is used at both high and low pressures. 
 the volume of the vacuum space may change many fold, and in srn-h 
 a case any error due to imperfect vacuum is far from being constant . 
 
 (3) Correction for instrumental or scale error. Errors arising from 
 several independent sources are embraced under this designation, as, 
 for example: (a) The graduated scale may not be adjusted so per- 
 fectly that its divisions are at exactly .the right distance from the 
 ivory point : (6) the sighting edge of the vernier may not be true of 
 in proper correspondence with its zero graduation line; (c) the 
 unavoidable errors and irregularities hi the graduations of the barom- 
 eter scale itself also introduce different errors from point to point 
 along the scale. Nevertheless, sufficient precision in scale gradua- 
 tion is easily attainable even in a scale that is only fairly good, and 
 sueh errors are generally so small as to be unimportant in ordinary 
 barometric observations and are seldom considered. 
 
 As stated in a previous paragraph, the combined effect of such 
 sources of error as those just mentioned gives rise to what is generally 
 called the correction for instrumental or scale error. 
 
 The manufacturer, in adjusting a good barometer, endeavor- to 
 eliminate as completely as possible, or at least to reduce to a very 
 
28 BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 
 
 small quantity, the several corrections mentioned above, viz, cor- 
 rection for capillarity, for imperfect vacuum, and. for instrumental 
 error. This he can do by sliding the scale up or down a small frac- 
 tion of an inch until he finds by repeated trials and comparative 
 readings with a standard instrument that the new barometer, when 
 corrected for temperature, as described below, gives the same or 
 nearly the same readings as the standard. Any slight outstanding 
 difference that may finally remain then becomes the " correction for 
 instrumental error, including capillarity/' or briefly, " correction for 
 scale errors and capillarity.'' 
 
 By comparing a barometer in a partial vacuum, so as to ascertain 
 the " correction for scale errors and capillarity" at several pressures, 
 such, for example, as at each inch between 20 and 30 inches of 
 pressure, it has been learned in a few interesting cases that very 
 great differences in the correction may be found at different points 
 of the scale. These differences amounted in one case to eighty- three 
 thousandths of an inch between 25 and 30 inches, and could not be 
 explained by any error of the scale or by any influence other than that 
 of the irregular capillary action at different points of the tube. These 
 investigations demonstrated the necessity of ascertaining the correc- 
 tion for scale errors and capillarity of each instrument for the par- 
 ticular pressure at which that instrument is to be used. 
 
 (4) Correction for temperature. The temperature of a barometer 
 affects the accuracy of its readings in two ways. First, the metal 
 scale expands and contracts with changing temperatures, and is, 
 therefore, continually changing its length. Second, the mercury 
 itself expands and contracts much more than the scale. The 30 
 cubic inches of warm mercury in a barometer tube at, say, a tempera- 
 ture of 80 F., will be more than 1 ounce lighter than the same volume 
 of mercury at the freezing temperature. 
 
 The true pressure of the air, therefore, is not shown by the observed 
 height of the mercurial column until we take into account both the 
 temperature of the scale and the density of the mercury. 
 
 For this reason barometric readings require to be reduced to a read- 
 ing which would have been obtained had the mercury and scale been 
 at certain standard temperatures. 
 
 The standard temperature adopted for the mercury is always that 
 of melting icethat is, C., or 32 F. 
 
 When the readings of the scale are taken in inches, the standard 
 temperature for the scale reduction is then 62 F. If, however, the 
 metric unit of length is used, the standard temperature is then C. 
 In the latter case the same temperature serves for both the scale 
 and the mercury. 
 
 There is thus a disparity between the temperatures at which Eng- 
 lish and metric scales are of standard length; moreover, tables of baro- 
 
BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 29 
 
 metric corrections for temperature usually give the reduction for 
 both the scale and the mercury hi one correction, whence it follows 
 from these two circumstances that the corrections in English and in 
 metric tables are not mutually con vert able. An error is therefore 
 introduced if the unconnected reading of a mercurial barometer 
 expressed in metric units is converted into English units, or vice 
 versa, and a temperature correction afterwards applied to the result. 
 The conversion of barometer readings from English to metric or from 
 metric to English units can only be made correctly after each read- 
 ing has been fully corrected for temperature. A further discussion of 
 this point will be found in the Monthly Weather Review for July, 
 1898, page 302. 
 
 42. Barometer correction cards. Each barometer of the Weather 
 Bureau, when sent out, is accompanied by a coirection card (Form- 
 
 /No. 1059-Met'l) showing the correction for instrumental error, and* 
 also the corrections of the attached thermometer. If these latter correc- 
 tions are as large as half a degree, which is, however, rarely the case, 
 they should be applied to the reading of the attached thermometer 
 before taking the correction for temperature from the table. 
 
 43. Tables of temperature corrections. Tables of correction for tem- 
 perature are computed by simple formulae taking into account the 
 known coefficients of expansion of the mercury and of the metal or 
 material of which the scale is made. The scale in this sense includes 
 all the metal parts between the ivory point and the top of the column 
 of mercury. It is generally assumed that the temperatures of the 
 scale and mercury are the same, and that the temperature is given 
 by the indications of the attached thermometer. 
 
 For barometers with brass scales the following formula is used for 
 computing corrections: 
 
 ,. -, Z- 28.630 
 
 -*1.1123+ 10,978 
 
 in which h is the observed reading of the barometer in inches, and t 
 is the temperature of the mercury and scale in degrees Fahrenheit. 
 
 The numerical factors in this equation are obtained by using the 
 folio whig values for the expansion of mercury and brass, viz: 
 
 Cubical expansion of mercury, 0.0001010 per degree Fahrenheit. 
 
 Linear expansion of brass, 0.0000102 per degree Fahrenheit. 
 
 In Section VIII are given full tables of corrections computed by 
 the above formula. 
 
 44. Correction for density of mercury. If the density of the mercury 
 is not the same hi two barometers that are exactly alike in every other 
 respect, the heights of the mercurial columns will not be the same 
 for the same pressure. In such a case a reduction to mercury of a 
 standard density will be required. The presence of 1 per cent of lead 
 with mercury causes a change in density that would require a correc- 
 
30 BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 
 
 tion of about 0.051 of an inch. On the other hand, mercury contain- 
 ing even so little as one one-hundredth of 1 per cent of lead is ren- 
 dered so exceedingly foul that it could not be used for barometric 
 purposes. It is therefore easily seen that a correction for standarc 
 density is a refinement which need not ordinarily be considered. 
 
 NORMAL BAROMETER STANDARD BAROMETER. 
 
 45. It is easily understood, after what has been said above aboul 
 errors of graduation, errors due to capillarity, to imperfect vacuum 
 to instrumental imperfection, etc., that even the best of ordinary 
 barometers is liable to be quite incorrect until corrections for these 
 errors have been determined. Moreover, from the nature of things 
 we can not determine these corrections except by comparison with a 
 standard barometer, and the question might properly be asked, How 
 do we know the standard barometer is right? We will answer this 
 by saying that the standard barometer ought to be a normal barom- 
 eter. So few understand clearly the distinction between these words 
 " standard" and " normal 7 ' in the present connection that somd 
 explanation is necessary. In the first place, the expression " normal 
 barometer" is used a great deal by the Weather Bureau and meteorol- 
 ogists in general when, strictly speaking, the expression should be 
 normal barometric pressure; by which is meant the average of a 
 great many years' observations of atmospheric pressure at a single 
 station. In the present case the word " normal" has an entirely 
 different meaning. 
 
 A standard barometer need not necessarily be anything more than 
 an instrument which has been pronounced to be correct by some 
 special authority. For instance, the Congress of the United States 
 might say that the indications of such and such an instrument 
 represent the true atmospheric pressure and that the particular 
 barometer in question is the standard of the nation. Such an instru- 
 ment, although formally pronounced to be a standard, might, never- 
 theless, possess little more than the average accuracy and its indica- 
 tions still be more or less erroneous. Since the several errors to 
 which barometers are subject can not, in the majority of cases, be 
 determined except by comparison with an instrument whose errors 
 are all known, a standard based only on the dictum of some authority 
 can not necessarily be regarded as giving true indications. A normal 
 barometer, however, is one the construction of which is such that the 
 instrument, fundamentally and independent of all other similar instru- 
 ments, gives a true measure of the pressure of the air. 
 
 Standard barometers should therefore generally be also normal 
 barometers. It must not be understood that a normal barometer is 
 absolutely without any error. The construction, however, is such 
 that those errors which can not be wholly eliminated can yet be 
 ascertained from the indications of the instrument itself. The error 
 
BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 31 
 
 for capillary action, for example, is wholly eliminated by employing 
 a tube of very large diameter. On the other hand, if the vacuum is 
 not sufficiently perfect, the error from this cause can still be ascer- 
 tained, for the barometer will be constructed so that readings can be 
 made when the vacuum chamber is large, and again when it is many 
 times smaller and the pressure of the remnant of air therein propor- 
 tionately increased; from such readings the desired corrections can 
 be computed. So, also, other errors are either eliminated or are 
 ascertained by special investigations, and the reading of the barometer 
 after all known corrections are made is regarded as fundamentally 
 correct. 
 
 Barometers of this type are generally elaborate of construction 
 and will not be described here. Several of the European normals 
 are fully described by Prof. Abbe in the Annual Report of the Chief 
 Signal Officer, 1887, Part II. 
 
 II. ANEROID BAROMETERS. 
 
 46. Figures 16 and 17 represent two of the more important types 
 of aneroid or holosteric barometers, showing, principally, the internal 
 mechanisms. The first is a more common form, but the second 
 i-. in general, somewhat better. The essential feature is the same 
 in both instruments and consists of the small metallic box or cell, 
 J/, the upper and lower walls of which are made of very thin circular 
 sheets of corrugated German silver, which are soldered together 
 on their outer edges, forming a very short cylinder. The air is 
 thoroughly exhausted from this cell through a tube at one side, 
 which, when the vacuum is as perfect as desired, is pinched tightly 
 together, cut off, and hermetically sealed with solder, producing the 
 projection seen at c. The flexible corrugated surfaces, which tend to 
 be collapsed by the pressure of the outside air, are forcibly held apart 
 by the action of a strong steel spring, R. As the pressure of the air 
 increases the spring is compressed and the corrugated surfaces ap- 
 proach each other slightly, returning again or separating still farther 
 with diminution of pressure. To measure the changes in atmospheric 
 pressure, it is only necessary to measure the minute movement- of 
 this flexible cell. 
 
 The two forms of aneroid figured herein differ simply in the man- 
 ner by which the minute alterations in the elastic yielding of the 
 spring are magnified and rendered measurable. 
 
 47. In the common aneroid a lever, Z, attached directly to the spring 
 connects by a link, ra, with a very short arm of a sort of bell-crank 
 lever, r, t, having a horizontal axis on pivots at each end. 
 
 The longer arm, t, of this bell-crank lever is connected by means of 
 a wire, s, with a very fine chain, the other end of which winds around 
 a -mall wheel or drum on the axis, a, upon which is mounted the hand 
 
32 BAROMETEES AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 
 
 as seen. At 6 is shown a small spiral steel spring, like the hair- 
 spring of a watch, which serves to take up the slack in the loose con- 
 nections of the numerous joints, levers, and links. 
 
 At r is shown, also, a small counterpoise weight attached to the 
 bell-crank lever to aid in securing a more stable position of the index 
 when the barometer is placed in different positions: that is, whether 
 the dial is horizontal, or vertical, or turned to one side or the other. 
 
 The point of attachment of the link, ra, to the bell-crank lever is 
 sometimes adjustable so that the movements of the hand can be made 
 to correspond to the value of the scale graduations. 
 
 The steel spring, R, is also slightly adjustable by means of a screw 
 from the underside threaded into the part, N. This permits adjusting 
 the hand to any particular point of the scale to give correct readings. 
 
 FIG. 16. Aneroid barometer. 
 
 48. Effects of temperature. The steel spring and the feebler elastic 
 reaction of the composition metal of the vacuum chamber are appre- 
 ciably weakened by increase of temperature, so that in some cases j 
 a rise of the pressure may seem to occur which is really caused by 
 the weakening of the spring. In some cases efforts are made to com- 
 pensate for this by leaving a small quantity of air in the vacuum 
 chamber, which when heated increases its pressure upward and tends i 
 to offset the weakening effect upon the springs. A better plan is to I 
 make the lever, Z, of two different metals, viz, brass and iron, firmly 
 brazed together. The differential expansion of these two metals with 
 temperature changes produces flexure in the lever. By filing and 
 adjusting the bimetallic bar, the flexure due to temperature can be 
 made very nearly to balance the effect of temperature on the spring. 
 
BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 33 
 
 The aneroid is then said to be " compensated" and this word is often 
 found on the dial. In many cases this word is there when the com- 
 pensation is very imperfect. 
 
 49. Dejects. The friction and looseness in the joints of the links 
 and the lack of perfect balance in the various parts give rise to con- 
 tinually changing errors hi the reading of the aneroids. This will 
 be shown by tapping the aneroid from different sides and holding it 
 in a variety of positions; a different reading will be given for each 
 condition. 
 
 50. Goldschmidt's aneroid. The numerous levers and links in the 
 common aneroid are dispensed with in this form, and the minute 
 movements of the cell and 
 
 spring are measured di- 
 rectly by means of a mi- 
 crometer screw. 
 
 This is accomplished in 
 several different ways by 
 manufacturers, a common 
 form of instrument being 
 shown in figure 17, where 
 the parts have been sepa- 
 rated for a better view. 
 The plate, B, with its at- 
 tached mechanisms, is se- 
 cured in the bottom of the 
 box, A. The micrometer 
 screw, S, works through 
 the cover of the box. The 
 corrugated aneroid vacu- 
 um chamber, M, is held 
 distended in the usual 
 manner by the steel 
 spring, R. A sharp knife- 
 edge projection, a, of a 
 double-formed lever, I, 
 rests upon a smooth pol- 
 ished spot near the outer end of the spring. This spot is some- 
 tunes a bit of glass or agate. The lever, Z, is pivoted delicately 
 upon an axis at r and is formed of two parts joined near the axis. 
 The upper piece of this lever is a very delicate steel spring, with a 
 flat polished surface at o, which by the springiness of the arm presses 
 against the point of tfie micrometer screw, S'. At the ends the 
 spring and lever are formed with little flat surfaces, each having a 
 fine line engraved across the middle. This construction is not 
 clearly seen in the drawing. To observe the air pressure the aneroid 
 2293712 3 
 
 FIG. 17. Goldschmidt's aneroid. 
 
34 BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 
 
 must be "set" by bringing the above-mentioned lines into coincidence. 
 For this purpose, and at the same time to measure the movement 
 necessary to bring about such a coincidence, the finely cut microm- 
 eter screw, ', is provided. The large head, , having a scale of 
 graduations engraved upon its outer rim, being turned, the point 
 of the screw presses against the spring at o, deflects it so that the 
 lines upon the ends of the spring and lever may be placed in exact 
 coincidence. To facilitate making this adjustment accurately, a 
 small magnifying glass, L, is generally provided. The small scale, P, 
 is opposite the ends of the lever, Z,'when the mechanisms are in their 
 normal position and indicates the whole number of turns made by 
 the screw, or, what is the same thing and more convenient, shows 
 the pressure corresponding to the successive positions of the screw. 
 The fractions of a turn are indicated accurately by the graduations 
 on the head of the screw. 
 
 51. Temperature effects. The Goldschmidt aneroid is not compen- 
 sated for temperature, but is generally accompanied by a table of 
 corrections therefor, the temperature being indicated by a small 
 thermometer, the scale of which, in the aneroid shown, is visible j 
 through an opening at T. 
 
 52. Reading, how made. Aneroids of this pattern are read by first 
 turning the micrometer screw until the lines upon the spring and 1 
 lever come into exact coincidence. The reading on the scale, P, is j 
 noted, and to this is added the part taken from the graduations on j 
 the head. 
 
 53. Consult paragraphs 104 to 106 for information respecting the 
 use of the aneroid in determining elevations. 
 
 54. How adjusted to standard pressures. The aneroid barometer, no | 
 matter how perfectly constructed, does not indicate any particular 
 pressure until by careful comparison with a standard barometer its 
 index is adjusted to give as nearly as possible the same reading as 
 the standard. This adjustment is made by means of the screw, which | 
 in nearly all aneroids is seen just within a small hole in the back of j 
 the case. The graduations of the dial must of course be such as to 
 show changes of pressure on a scale of millimeters or inches. 
 
 The Goldschmidt aneroid is similarly adjusted in a variety of ways, ! 
 of which a common one is to shift the zero or index line at which the j 
 reading of the micrometer screw is made. 
 
 55. Errors and dejects oj aneroids in general. Alter being once j 
 adjusted to give accurate pressures, as already described, the aneroid | 
 should be handled with great care. Violent knocks and shaking i 
 will, especially with the common aneroid, almost certainly change 
 or shift the various links and levers in their joints and change, more 
 or less permanently, the position of the index. For such reasons 
 aneroids are very liable to acquire unknown and often large acci- 
 
BAROMETEES AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 35 
 
 dental errors, and can not, therefore, be regarded as very satisf actory 
 instruments. 
 
 56. Errors due to very slow changes, "creeping." If an aneroid 
 adjusted to read correctly under ordinary air pressures is placed 
 within the receiver of an air pump, the index will quickly fall to a 
 lower pressure when a partial vacuum is formed. If, however, the 
 vacuum be maintained constantly at the same pressure for many 
 days in succession, the reading of the aneriod will be found grad- 
 ually to become lower and low r er, but after three or four weeks 
 further changes cease or are very small. The amount of this slow 
 change differs greatly and may be from one-half inch or less to over 
 an inch, according to the diminution of pressure and other circum- 
 stances. Again, when the barometer is removed from the air pump 
 it does not immediately return to its original correct reading, but its 
 indications will be found to be too low, several weeks being again 
 consumed in a slow return to approximately its former correct reading. 
 
 This "creeping" action depends, no doubt, upon some molecular 
 changes, as yet not clearly understood, that take place within the 
 materials of the aneroid box and steel springs. In any case the read- 
 ings are liable to be very seriously in error, and tourists and others 
 who carry with them aneroids for the purpose of ascertaining eleva- 
 tions should have means to determine and eliminate the very serious 
 errors referred to above. A further discussion of these errors will 
 be found in the Monthly Weather Review for September, 1898, 
 page 410. 
 
 ")7. The aneroid barometer is a convenient instrument for showing 
 more or less accurately the character and the amount of barometric 
 cluniffps going on from day to day, but the mercurial barometer is the 
 oniv instrument that gives atmospheric pressures with that degree 
 of precision required in simultaneous meteorological observations. ' 
 
 ">s. Test of condition of aneroid, Aneroids, seemingly good, are 
 often defective, because some of the joints of the levers and pivots 
 are too tight, causing the hand to stick and not move with the perfect 
 freedom it should. The condition of an aneroid can be quickly tested 
 in this respect by tapping the instrument on the side or bottom with 
 the fingers or knuckles, or perhaps better by lifting the instrument 
 about one-fourth of an inch from a table or cane-seated chair and 
 placing it back again somewhat sharply. Under this treatment, if 
 the joints and levers are perfectly free, the hand will jump away and 
 t lieu return quickly with a vibratory movement to its original position. 
 If the instrument is defective, the hand in some cases will not respond 
 to the slight knocks, or will do so without exhibiting any vibratory 
 movement, or upon being disturbed it may move a little, but will not 
 return to its original position. 
 
36 BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 
 m< MISCELLANEOUS BAROMETERS. 
 
 59. Many curious and interesting forms of barometers have been 
 devised for the purpose of showing the changes of air pressure in a 
 much more magnified manner than is possible with ordinary barome- 
 ters, especially of the mercurial pattern. A few of these will be 
 briefly mentioned. Many others equally curious and meritorious are 
 necessarily omitted. 
 
 60. Water barometer. This may be constructed in practically the 
 same manner as the mercurial barometer, except that water is used 
 instead of mercury. Now, as mercury is 13.6 times as heavy as water, 
 it will result that the water column will be 13.6 times higher than the 
 mercurial column, or about 34 feet high; also a change of 1 inch in 
 pressure by the mercurial barometer will appear as a change of 13.6 
 inches in the water barometer. 
 
 The great defect of the water barometer, aside from its inconven- 
 ient proportions, is the shortening of the column, due to the pressure 
 of water vapor in the vacuum. This shortening amounts to about 10 
 inches at a temperature of 70 F., and if the temperature were to rise 
 10 the pressure would seem to fall nearly 4 inches, when really no 
 change of pressure had occurred. 
 
 61. Glycerin barometer. Glycerin, sulphuric acid, or nonvolatile 
 oils may also be used in place of mercury, each affording a magnified 
 indication of pressure changes, as in the case of the water barometer, 
 and with the advantage that the errors due to vapor pressures are 
 much less, or are quite inappreciable. 
 
 62. Diagonal barometers. It is easily seen that if the top por- 
 tion of a barometer tube be bent off at a small inclination upward 
 from a horizontal position, the movement of the mercury along this 
 diagonal portion will exhibit, in a magnified manner, the ordinary 
 fluctuations of atmospheric pressure. The same result is secured by 
 forming the top portion of a barometer tube into a helical coil having 
 suitable graduations. 
 
 63. Dial barometers. Probably the most practical expedient for 
 magnifying the indications of an ordinary siphon mercurial barometer 
 is the form known as the "dial barometer/' shown in figure 18. 
 
 The action of the instrument will be readily understood from the 
 diagram without further explanation. 
 
 64. Sympiesometer. This is another form of sensitive barometer, 
 in which the pressure of the air, acting through a short column of 
 liquid of low specific gravity, is made to compress a portion of air 
 confined within the instrument. In this case it is necessary to make 
 allowance for the expansion and contraction of the confined air with 
 changes of temperature. This is conveniently accomplished by pro- 
 viding the instrument with two scales, one of which, containing the 
 readings of pressure, is set to a certain line on the other scale accord- 
 
BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 37 
 
 ing to the temperature indicated by the attached thermometer of the 
 apparatus. 
 
 65. Howson's barometer. This ingenious and novel device is shown 
 in section in figure 20. The barometer tube is large in diameter and 
 longer than usual. The cistern is recurved and extends as a long core 
 up inside the barometer tube, reaching within 3 or 4 inches of the top 
 of the column of mercury. The proportions are arranged to be such 
 that the upward pressure of the air on the under side of the cistern is 
 sufficient to sustain the cistern and contents suspended from the end 
 of the barometer tube. 
 
 When a change of pressure occurs the cistern 
 is caused either to ascend upon a barometer 
 tube or to move down to a new position of equi- 
 librium, and, by selecting suitable proportions 
 between the internal diameter of the barometer 
 tube and the thickness of its walls in relation to 
 the diameters of the core and cistern, the move- 
 ment of the latter up and down the tube with 
 changes of atmospheric pressure can be made to 
 represent pressures upon a greatly magnified scale. 
 
 66. Magnifying siphon barometer. Figure 21 
 illustrates still another expedient by which the 
 indications of a plain mercurial barometer may 
 be greatly magnified. The short leg of a siphon 
 barometer is extended upward, in the manner 
 shown. The small-bore tube, a, is enlarged at 
 the top to have the same diameter as the cistern 
 portion. The portions, a, 6, c, of the short leg 
 are filled up to a point at about Ji with colored 
 water, and then above this with kerosene or some 
 other liquid of nearly the same density as water 
 and with which it does not mix. It is plain 
 that when the level of the mercury hi the short leg 
 changes, the meniscus separating the oil from the 
 
 water in the small-bore tube will be seen to change its position by 
 an amount enlarged in proportion to the relation between the areas 
 of the cistern and the small tube. 
 
 67. General comments on magnifying barometers. The special barom- 
 eters described above are interesting and in many respects curious, 
 but they can not be regarded as anything more than philosophical 
 toys and curiosities. It is impracticable in any of the magnifying 
 instruments to determine the absolute pressure with great precision, 
 for the expedients of magnification introduce sources of both constant 
 and accidental errors that affect the results to a magnified extent, so 
 that even less precision generally results than is attained with well- 
 made mercurial barometers of the simple pattern. 
 
 FIG. 18. Dial barometer. 
 
38 BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 
 
 Fro. 19, Syrtipiesometf-r. 
 
 
 FIG. 20. Howson's 
 barometer. 
 
 
 FIG. 21. Three-liquid 
 barometer. *fffl 
 
BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 39 
 IV. BAROGRAPHS, OR CONTINUOUSLY RECORDING BAROMETERS. 
 
 68. The barograph is a form of barometer with the addition of 
 parts by which a continuous record of the barometric oscillations is 
 traced upon a sheet of moving paper or obtained by photographic 
 processes upon sensitized plates. Many different forms of apparatus 
 have been devised, nearly all of which are more or less elaborate, in 
 general, but not correspondingly accurate. In most cases the chang- 
 ing temperature to which the instruments are subjected introduces 
 small errors, and the mechanisms effecting continuous registration 
 either obstruct the free action of the barometer proper or do not 
 transmit to the record sheet a trace representing exactly the original 
 fluctuations of pressure. The absolute value of the pressure indi- 
 cated by the position of the tracing point is also subject to uncertain- 
 ties, and while changes of pressure are shown with greater or less 
 accuracy, it is impossible, even with the best instruments, to record 
 the absolute pressure with a precision equal to that of an eye reading 
 of a standard barometer. In general, therefore, the indications of 
 automatic instruments are checked and corrected by reference to occa- 
 sional eye readings of a standard barometer. 
 
 69. It hardly requires to be said that the mercurial barograph is 
 more reliable and gives more accurate results than those of the 
 aneroid type. Many ingenious mechanisms are employed to effect 
 the continuous registration, thus: 
 
 (1) In some cases this is accomplished by directly photographing 
 on a moving sheet of sensitized paper or a moving plate the changing 
 positions of the summit of a barometric column, notably the case at 
 the Kew Observatory, England. 
 
 (2) In other forms, a float resting upon the mercury in the open leg 
 of a siphon barometer communicates the motion directly to a lever 
 which carries the recording pen at its end and produces the record on 
 a magnified scale. It is difficult to overcome the friction of the pen 
 and magnifying levers sufficiently to obtain correct records; the con- 
 struction in some forms is therefore modified and clock movements or 
 electromagnets are caused to perform the real labor of producing the 
 record. Under this category we have such cases as follow. 
 
 (3) The motion of the float is communicated to a very delicately 
 poised lever, the slightest movement of which sends a current of elec- 
 t ririty through properly disposed magnets, whiph either alone or act- 
 ing in conjunction with clockwork perform the real labor of moving 
 the pen mechanisms and preserving a proper condition in the equilib- 
 rium of the float. 
 
 (4) A distinct class of barographs is obtained by constructing 
 mechanisms which measure and record the barometric oscillations by 
 weighing the changing quantities of mercury within a poised barome- 
 
40 BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 
 
 ter tube or cistern. The weighing, is effected by suspending .the 
 barometer tube or its cistern from the beam of a balance which is con- 
 tinuously preserved in a condition of equilibrium by the automatic 
 movement of a counterpoise traveling along the beam of the balance. 
 The movement of the counterpoise is effected either by clockwork, 
 or by electromagnets, or by both. 
 
 (4a) A modification of the weighing principle consist in substi- 
 tuting jx>r the balance mechanisms, coiled steel springs by the deflec- 
 tion of which the changing weights are measured and recorded. 
 
 The following descriptions, given in some detail, of representative 
 types of mercurial barographs that have been maintained in opera- 
 tion by the Weather Bureau will enable the student interested to 
 understand more fully how such mechanisms operate : 
 
 70. Foreman's barograph. This is shown in figure 22. It belongs 
 to the class mentioned above under (3). Prof. G. W. Hough, director 
 of Dudley Observatory, Albany, N. Y., about 1862, perfected baro- 
 graphs recording on this principle, the form here figured being designed 
 by Mr. H. L. Foreman, who was at one time Prof. Hough's assistant, 
 The glass siphon tube of the baragraph is at the back at B, and is 
 only partly visible, the bend being hidden behind the record cylinder, 
 A. The open end of the siphon tube is seen at 0; an iron float rests 
 lightly upon the surface of the mercury within, being sustained by 
 means of a fine wire, t, from the short end of the lever, Z, which is deli- 
 cately poised upon steel knife edges at r. The long end of this lever 
 at h is tipped with platinum and placed between two platinum- 
 pointed screws, both of which nearly, but not quite, touch the tip of 
 the lever when the latter is poised in proper equilibrium. The upper 
 screw is connected by the wire, TT, with an electromagnet at the back 
 of the instrument; a corresponding electromagnet, partly seen at M, 
 is connected by the wire, W', with the screw just beneath the tip of 
 the lever. P is a strong clockwork driven by the cord, T, and per- 
 mitted to run intermittently whenever released by the action of the 
 electromagnets at M. 
 
 The clock movement, D, regulated by the pendulum, F, gives 
 motion to the recording cylinders, A, A'. The cylinder, A, makes 1 
 revolution in 24 hours, whereas A' revolves at the much slower rate 
 of about 1 revolution in 16 days. 
 
 The lever, I, together with the platinum-pointed screws and elec- 
 trical connections, W, W, are all mounted upon a carrier, R, which is 
 moved by the fine- threaded screw, S, and guided by the columns a, a. 
 The coarse-threaded screw, S', is provided with the double-pen carrier, 
 R', the screws, S' and S, being geared with each other by means of 
 suitable wheels. We will suppose that the mechanisms have been 
 properly set so that the iron float is normally sustained upon the 
 surface of the mercury and the lever, I, poised in equilibrium, in which 
 
BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 41 
 
 FIG. 22. Foreman's barograph. 
 
42 BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 
 
 case the platinum tip, h, will stand about midway between the plat- 
 inum-pointed screws above and below it, respectively. The pen car- 
 rier, R', must also then occupy such a position on the screw that the 
 tracing pens will indicate the true barometric pressure upon the 
 rulings on the record sheet. Appropriate connections with an 
 electric battery being made, the action of the mechanisms will be as 
 follows : 
 
 Any minute change in the level of the mercury will alter the posi- 
 tion of the iron float, in consequence of which the platinum tip of the 
 lever, Z, will move into contact with one or the other of the platinum- 
 tipped screws, causing a current of electricity to be directed through 
 the electromagnet connected therewith. The action of either electro- 
 magnet releases the clockwork, P. In doing this, however, the one 
 magnet shifts the lever, L, laterally toward the back of the instrument, 
 while this lateral movement will be toward the front if brought about 
 by the action of the other magnet. The movement of the clockwork 
 causes the lever, L, to advance or recede so that a pawl upon the end, 
 engaging a tooth of a ratchet wheel upon the end of the screw, $', 
 revolves the latter a fraction of a turn. This fractional turn will be 
 in one direction if the lever, L, is drawn backward or in the opposite 
 direction if L is pushed forward, according as the lever, Z, has made 
 contact with the upper or under screw. This movement of the screw, 
 $', shifts the pen carrier, R' , and the pens upon the record sheet, and, 
 being communicated to the screw, S, causes a proportionate change 
 to take place in the float-carrier, R. The clockwork automatically 
 stops after one such cycle of actions. If after these movements the 
 lever, /, is again poised in equilibrium no further action ensues until 
 the contact of h with one or the other of the screws is again made, 
 whereupon the cycle of actions will again be set up and, if necessary, 
 repeated in quick succession until the equilibrium of the poised lever, I, 
 is restored. The movement of the pen carrier, R f , corresponding to a 
 change in the position of the float is four times as great as the change 
 in the height of the mercurial column. A change of 1 inch in pressure, 
 therefore, is represented as a change of 4 inches on the sheet. Each 
 closure of the circuit producing ^ revolution of the screw, S f , or 5 V 
 revolution of S represents a change in the height of the mercurial 
 column of 0.001 inch, which is the nominal sensitiveness of the instru- 
 ment. Owing, however, to unavoidable imperfections in screw 
 threads and electric contacts and to the capillary action of the mer- 
 cury in the barometer tube, the probable error of the instrument is 
 much greater than this, no doubt amounting to at least 0.01 inch. 
 
 By selecting proper proportions for the long and short legs of a 
 siphon barometer, the effects of temperature can be almost perfectly 
 eliminated. This, however, appears not to have been considered 
 when Foreman's barograph was designed, and the records are subject 
 
BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 43 
 
 to small periodic errors due to temperature changes. For the further 
 elucidation of the automatic compensation of siphon barographs for 
 temperature see paragraph 80. 
 
 Owing to the presence of the float on the surface of the mercury 
 in the short leg, and to other causes, it is practically impossible to 
 make a direct measurement of the actual height of the column of 
 mercury. When, therefore, it is desired to set the recording pens or 
 check their positions in relation to the true air pressure, it is neces- 
 sary to make a reading of the standard barometer. In the barograph 
 next described the effect of temperature is inappreciable, and the 
 actual height of the mercurial column may be directly measured at 
 any time, thus dispensing with the extra barometer required with 
 Foreman's barograph. 
 
 71. Marvin's normal barograph. This instrument is shown in 
 figures 23 and 24. It belongs to the class mentioned above under 
 (4), wherein the mercurial column is directly weighed upon a balance. 
 
 The glass tube which, with the top portion of the mercurial column 
 may be seen at B, is freely suspended by the hook, h, from the balance, 
 A. The point of the tube dips into the mercury contained in the 
 cistern, (7, which is suspended by a gimbal joint from the columns, d, 
 d, by means of the metal tube, B' ', which forms a sheath and protec- 
 tion for the glass barometer tube proper. The weight of the barome- 
 ter tube on the short arm of the beam, A, is balanced by the rolling 
 carriage, W, and a fixed weight (not shown) on the end of the long 
 arm of A. Whenever a change occurs in the height of the mercurial 
 column, the weight changes, and the carriage, W, must be moved to a 
 new position if equilibrium is to be preserved. In order to make the 
 motions of the carriage, W, automatic, a platinum-tipped contact 
 spring is attached to the balance beam at the extreme end, r, of the 
 long arm. The slightest displacement of the beam from its position 
 of equilibrium causes the spring to move into contact with one or the 
 other of two platinum-pointed screws, shown enlarged in figure 24 at 
 m, m f . These are electrically connected, respectively, with the mag- 
 nets, M and M', so that when the spring, r, makes contact with m or 
 m', an electric batter} r being in proper connection, a current is caused 
 to flow through the corresponding electromagnet, the action of which 
 causes the pin, N or N', to engage the teeth of the notched wheel, D, 
 in such a manner as to revolve it tooth by tooth. The long screw, 
 S, figure 23, carries the wheel, D, fixed at its end so as to be revolved 
 thereby. The threaded carrier, W, fitted to the screw, 8, is connected 
 by a double universal linkage to the rolling carriage, W. The electro- 
 magnets thus act very directly through the wheel, D, and the screw, 8, 
 to automatically move the carriage, W, into such positions as may 
 be required to maintain the equilibrium of the balance: that is, t< 
 prevent the contact spring on the beam from remaining continuously 
 in contact with either screw m or m 1 '. 
 
44 BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE 
 
 FIG. 23. Marvin's normal barograph. 
 
BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 45 
 
 The motor mechanisms act in such a manner that, whenever the 
 equilibrium is disturbed and the electric circuit closed, the armature 
 of whichever electromagnet is affected makes stroke after stroke, 
 revolving the wheel, D, until the equilibrium is restored. Generally 
 one or two strokes only are necessary, representing a change in the 
 
 FIG. 24. Electric-motor mechanisms. 
 
 carriage corresponding to only the ten- thousandth part of an inch of 
 pressure, as explained below. 
 
 The continuous record of the pressure, as indicated by the succes- 
 sive positions of the rolling carriage, is obtained in a very direct and 
 simple manner. A suitable spring, adjustably attached to the threaded 
 
46 BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 
 
 carrier, W, is fitted with a pen, p, figure 23, and traces the pressure 
 curve upon a large cylinder, not shown in the figure, but mounted 
 with its axis m the bearings a, a f . The cylinder is revolved regularly 
 by the clock movement, C'. 
 
 If the height of the mercurial column changes 1 inch, the rolling 
 carriage and recording pen will move 5 inches, thus giving a sufficient 
 magnification to render estimations of the pressure to the one-thou- 
 sandth part of an inch practicable. 
 
 The portion of the long arm of the beam over which the rolling 
 carriage moves is provided with a scale of 20 subdivisions to the inch 
 which represent hundredths of an inch of pressure. Still further 
 subdivision is effected by reference to the graduations on the face o: 
 the notched wheel, D. These represent the ten-thousandth part of an 
 inch in pressure. Thus a mere inspection of the position of the car- 
 riage on the beam, together with the reading on the notched wheel 
 gives the air pressure to four decimal places. Owing to the frictiona 
 resistances and other influences unavoidable in all such mechanisms 
 the fourth figure of decimals can not be regarded as having a rea 
 pressure significance. 
 
 The readings are as accurate, probably, as the best eye readings oJ 
 a good mercurial barometer, that- is, to about one-thousandth part oJ 
 an inch . 
 
 The record is not appreciably affected by changes of temperature 
 that affect the whole instrument uniformly. 
 
 72. Compensated siphon barograph, Marvin system. This instru- 
 ment is illustrated in figures 25 and 26 and belongs to that class in 
 which the record is made mechanically without interposition of any 
 clockwork or. electric mechanisms to overcome friction, etc. To secure 
 satisfactory records on a highly magnified scale by this method it is 
 indispensable that the friction involved in writing the magnifier 
 record be removed to the last degree. Experience has demonstratec 
 that this has been accomplished in the arrangement described, anc 
 this instrument proves to be exceedingly accurate and far more 
 reliable than any of the types heretofore employed. The clock and 
 electrical mechanisms required in the older instruments act in a 
 certain sense indirectly and are the cause of some errors. The 
 weakening of batteries or failure of electric mechanisms from time to 
 time also results in interruptions in tne record that do not occur in 
 the system of direct mechanical registration now to be described. 
 
 73. Compensated siphon. The barometer of this instrument is a 
 special form of siphon clearly shown in figure 25 and with dimensions 
 marked in figure 26. The long and short branches consist of sim- 
 ple, straight tubes. These are narrowed down at the lower ends 
 where they are fitted into the upturned branches of the bend, or U. 
 The tubes, in fact, form hollow stoppers carefully fitted and ground 
 
BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 47 
 
 T 
 
 FIG. 25. Compensated siphon barograph, 
 
 Man-in system. 
 Glass cover case removed. 
 
 FIG 26. Siphon tube. 
 
48 BAROMETERS AND MEASUREMENT .OF ATMOSPHEEIC PRESSURE. 
 
 in. The tops of the U above the ground joints are provided with 
 bells, or cups, of ample size, which have a lip formation on one side. 
 This three-piece construction enables the barometer to be filled in a 
 most satisfactory manner, but more especially the siphon after being 
 once filled can be assembled or dismantled and transported without 
 loss of the vacuum. The mercury in the open leg of the siphon in 
 the course of time becomes more or less fouled with oxidation, the 
 accumulation of dust, etc. The construction described permits of 
 removing the short branch of the siphon at any time with very little 
 trouble. The tube and excess of mercury can then be thoroughly 
 cleaned and replaced. 
 
 74. Filling and installing the siphon. The ordinary siphon tube 
 made in one piece of any considerable size is very difficult to fill and 
 secure a good vacuum, and it can not then be easily cleaned or trans- 
 ported. The three-piece construction already described overcomes 
 these difficulties and requires only that the long straight branch be 
 carefully filled. This may be done by almost any of the methods 
 already described in paragraphs 33 to 38, but the air-pump method 
 is undoubtedly the best. 
 
 When the siphon is to be installed it will be well to prepare the 
 ground joints by the application of a little lubricant, such as vaseline, 
 tallow, or, if available, special stopcock lubricant, very sparingly 
 rubbed over the external surfaces of the tubes. A little pure mercury 
 is next filtered into the bend, or U-shaped section. Small air bubbles, 
 if any appear, should be excluded by tilting the tube and causing the 
 mercury to flow about in a manner that will accomplish this result. 
 When the mercury covers the ground surfaces the short branch of 
 the siphon should be carefully inserted 1 and the whole secured to the 
 instrument in the manner provided. More pure mercury is now 
 added to the open cup until it is filled nearly to the brim. As some 
 mercury is likely to be spilled in the course of subsequent operations, 
 it is a good plan to have a clean porcelain or glass photographer's 
 tray close underneath the plate supporting the bend. This will serve 
 to catch any mercury that may escape. 
 
 75. The long branch of the siphon, completely filled with clean 
 mercury, is now lifted, and, while the open end is temporarily closed 
 firmly with the finger tip, the tube is carefully inclined in a manner 
 that will permit the finger and point to be dipped below the free 
 surface of mercury in the cup. Still supporting the weight of the 
 heavy tube so that the submerged end does not bear with undue 
 pressure upon the parts of the cup, the whole is carefully and slowly 
 brought into a vertical position. When the elevation of the tube 
 has reached the point at which the mercury begins to leave the top 
 
 i The alignment of the ground joints of the siphon can never be made quite perfect, and the two branches 
 are marked with a side that when faced to the front gives the best results. In seating the tubes they 
 should be faced in this position. 
 
BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 49 
 
 of the tube, an assistant should be ready to catch in a suitable vessel 
 (a dry, clean, drinking glass will answer very well), the excess of 
 mercury that overflows from the open cup. 
 
 The heavy tube must be fully supported until quite vertical, and 
 the end only then inserted into the ground joint and rotated a little 
 as it is faced to the front. 
 
 76. Certain precautions must be observed throughout the opera- 
 tions we have just described. (1) The tip end of the tube must not, 
 under any circumstances, be lifted out of the mercury after the finger 
 is removed. (2) After the flow of mercury has started the elevation 
 of the tube must be made gradually; otherwise the column of mer- 
 cury will tend to oscillate or surge up and down and ma}' uncover 
 the point of the tube in the cup. (3) Any lowering of the tube causes 
 the mercury to recede into the vacuum, and will empty the cup unless 
 the supply is kept up by pouring back some of the excess that has 
 already overflowed. 
 
 77. Having finally seated the long branch, some of the excess of 
 mercury must be restored to the siphon and the level brought up to the 
 proper point in the open leg. At the completion of these operations 
 one of the cups of the bend is full to overflowing with mercury, and 
 the other is nearly or quite empty. Some of the mercury in the full 
 cup can easily be removed by splashing it out into a cup held to 
 receive it, using a piece of card or ivory paper folder for the purpose. 
 A little mercury may be added to the empty cup. 
 
 78. To clean the mercury. When the glass and mercury in the 
 open leg become soiled through prolonged use, all that is necessaiy 
 ofter removing the float is to loosen carefully the short branch of the 
 siphon and permit the excess of mercury to overflow into a clean 
 glass. When thus emptied the open branch may be removed, thor- 
 oughly cleaned, and replaced. Most of the dirt will come away with 
 the glass tube, but the mercury may easily be filtered and replaced 
 clean and bright. 
 
 79. To dismantle the siphon. If it is desired to take down the 
 siphon, it is first necessa^ to remove the short branch, carefully col- 
 lecting the excess of mercury, and then, after separating the ground 
 joint of the long arm, the latter is slowly inclined, while an assistant 
 steadily pours mercury into the open cups to replace what flows into 
 the vacuum. When the tube is entirely filled, the finger may be 
 slipped over the open end while submerged in the mercury and the 
 whole tube removed. 
 
 80. Temperature compensation of siphon. It has already been 
 mentioned that by giving the siphon barometer proper dimensions 
 the influence of temperature can be eliminated for all practical pur- 
 poses. The compensation operates so that changes of tempera- 
 ture affecting the whole instrument uniformly produce no sensible 
 
 2293712 4 
 
50 BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE 
 
 change in the level of the mercury in the short or open branch of th 
 siphon. The actual difference of level of mercury in the two branche 
 will, of course, be affected by temperature in the usual way, but no 
 the absolute position of the surface in the open leg. Since all meas 
 urements are made only on this surface in many forms of mercuria 
 barograph, it is very desirable to realize in the design of such instru 
 ments this condition of automatic compensation for temperature. 
 
 The physical principle utilized for this purpose is found in th( 
 different rates of expansion of mercury and glass or whatever mate 
 rial is used for the tube or envelope for the mercury. If the coefficien 
 of expansion of the envelope were zero, the mercury would rise slightly 
 in the open leg with rise of temperature, and vice versa. As th< 
 theory of this temperature compensation is not stated in the ordi 
 nary textbooks of physics and meteorology, and, in fact, does no 
 appear to be widely known, it seems worth while to present it her< 
 briefly. The theory was developed by Prof. G. W. Hough 1 in 1862 
 and later by Goulier: 2 
 
 Let m = Cubical expansion of mercury per unit temperature. 
 
 Let g = Cubical expansion of glass per unit temperature. 
 
 Let F = Volume of mercury in instrument at temperature t Q . 
 
 Let d = Diameter of tube at top of column in vacuum. 
 
 Let HQ = Height of column, at temperature t Q . 
 
 Let H = Height of column, at temperature t. 
 d x , d 2 = Diameter of the two branches of the siphon at the level o 
 the top of the column in the open branch. 
 
 We assume that the pressure remains constant. Therefore th< 
 barometric column for a change of temperature must change it*. 
 length by an amount represented by the expression 
 
 m(t-t )H ; 
 otherwise its hydrostatic pressure will be altered; that is, 
 
 Neglecting small quantities of a second order of magnitude, the vol 
 umetric increase in the barometric column will be the expression 
 
 which is the change necessary to preserve hydrostatic equilibrium 
 
 1 Hough, Prof. G. W. Annals of the Dudley Observatory, Albany, N. Y., Vol. I, 1866, p. 88. 
 
 2 Goulier, C. M. Comptes-rendus, vol. 84, 1877, p. 1315. 
 
BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 51 
 
 Now, the apparent change in the volume of mercury in the tube 
 will depend upon the differential expansion of mercury and glass, 
 and is given by the expression 
 
 V Q (m-g) (t-t ). 
 
 When this increase is just equal to that necessary to preserve hydro- 
 static equilibrium, all the expansion will seem to take place in the 
 vacuum chamber and no change will occur in the level of the mercury 
 in the open leg. To realize this condition we have 
 
 
 V (m-g) (t-t ) = frd*m (t-t ) 
 m 
 
 4 rn g 
 The expression j 
 
 is the volume of the barometric column, supposing the diameter to be 
 the same throughout as at the top. 
 
 The cubical coefficient of expansion of mercury, m, is a very definite 
 quantity and for barometric work may be taken to be 0.0001010 
 per degree Fahrenheit. The expansion of glass is much smaller and 
 varies considerably, ranging, according to Regnault's measurements, 
 from 0.0000145 for common white tubing to 0.0000118 for the hard 
 French and crystal tubes. That is to say, the whole volume of mer- 
 cury in a siphon to be compensated must be about 
 
 if made of common tube, 
 
 or F = 1.132 
 
 if made of French crystal. 
 
 Xo great exactness is necessary in the volume of F . It will suffice 
 [to assume H = the mean barometric pressure at the place of observa- 
 tion, and the total volume of mercury should be about 17 per cent 
 [more than requisite to fill a column of height H and diameter d. 
 If the bend of the siphon is of wide bore, the open leg and bend 
 ust be very short (for example: 30x0.17 = 5.1 inches); otherwise 
 F will be too large. For this reason, as well as for convenience of 
 :onstruction, the bend is best made of smaller diameter than the main 
 ube, as shown in the illustration. 
 
 81 The theory given above takes account only of the influence of 
 uperature on the mercury and glass tube. The effects that result 
 changes in the mechanisms described later for transmitting and 
 inscribing the record, and for holding the glass barometer tube itself, 
 
52 BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 
 
 all require consideration, but fortunately these are in the main sc 
 small, especially when considered in relation to the highly magnified 
 scale on which the record is inscribed, that they may be neglected. I 
 any case they can be incorporated with the mercury effect so that b 
 adding or removing small amounts a certain total volume, F , of 
 mercury at temperature t may be employed, and thus all uniform 
 effects of temperature on the whole apparatus will be automatically 
 compensated. 
 
 If .the siphon is not compensated, then the volume of mercury at 
 temperature 1 = V l} which in general will be greater than F , but 
 may be less, and a small correction will be required, the amount of 
 which will be simply the apparent expansion of the excess of mercury 
 occupying the bend and short leg of the siphon. This expansion 
 may be imagined simply to lift the whole column of mercury a small 
 amount, All. 
 
 The volumetric expansion will be 
 
 (Fj-Fo) (m-g) (t-Q, 
 and the rise of mercury, All, is given by the expression 
 
 ^(dl + dl) Ah=(V,-V) ( m -g)(t-t Q ). 
 
 In general, d^ and d 2 will be made sensibly equal, and, in fact, =d. 
 Hence: 
 
 Jft = 9(F F) ( m -.9)(*-O 
 1 nd 2 
 
 Let y= the amount by which the mercury in the open leg of tli 
 siphon stands higher than required for compensation. Then, sine 
 an equal excess of mercury occupies the opposite branch of the I 
 we have: 
 
 Hence, 
 
 A~h=y (m-g) (t-Q, 
 or, for ordinary glass: 
 
 Ah=. 0000865 ?/ (t-t ). 
 
 That is, the temperature correction required is simply the apparen 
 expansion of the short column y of excess of mercury. Now, th 
 temperature variations affecting a barograph throughout the perio 
 of a single record sheet, or rather during intervals when the recor 
 may be checked by eye observations of a standard barometer, wi 
 rarely exceed 10 or 20 F. If y=l inch, for example, then All fo 
 20 =.00173, a quantity which, in general, may be neglected. 
 
BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 53 
 
 82. Magnifying and recording mechanisms. In the barograph illus- 
 trated, the barometric changes are magnified five times and recorded 
 on a vertical drum adapted to embrace a change of 2 inches of pres- 
 sure and revolving once in three days, moving at the rate of nearly 
 a quarter of an inch per hour. A long experience with a variety of 
 scales indicates that records on time and pressure scales of about the 
 above proportions give, on the whole, the most satisfactory and 
 graphic picture of ordinary barometric oscillations. Even the sud- 
 den changes that sometimes occur with' thunderstorms are very well 
 brought out; but for the most detailed effects of this character a 
 more rapid time scale is necessary. The magnification is sufficiently 
 great to show clearly the small fluctuations of from a few thousandths 
 to some hundredths of an inch that sometimes occur for hours at a 
 time. 
 
 In the siphon form of barometer the change of level of the mercury 
 in either leg is only half of the whole change, assuming both legs to 
 have the same diameter, and, since we measure effects in the open leg 
 only and desire a fivefold magnification, it follows that an actual ten- 
 fold magnification of the movements of the float is necessary. This 
 is accomplished by a large and a small wheel operating on the prin- 
 ciple of the wheel and axle, as may be clearly seen in figures 25 and 27. 
 This construction provides a perfectly balanced system which is itself 
 neutral in all positions and, at the same time, admits of a wide range 
 of movement, results impossible to secure with lever systems com- 
 monly employed in cases of this kind. 
 
 In order to secure strength of construction and at the same time 
 reduce friction to a minimum, the multiplying wheel and axle are 
 mounted on carefully designed and constructed ball bearings, each 
 cell containing only six balls, each one-sixteenth inch in diameter. 
 The ends of the axis entering the ball cups are 70 cones. 
 
 A conical steel float, with the base somewhat hollowed out so as 
 to conform fairly well with the shape of the surface of mercury rests 
 lightly upon the top of the column and is suspended from the small 
 drum of the wheel-and-axle system by means of a narrow platinum 
 ribbon about 0.001 inch thick. The pen carrier is suspended by a 
 very fine copper wire running in a groove in the rim of the large 
 wheel, the diameter of which is approximately 5 inches, while that 
 of the drum is one-tenth as great. The exact ratio of these wheels is 
 made so as to realize a fivefold magnification of pressure changes; 
 due account being taken of any slight dfferences in the diameters 
 of the open and closed chambers of the barometric column. 
 
 To realize a condition of minimum friction great attention is neces- 
 sary in the design and arrangement of the pen carrier. First, the 
 weight must be the least practicable, since the mass of the float must 
 be somewhat in excess of 10 times that of the pen carrier, and any 
 
54 BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 
 
 unnecessary weight in these parts introduces avoidable pressure and 
 friction on the axle. Second, while the pen carrier is guided and 
 constrained to move without sensible looseness in a definite vertical 
 line by sliding along a fine, stretched wire, nevertheless the whole 
 arrangement is so poised and balanced that if not disturbed by exte- 
 rior influences the carrier will rise and fall in exactly the same ver- 
 tical line, as nearly as may be, even when the wire is removed. This 
 adjustment serves to eliminate any sliding friction experienced by 
 the pen carrier not absolutely essential to constraing the pen to 
 the desired vertical line. Finally, the contact pressure of the pen on 
 the record sheet is no greater than essential, and results from a small 
 residual gravitational tendency of the carrier to rotate the pen point 
 against the record sheet with a very gentle pressure. The siphon 
 tubes are about 1J inches in diameter and the float is only slightly 
 smaller. This gives a moving force capable of overcoming the una- 
 voidable friction in a highly satisfactory manner, and the absence of 
 any complicated mechanisms renders false and interrupted records 
 almost an impossibility. 
 
 83. Time checks on record sheet. As thus far described the baro- 
 graph is complete, and with the aid of the driving clock and drum, 
 which require no further description, gives highly accurate and 
 continuous records. The detailed analysis of barometric records 
 generally requires hourly readings. When record sheets are employed 
 with ruled scales for pressure and time, there is always a difficulty in 
 setting the record so that the ruled hour lines on the sheet indicate 
 the true time. A similar difficulty arises in setting the pen to the 
 correct point on the pressure scale. This, however, is of slight con- 
 sequence if sheets are properly printed and cut with uniform margins 
 and carefully placed on the cylinder. 
 
 To secure easily an equally satisfactory result with the time record, 
 the driving clock is provided with a dial and hands in the usual fashion. 
 These moving continuously day after day enable the clock to be reg- 
 ulated to keep correct time, a result very hard to secure when the 
 rating is done on record sheets that are frequently changed. More 
 especially, however, the barograph is equipped with a special time- 
 marking device which automatically operates once each hour at the 
 instant the minute hand of the clock reacher XII, or the zero point 
 of the hour. Nearly all the time lines are omitted from the printed 
 rulings of the record sheet, and the marker operates so as to lift the 
 float a few hundredths of an inch and immediately release it. This 
 causes the recording pen to oscillate a few times up and down and 
 to inscribe a short transverse line across the pressure record. These 
 transverse strokes are, in fact, the hour lines for the entire record 
 and are inscribed with all the accuracy required. 
 
BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 55 
 
 84. There is a further advantage from the action of the time 
 marker. The float is raised from the mercury momentarily and 
 subsequently oscillates slightly. This causes a general renewal of 
 the forces of buoyancy and capillarity which determine the exact 
 position of the float, and any failure of the pen at the end of the oscil- 
 lations to return exactly to its original position is an index of the 
 magnitude of errors that arise partly from friction and partly from 
 variation in the capillary and buoyant forces. 
 
 Discontinuities of several thousandths of an inch, due to these 
 causes, are sometimes found in the records, "rendering apparent the 
 existence of small errors which would otherwise be only conjectured. 
 
 FIG. 27. Time marker. , 
 
 85. The time marker is shown in figure 27, and consists of an elec- 
 tromagnet, the circuit of which is momentarily closed by a spring 
 contact operated by the minute hand of the clock as it passes the 
 XII point of the dial. 
 
 The armature of the magnet is L-shaped, as seen in the picture. 
 A long, light, horizontal rod is pivoted at the depressed end of the 
 L-formed armature, and is also partly lifted by the pull of a spring 
 carried wholly on the armature. The outer end of the long arm is 
 tipped with a bit of soft rubber, and is further loaded with a small 
 counterweight, which rests lightly on a small post or stop provided 
 for that purpose. 
 
56 BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 
 
 86. The action of the marker is as follows: When the armature is 
 suddenly pulled down upon the magnet the rubber-tipped rod is 
 thrust forward against the rim of the large wheel. The inertia of 
 the counterweight suffices to overcome for an instant, but only for 
 an instant, the pull of the spring previously mentioned. In ' this 
 instant, however, the rubber-tipped end of the rod has engaged the 
 rim of the wheel and the slower-acting pull of the spring then lifts 
 the rod and thus turns the wheel a small distance (one to two tenths 
 of an inch) . As soon as the armature is released by the breaking of 
 the contact in the clock, the wheel and float are released and oscil- 
 late freely for a moment, producing the results already fully 
 explained. 
 
 FIG. 28. Richard's aneroid barograph. 
 
 87. The recording drum makes a complete rotation in 74 hours, 
 i. e., 3 days and 2 hours. Sheets ordinarily are changed at any time 
 between 11 a. m. and 12 noon, preferably shortly after 11 a. m. 
 The new record is therefore fully started before noon, and a check 
 reading of the standard barometer is made as nearly as possible at 
 noon. This furnishes a check observation for determining the start- 
 ing error of the barograph. Further checks may be obtained sub- 
 sequently from the regular observations at 8 a. m. and 8 p. m. 
 
 88. Aneroid barographs. Extremely simple and portable baro- 
 graphs are constructed upon the aneroid principle, of which that of 
 Richard, being widely used, is fully described. (See fig. 28.) 
 
 It consists of a cylinder, A, on which the recording paper is 
 wound, revolving once a week by means of a clockwork contained 
 
BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 57 
 
 inside. A series of corrugated metallic shells, B, eight in number, joined 
 one above the other and exhausted of air, forms an aneroid system 
 eight times as sensitive as a single chamber. The movement of the 
 shells is still further greatly magnified and is transmitted to the record- 
 ing pen, Cj by a series of connecting levers. The pen may be released 
 from contact with the paper by pushing the lever, D, to the right. 
 
 The corrugated shells are the same as used in ordinary aneroids, as 
 described in paragraph 46, the steel springs for distending the shells 
 being placed inside. The shells are made into a vertical column by 
 screwing the one on the other. The lower base of the column being 
 1i\cd, the upper end rises and falls with every variation in the atmos- 
 pheric pressure, by a quantity which is the sum of the displacements 
 of the elementary shells. 
 
 The compensation for temperature is accomplished by leaving a 
 sufficient quantity of air in one of the shells, ascertained by experi- 
 ment when the instrument is made, so that with a rise of temperature 
 the tendency of the barometer to register too low on account of the 
 weakening of the springs, and the expansion of the levers and other 
 parts is counteracted by the increased pressure of the air in the shell. 
 However, the instrument should be kept at a uniform temperature 
 as far as possible. 
 
 V. GENERAL INSTRUCTIONS. 
 (A) FOR CARE AND USE OP BAROMETERS. 
 
 89. Exposure of barometers. The two important considerations in 
 selecting a proper location for a barometer are (1) that the cistern 
 and top of the mercurial column may be in a good light and (2) that 
 the temperature may be as constant as possible. The best conditions 
 for light are obtained when the barometer can be placed between the 
 observer and a window, preferably a north one, covered either with 
 tissue paper or fitted with ground glass. Very nearly as good results 
 are obtained by a light from one side reflected from clean white 
 paper or white glass immediately back of the barometer. The top 
 of the column should be about the height of the observer's eye. The 
 barometer should not be exposed either to the direct rays of the sun 
 or to the air currents that are always found in the vicinity of cracks 
 and crevices in windows. 
 
 In establishing stations, officials will use special care in selecting 
 the exposure of the barometers and satisfy the conditions stated above 
 as nearly as possible. In general, it will be necessary to avoid expo- 
 sures near windows, as proper temperature conditions can not be 
 found in such locations. 
 
 As houses, no matter how tightly built, always permit the free flow 
 of air in and out through crevices, ventilators, chimneys, etc., it 
 
58 BAROMETERS AND MEASUREMENT OP ATMOSPHERIC PRESSURE. 
 
 results that the air pressure within is exactly the same as without, 
 except possibly for very slight differences of very short duration. If 
 such were not the case it would be necessary to expose barometers 
 out of doors to obtain the real air pressure. 
 
 89a. Pumping of barometers. Notwithstanding what has just 
 been said about the pressure indoors and out being the same during 
 very windy , gusty weather barometers within doors are often sub- 
 jected to very rapid and irregular oscillations of pressure, caused by 
 gusts of wind blowing into doorways, windows, or chimneys, and 
 momentarily increasing the pressure, or by blowing across chimney 
 tops and otherwise, so as to produce a sort of suction that momen- 
 tarily diminishes the general pressure. In consequence of these 
 effects the mercurial column of a barometer may be observed on 
 such occasions to rise and fall irregularly within narrow limits, the 
 motion in many cases being little more than changes in the curvature 
 .of the meniscus. This action is called the "pumping" of barometers, 
 and,, of course, interferes with accurate pressure observations. 
 
 The term "pumping" is also applied to much more violent oscil- 
 lations of the mercurial column, such, for instance, as will occur 
 when an ordinary barometer is exposed on a vessel at sea, or when 
 carried in an upright position in the hand. In barometers for use on 
 shipboard this action is prevented by making the lower portion of 
 the glass tube of very fine bore, so that the movement of the mer- 
 cury is necessarily too slow to follow sudden and irregular oscillations 
 of pressure. 
 
 90. Verticality of barometers. For accurate results it is necessary 
 that barometers should be exactly vertical when the adjustments for 
 reading are made. For this purpose the better forms of barometers 
 are arranged to be suspended from rings at the top, so that the instru- 
 ment itself acts as a plumb line and takes a vertical position with 
 sufficient accuracy. It is desirable, however, for convenience in set- 
 ting the barometer, as well as to insure the permanent verticality 
 of the instrument, to steady it in supports which are first adjusted, 
 once for all, so that the barometer is accurately vertical, as determined 
 by a plumb line applied alternately at the front and one side. 
 
 91. Improved barometer box. The standard pattern of barometer 
 box now in use by the Weather Bureau is shown in figure 29. 
 
 This box must be securely attached to the wall in a location afford- 
 ing good light and not subject to sudden changes of temperature. 
 In many cases it will first be necessary to fasten to the wall hard- 
 wood strips, to which the top and bottom of the barometer box can 
 be secured by screws passing through metal plates provided for this 
 purpose. Place the screw for holding the top of the box in the { 
 center of the top strip and suspend the box thereon. When the 
 
BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 59 
 
 FIG 29. Improved barometer box. 
 
60 BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 
 
 box is set about vertical (as determined by a plumb line) secure it 
 firmly at the bottom by a screw passing through the metal plate into 
 the wooden strip. 
 
 The hooks in the top of the barometer box will, upon examination, 
 be found to be adjustable in their positions. Place them in their 
 central positions and hang thereon the ''station" and "extra" barom- 
 eters. Next, find the positions at the bottom for attaching the ring- 
 shaped guides, one of which is shown in figure 30. These rings 
 should be slipped over the lower end of the barometer, and placed 
 about the mid height of the cylindrical part of the cistern. Mark 
 the screw holes and fasten the rings to the back of the box. The 
 barometer when at rest should hang freely within this ring, and may 
 
 be adjusted to do so by the hook at the 
 top. When the rings are fitted with cen- 
 tering screws these should be careful!}' 
 screwed up until the barometer cistern is 
 very gently clamped and held steadily, 
 but not in the least deflected from its 
 vertical position when free. 
 
 Barometers that swing slightly free 
 within the ring support should, in setting, 
 be steadied against one side of the ring, 
 as shown in figure 30. The following 
 caution must, however, be observed. 
 
 92. Caution against error. In adjusting 
 the barometer for reading it is very im- 
 portant that it be steadied against the 
 ring in the proper manner, otherwise an 
 appreciable error is introduced, because of 
 imperfect verticality. 
 
 If 7, figure 30, is the position of the 
 ivory point, then steady the cistern against 
 the ring at A or at B, but never at C or I), or at other points. A 
 and B should always be in a line exactly at right angles to a line through 
 the ivory point and the center of the cistern. 
 
 A little thought will show the necessity for this. If, for illustra- 
 tion, we imagine the ivory point at one side and just in contact with 
 the mercurial surface and the barometer vertical, it is very clear that 
 if we swing the barometer out of vertical a little either to one side 
 or the other, the ivory point will either dip into the mercury a little 
 or rise above the surface, whereas, if the barometer be deflected back- 
 ward or forward, there will be little or no perceptible change in the 
 level of the mercury at the ivory point. 
 
 FIG. 30. Barometer cistern and 
 ring support. 
 
BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 61 
 
 93. Old-style barometer boxes. The simplest form of box used by 
 this bureau is shown in an improved form in figure 31. The top end 
 of the box is made of thick material, which is recessed to receive the 
 top portion of the barometer when suspended upon the long hook, 77, 
 screwed into the top. For reading, the barometer is drawn out upon 
 the hook, as shown in the figure, and returned to the box after the 
 observation. The lid of the box is fitted to close 
 
 under the hook so that the whole barometer is 
 thoroughly encased and well protected. Except that 
 the barometer must be shifted out of and into the 
 box every time readings are made, and the further 
 disadvantage that the cistern necessarily swings free, 
 this style of box answers the purpose in a very satis- 
 factory manner. 
 
 94. The barometer should be' carefully lifted along 
 the hook and not made to slide roughly or permitted 
 to knock against the guides in the box. 
 
 95. When boxes such as described in the preced- 
 ing paragraphs are not furnished the barometer may 
 be suspended from almost any suitable hook securely 
 fastened to the wall in such location as will satisfy, as 
 far as possible, the conditions of paragraph 89. 
 
 96. Marine barometer box. This has already been 
 described in previous paragraphs 26 to 31. We need 
 only add that in attaching this box to the wall the 
 height must be regulated so that the scale comes at 
 the level of the eye when the hinged bracket is 
 lowered with the barometer in the reading position. 
 The barometer will be several inches too high to 
 read conveniently when folded into the box. 
 
 97. How to set and observe the barometer. Having 
 in mind the various sources of error affecting barome- 
 ters, and other peculiarities of the instrument, we may 
 next consider how best to secure accurate readings. 
 The presence of the observer's body near the barome- 
 ter ends to increase its temperature. The scale and 
 outer parts are affected first, then the thermometer, 
 and much more slowly the mercurial column. Gen- 
 erally, however, this effect is slight, as only a few 
 
 minutes are required in making a reading. It is best to read the 
 attached thermometer first. Next, if the barometer is freely sus- 
 pended, jostle the cistern a little, so that the mercurial surfaces may 
 be detached if they tend to cling to the glass walls. To "set" the 
 cistern of the barometer, the level of the mercury should be lowered 
 a little by turning the milled head, 0, figure 3, and raised again until 
 
 Fro. 31. Barometer 
 box. 
 
62 BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 
 
 it is just in contact with the ivory point. To make this adjustment 
 of the mercury to the ivory point accurately requires care and sharp 
 scrutiny. 
 
 Adjustment of cistern, first method. One way is to sight between the 
 point and the mercury and watch for the slightest thread of light that 
 can be detected. The screw should be turned very carefully until 
 this thread of light just disappears. This method is believed to be 
 best and is uniformly practiced at the central office. It is equally 
 applicable to new barometers with bright mercurial surfaces and to 
 older ones the mercury of which is more or less oxidized. The light 
 should be strong from behind the barometer and the front of the 
 cistern should be in shadow. 
 
 Second method. The adjustment of the mercury to the ivory point 
 may also be determined by watching the formation and disappearance 
 of the small, dimplelike depression made in the mercury when the 
 ivory point is pressed into the mercury a little and again withdrawn. 
 When the dimple just disappears the surface may be supposed in con- 
 tact with the ivory point. The mercury often clings to the ivory 
 point, especially when the ivory is newly cut. This method, however, 
 is not so reliable and accurate, and, in general, can be followed only 
 with clean mercury. Moreover, it is not good practice to lower the 
 mercury any slight amount after it is once raised to the ivory point. 
 The effect of this generally is to change simply the convexity of the 
 meniscus at the top of the column, and this gives rise to a new and 
 unknown correction for capillarity. The most uniform results are 
 obtained by gradually raising the mercury until precise contact is 
 secured. If it is imagined the mercury has been raised too much, 
 lower it until entirely free from the point and adjust again. 
 
 Third method. Another method that is often given is to watch 
 closely until the reflected image of the ivory point coincides with the 
 point itself. This also requires clean, bright mercury and is there- 
 fore not a general method. Great precision in the adjustment of the 
 contact of the ivory point with the mercury may be attained with a 
 little care and practice, and observers may scarcely be conscious of 
 precisely the manner of making the adjustment. 
 
 98. Adjustment of the vernier. The level of the mercury being 
 adjusted to the ivory point, the vernier must next be brought to the 
 top of the column. Greater uniformity and accuracy are insured if 
 the fingers be now tapped smartly against the side of the metal 
 barometer tube. This aids the mercury in detaching itself from the 
 glass and forming into a normal meniscus. The proper setting of the 
 vernier is made when the light is just cut off from across the extreme 
 summit of the meniscus. The figures on page 13 indicate how the 
 vernier should be set to the mercurial meniscus. The lower edge of 
 the vernier must be brought just to the level of the extreme summit of 
 
BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 63 
 
 the meniscus. The eye must be held so that both front and back edges 
 of the vernier are in the line of vision. 
 
 99. It is needless to say that throughout the setting of the barome- 
 ter, as described above, the column must be maintained vertical, 
 either by means of the fixed supports or by the skillful handling of the 
 freely suspended barometer, so that at the critical moments when 
 contacts are judged to be made the instrument is truly vertical. 
 
 100. After the cistern and vernier are adjusted in the manner 
 described above it remains only to read the scale and vernier in 
 accordance with the instructions in paragraphs 13, 14, and 15. 
 
 101. This reading may next be corrected for temperature by apply- 
 ing the proper correction taken from Table I, corresponding to the 
 temperature shown by the attached thermometer, and further cor- 
 rected, if necessary, by the addition or subtraction, as the case may be, 
 of all other corrections known for the instrument, such as correction 
 for capillarity, instrumental error, imperfect vacuum, gravity, etc. 
 
 102. Reduction to standard gravity. The following will elucidate 
 the nature of the gravity correction as applied to barometric observa- 
 tions an important matter that is often but indifferently considered 
 in the ordinary textbooks of meteorology : 
 
 By the well-known principle of hydrostatics on which the action 
 of the mercurial barometer is based the pressure of the atmosphere 
 is equal to the pressure of the column of mercury that it will support. 
 But this latter pressure is only another name for the weight of the 
 mercury, and for columns of equal section the weight varies both 
 with the height of the column and with the force of gravity. 
 
 The force of gravity varies with latitude and altitude; therefore the 
 height of the barometer, even when corrected for temperature and 
 instrumental error, does not give us a true measure of the atmospheric 
 pressure unless we first eliminate the small variations that are due to 
 gravity; that is, observations taken over a widely extended region to 
 be strictly comparable must be reduced to a standard force of gravity. 
 
 The standard gravity adopted by physicists is that at the level of 
 the sea in latitude 45. 
 
 Tables of corrections for gravity are given in Section VIII. 1 
 
 103. Reduction to sea level. It was mentioned in paragraph 5 that 
 the atmospheric pressure was in the main nothing more than the 
 weight of a vertical column of air extending to the limits of the at- 
 
 i According to the formula for the force of gravity adopted by the International Bureau, of Weights and 
 Measures we have for the variations in gravity due to the latitude 
 tt 0=046 (1-0.00259 cos 2 0) 
 
 The variation in the force of gravity for different altitudes is small, and is given by the formula 
 
 G"h=0 (1-0.0000000597 h) 
 h being the elevation in feet. 
 
 Neglecting this latter factor, the application of the gravity correction is equivalent to multiplying the 
 height of the barometer by the factor 
 
 (I -0.00259 cos 29) 
 
64 BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE 
 
 mosphere. It naturally follows that as we go above the general sur 
 face of the earth, whether in balloons or by ascending mountains 
 the atmospheric pressure becomes less and less as we leave more anc 
 more of the air beneath us. When it is desired to chart and com 
 pare simultaneous observations of atmospheric pressure over extendec 
 areas, and at various elevations above sea level, no inference can 
 easily be drawn from the actual pressures themselves, but each must 
 be reduced to some standard level. The sea level plane is most gen- 
 erally adopted, but the selection of a plane 2,000 to 5,000 feet above 
 the sea offers more rational conditions in certain respects and is 
 sometimes advocated. For comparative purposes, therefore, baro- 
 metric observations from different stations require a " reduction for 
 elevation." 
 
 We can form a clear idea of what is wanted by confining our atten- 
 tion to the case of a barometer in a balloon at an elevated point 
 above the sea. The reduction for elevation is simply a measure, ex- 
 pressed in inches of the mercurial column, of the weight of the column 
 of air between the balloon that is, the barometer cistern and sea 
 level. ' This weight evidently depends not only upon the elevation 
 above sea, but also upon the mean temperature of the air below the 
 balloon and the amount of moisture it contains. 
 
 The temperature and moisture conditions are easily conceived of 
 in the case of a barometer in a balloon with a great ocean of air 
 directly beneath, but when we consider the reduction for elevation of 
 barometric observations taken over extended plateaus and at great 
 distances from sea level, such, for example, as the reduction of obser- 
 vations at Denver, Colo., no clear meaning attaches to the tempera- 
 ture and density of the air column; in fact, the air column can not 
 have any real existence, and this constitutes a considerable difficulty 
 in computing satisfactory values for the reductions for elevations. 
 Approximate values only, therefore, are possible. Considerations 
 such as these lead us to see the advantage of making all reductions to a 
 plane, say, 5,000 feet above sea level, in which case an air column 
 actually exists, and has a definite mean temperature, humidity, etc. 
 
 NOTE. The diminution of gravity as we go from latitude 45 to the Equator causes the mercury in the 
 barometer to weigh less, and hence for a given pressure in the atmosphere the mercury in the barometer 
 stands higher than it would if the force of gravity preserved the uniform standard value. Therefore, the 
 farther a barometer is removed from latitude 45 the greater its correction becomes, so that at the Equator 
 a pressure that appears to be 30 inches (at sea level) is really about 29.92 inches. 
 
 It should be noticed that when the barometer is thus corrected for its peculiar error due to the influence 
 on it of variations of gravity the pressure that it then gives is the actual pressure of the air at each latitude 
 expressed in terms of an absolute and not a variable standard. 
 
 It is important to remember that the barometric pressure is due not only to the weight of the air, but 
 also to the prevailing winds, the rapid heating or cooling and consequent expansion or contraction of low 
 layers of air, and to other causes. . 
 
 In the mercurial barometer we balance this elastic pressure by weight of quiescent mercury; a change of 
 the force of gravity will change the weight of the column of mercury without necessarily changing the 
 atmospheric pressure. 
 
 ' 
 
BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 65 
 
 Various more or less arbitrary computations of the temperature to 
 be used in finding the reduction for elevation have been tried from 
 time to time. At present the temperature selected for the reduction 
 of the observations of the Weather Bureau is the mean of the current 
 air temperature and that of the preceding 8 a. m. or 8 p. m. obser- 
 vation. 
 
 104. Determination of height by barometric readings. The calcula- 
 tion of elevation above sea level by barometric readings involves all 
 the principles and encounters all the difficulties of the "reduction to 
 sea level" described above. This method, therefore, of measuring 
 heights is to be used only when others are not possible, and numerous 
 observations should be made to eliminate the very large accidental 
 errors to which the method is subject. 
 
 As the aneroid barometer is used by tourists and others so largely 
 in connection with determination of elevation, some further discussion 
 of the subject is here given. 
 
 105. Determination of heights by the aneroid. A reading of a barome- 
 ter at a single station, without reference to the air temperature and 
 corresponding pressure at some adjacent points whose elevations are 
 known, gives only the crudest possible idea of the elevation of the 
 station, and the neatly constructed little pocket aneroids in morocco 
 cases on sale in the shops, having their dials graduated to feet of 
 elevation above sea level, are to be regarded as extremely inaccurate, 
 especially if the scale of altitudes is graduated upon the same metal 
 plate as the scale of inches. In many aneroids the scale of feet is 
 adjustable, and on this account may serve to some advantage for 
 showing small differences of elevation, but at the best all direct indi- 
 cations of elevation from readings on the dial of the aneroid are only 
 roughly approximate. 
 
 106. Adjustable scale of elevations, how used. If the scale of feet is 
 adjustable on an aneroid for moderate changes of elevation, it may 
 be conveniently used as follows: If a tourist is about to set out on a 
 short expedition to an elevated point, starting at a station of which 
 the elevation is known, let him set the movable scale of the aneroid 
 so that the proper graduation, marking the known elevation, stands 
 opposite the index hand. On reaching the elevated point the position 
 of the hand on the scale of feet will now indicate approximately the 
 new elevation, provided, of course, the scale has not been shifted, and 
 provided further that the real air pressure was uniform throughout 
 the vicinity and did not change at all during the time occupied in the 
 expedition. If this time was several hours or the distance consider- 
 able, the result may be very greatly in error. Suppose we know our 
 elevation to be 500 feet and we set the scale to this point. To-morrow 
 
 2293712 5 
 
66 BAKOMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 
 
 the aneroid may indicate only 200 or even 800 feet, the new value 
 being a direct result simply of the changes in air pressure. 
 
 A variation of one-tenth of an inch in the barometric pressure 
 affects an observation of altitude by from nearly 100 to nearly 150 
 feet, according (1) to the elevation itself and (2) the temperature at 
 the time. On this account and because of the large accidental errors 
 to which aneroids are subject, as described above, they are unreliable 
 in the determination of elevations. 
 
 107. To determine heights as well as can be done by pressure meas- 
 urements, it is necessary that simultaneous observations, not only of 
 the barometer but of the temperature and humidity of the air, be made 
 at one or more adjacent stations of which the elevations are known. 
 These observations, by means of suitable tables, will give the differ- 
 ence in elevation of the stations, and the mean result from a large 
 number of such simultaneous readings will give a fairly accurate 
 value for the desired elevation, especially if the differences of elevation 
 and the distances between the stations are not very great. 
 
 108. The care and preservation of barometers. A barometer is a very 
 delicate instrument, and in general must be handled with great care ; 
 therefore observers in handling a barometer should first inform them- 
 selves as to the best methods to follow and the various precautions to 
 observe, as embodied in the instructions given below. 
 
 109. When a new 'barometer is received, in unpacking it should be 
 lifted cistern uppermost from the box and all wrappings removed after 
 placing the barometer in a horizontal position. When moved about, 
 the cistern end should be carried uppermost. 
 
 110. To turn the barometer tube end up, bring it first gradually to 
 a horizontal position, watching for a small bubble at the cistern. 
 This should never be very large, nor should it be absent, in which case 
 there may be serious pressure from within, tending to force the mer- 
 cury out through joints of the cistern, etc. If necessary, the adjusting 
 screw should be turned so that the bubble is not larger than a space 
 within which a 10-cent coin could be placed. The tube may then be 
 gradually elevated to an upright position. The mercurial column 
 should not be lowered until the instrument is safely suspended from 
 a hook. 
 
 111. Never remove a good barometer from its supports while the 
 mercurial column is at or near its normal height. Always screw up 
 the cistern until the top of the column is just visible at the top of the 
 opening in the brass case. Do not subject the barometer to quick 
 movements or sudden changes in its position; always move it about 
 slowly and regularly and change its position gradually. Do not 
 handle or carry the barometer in an upright position. Handle it 
 horizontally, or upside down as far as possible, preferably the latter. 
 The proper procedure to invert a barometer is as follows : 
 
BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 67 
 
 Examine the cistern to see if there is any special air vent as at d, 
 figure 9, in the Tuch cistern. Screw up the mercury until it reaches 
 the top of the cistern. Then close tightly the air vent, and continue 
 screwing up the cistern until the top of the mercurial column reaches 
 the summit of the opening in the metal tube. Always avoid screwing 
 up the cistern until the tube is entirely filled with mercury. It is 
 impossible to tell exactly when the tube is full, and a turn too much 
 of the screw is almost sure to force the mercury through the joints 
 of the cistern or even the pores of the leather bag and lead to very 
 serious injury of the barometer. Do not strain the screw if it goes 
 hard. Mercury may have leaked from the cistern, and what remains 
 be insufficient to fill the tube. A barometer can be safely inverted 
 even if there is quite a deficient supply of mercury in the cistern. 
 
 When the mercury is near the top of the tube, remove the barome- 
 ter from its supports and incline slowly, listening, meanwhile, for any 
 slight sound or "click" that may be emitted from the top of the 
 barometer. When the tube is nearly horizontal, watch for the 
 appearance of an air bubble at the cistern end, showing there is still 
 a small free space within. From the horizontal position the instru- 
 ment may be turned cistern end up without any special precautions 
 and may then be handled and carried with ease and safety. It is 
 even advisable now to loosen the cistern screw a turn or so, that there 
 may be plenty of free space in the cistern. 
 
 112. Special instructions for handling marine barometers. It will be 
 readily understood from the description of the marine barometer and 
 its cistern that special care must be employed in inverting such a 
 barometer. In this case the cistern is not and can not be filled with 
 mercury. Moreover, the constriction of the tube prevents the vac- 
 uum from filling quickly, so that to invert a barometer of this kind 
 the tube must be inclined 30 or 40 from the vertical and held in such 
 a position, and farther inclined, if necessary, until the vacuum cham- 
 ber is completely filled with mercury, whereupon the instrument may 
 be full}' inverted and handled without danger, cistern uppermost. 
 
 Since the cistern is only partly full ordinarily, the marine barometer 
 is more liable to injury in shipment than other barometers and, in 
 fact, is best transported by hand. 
 
 113. The "metallic click." The so-called " metallic click" is best 
 produced while the barometer is inclined at about 45, or possibly 
 still more nearly horizontal at high-level stations. The cistern must 
 not be screwed up too much. The "click" occurs just as the mer- 
 cury moving up the tube reaches the top and completely fills it. If 
 the barometer is quickly inclined, the violent shock of the mercury 
 against the top of the tube is sometimes sufficient to crack the tube. 
 Hence, sudden movements of this sort are always attended with 
 danger to the barometer. 
 
68 BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 
 
 Many think they can judge of the excellence of the vacuum in a 
 barometer by the character of the "metallic click." It is exceedingly 
 deceptive, however, and even experts are able to draw only approxi- 
 mately correct conclusions from its character. The greatest caution 
 should be exercised in producing the click, as, if the vacuum is first 
 class, it tends to injure the barometer. A good plan is to incline the 
 barometer, as described above, until the mercury almost reaches the 
 top of the tube; then, holding it in this position, move it somewhat 
 quickly, but very slightly and regularly, back and forth three or four 
 times exactly in the direction of its length, and, if necessary, changing 
 the angle of inclination and increasing, very cautiously, the intensity 
 of the shaking motion until two or three gentle clicks may be heard. 
 Too great care can not be exercised in this respect, and only the most 
 gentle clicks should be produced. Even then, with very perfect 
 vacua, the internal stress is very great, and barometer tubes that 
 have been subjected to boiling in the process of filling and are not 
 thoroughly annealed are sometimes in such a state of internal stress 
 as to be very easily cracked and injured. 
 
 114. Handling barometers at elevated stations. In the case of sta- 
 tions from 3,000 to 10,000 feet or more above sea level the top of tha 
 mercurial column, in extreme cases, is a long distance from the top 
 of the tube. It is not advisable, therefore, when it is desired to invert 
 such a barometer, to screw up the cistern immediately until the col- 
 umn reaches nearly the top of the tube. A better plan is to raise the 
 column only 2 or 3 inches, then, while gradually inclining the instru- 
 ment, continue to screw up the cistern until the column is about to 
 disappear from view at the top. The object of this is to avoid sub- 
 jecting the cistern to the considerable hydrostatic pressure that occurs 
 if the column is raised several inches above that which the air pressure 
 itself is capable of supporting. 
 
 At an elevated station the barometer must be in a much more nearly 
 horizontal position to produce the "metallic click" than at sea level. 
 
 115. The best possible care a barometer can receive is to be pro- 
 tected from accumulations of dust, etc., and left quite' alone. When 
 readings must be taken, and the barometer is suspended from a hook 
 upon which it is drawn out to a position 'convenient for reading, the 
 rough sliding of the barometer along the hook, together with the 
 springing movement up and down, and finally the knocks the cistern 
 is apt to receive when the instrument is returned to the box, are all 
 very injurious to the condition of the barometer and are to be avoided 
 by gentle and careful handling. 
 
 116. The results of comparative barometer readings conclusively 
 show that in spite of every care a difference of several thousandths of 
 an inch in the indications of two or more instruments can not ordi- 
 narily be avoided. Any change of instruments at a station therefore 
 
BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 69 
 
 is apt to make a more or less objectionable break in the strict con- 
 tinuity of the pressure reports from that station and obviously such 
 changes should be made only when absolutely necessary. 
 
 117. After continued use the mercury in the cistern of a barometer 
 loses its brilliant surface and becomes coated with a slight film of 
 oxide. This does not impair the barometer to any serious extent, 
 and very accurate readings can yet be made. It is a bad practice 
 to clean the mercury in barometers as s6on as it becomes slightly 
 dull and tarnished. Leaks are apt to be started in the joints of the 
 cistern, and slight changes in the position of the ivory point give 
 rise to new and unknown corrections for instrumental error. The 
 mercury itself is apt to become contaminated with impurities and 
 afterwards will remain bright only a very short tune. 
 
 118. Comparative barometer readings. Each regular Weather Bu- 
 reau station is supplied with two good barometers to lessen the chances 
 of a break in the record and to guard against erroneous reports from 
 the use of imperfect instruments. Monthly therefore, and on other 
 special occasions, as further specified hi paragraph 122, five com- 
 parative readings of all barometers on station should be made at 
 uniform intervals of hours, half hours, or quarter hours, as may be 
 most convenient to the observer. 
 
 119. As the object of the comparative readings is to ascertain 
 accurately the amount of discordance between the barometers and 
 enable the main office to replace defective instruments, it is impor- 
 tant that the observer use more than ordinary care to read the barome- 
 ters exactly as they are. He should not feel biased or disposed, in 
 the slightest degree, to make the readings come out one way more 
 than another. His whole endeavor should be to make the settings 
 and readings as accurately as possible, without any regard as to 
 how the readings may differ in the end. When the pressure is 
 found to vary rapidly, make the readings of the two or more instru- 
 ments as quickly as possible, and throughout the series endeavor to 
 keep the temperature stationary. 
 
 120. Before each reading the cistern of the barometer should be 
 unscrewed so as to lower the mercury one-sixteenth to one-eighth of 
 an inch below the ivory point and the setting then carefully made. 
 
 121. An interval of two or tnree hours should intervene after 
 barometers are unpacked, cleaned, or moved to a new office and 
 hung in position before comparative readings are commenced. 
 
 \'2'2. Comparative readings should always be made with new 
 barometers whenever received at a station, and also both before 
 and after instruments are removed from one location to another or 
 cleaned. 
 
 123. Suggestions and instructions for cleaning barometers. In a few 
 cases observers are authorized to clean cisterns of barometers that 
 
70 BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 
 
 are very dirty and can not easily be replaced. (See paragraph 187.) 
 The following instructions will then guide in the proper performance 
 of the work: 
 
 124. Take a series of five comparative readings before the work is 
 begun. 
 
 125. Provide one or more very clean, dry porcelain or glass cups 
 or saucers. Avoid the use of damp, unclean, or metal vessels. 
 Cleanse the vessels by thorough washing in soap and water and 
 wipe dry with a clean towel, finally polishing the vessel with tissue 
 or similar soft paper. Provide, also, some pieces of clean cloth 
 and sheets of tissue paper for cleansing the glass parts of the cistern, 
 also a few small sheets of clean white paper about 4 by 6 inches 
 for use in filtering the mercury. Calendered letter paper is not 
 so good as the ordinary so-called book paper. A most convenient 
 position for cleaning a barometer is to be seated in front of a desk 
 with a drawer at the top and side partly opened. This affords 
 convenient corners in which the barometer can be rested in upright 
 positions during the process. 
 
 126. The barometer will be removed from its box or support 
 and inverted, as described in paragraph 111. 
 
 Unscrew with one hand the portion of the cistern marked S, 
 figure 3, grasping with the other hand only the narrow flange R. 1 
 
 127. Next separate the two wooden portions of the cistern marked 
 i and j by loosening the four screws uniting the split-ring clamp 
 marked Z and M, in figure 3. It is important that each screw be 
 loosened a little in turn, otherwise an uneven strain may be thrown 
 upon some portion of the fragile wooden flange and chip out a piece. 
 After loosening each of the screws one may be taken out entirely, 
 and the whole system of split rings still interlocked by the screws 
 will generally unfold from around the cistern. Sometimes another 
 screw must be taken out. 
 
 If the rings are separated, they should afterwards be united again 
 precisely in the original relation. When removing the w( oden piece 
 j, to which the leather bag is attached, lift it cautiously directly up 
 from the part i so as not to spill the mercury, which is thereby 
 exposed and should just about fill i. Hold a clean, dry vessel close 
 under the flange of i and pour out steadily from the cistern all or 
 nearly all, the mercury it contains. The mercury will not leave the 
 open end of the barometer tube so long as the latter is not raised 
 
 1 If mercury has leaked out of the cistern this will generally be indicated by the presence of minute 
 globules of mercury adhering to the threads of the screw O. In such a case the cap at the extreme bottom 
 of the cistern should be unscrewed, instead of the portion marked 8, thereby preventing the escape of 
 the mercury which has leaked out. As this mercury, by reason of its contact with the metal parts of 
 the cistern, is impure, it must be emptied separately, and under no circumstances afterwards used in the 
 barometer or mixed with good mercury, as the whole will be rendered impure. 
 
 To empty thi? impure mercury from the cistern the finger must be used to force the kid-skin bag up 
 into the cistern while the barometer is inclined and the impure mercury poured out. 
 
BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 71 
 
 much above a horizontal position, and generally not then unless the 
 opening is large and the tube shaken or jerked a little. Care must 
 be taken to prevent the mercury from passing out of the tube. The 
 barometer is then returned to its inverted position and the remaining 
 parts of the cistern removed by loosening the screws P and P; here 
 again each screw must be loosened a little in turn to avoid chipping or 
 cracking the glass cylinder. If a small globule of mercury remains 
 in the glass cistern, allow the latter to rest in its position, while the 
 boxwood piece, i, the metal flange, R, and the screws, P, are removed. 
 Then holding the glass cylinder in position with the fingers, empty 
 what remains of the mercury in the cistern. In handling the little 
 leather washers taken from the parts of the cistern, avoid wrinkling 
 or creasing them or otherwise changing their form, as any injury of 
 this kind will probably result in leaks that can not be prevented 
 except by new washers. 
 
 128. The barometer tube and attached wooden piece G, figure 3,may 
 be next withdrawn from the metal sheath and all the parts thoroughly 
 cleaned. Before removing the tube notice exactly the position of the 
 ivory point in reference to the outside sheath so that it may be 
 returned to this position, otherwise a change may be introduced in 
 the correction for instrumental error. In all probability small quan- 
 tities of mercury will be spilled into or remain in various little cracks 
 and crevices while the cistern is being emptied. These, by all means, 
 should be thoroughly dislodged, especially from about the metal parts. 
 With the glass tube removed, the sheath should be tapped and shaken 
 smartly to remove all small globules of mercury. It may then be 
 wiped and cleaned thoroughly with cloths or chamois skin. In 
 case the scale is somewhat dull and tarnished it may be brightened 
 by suitable polishing, but this is a delicate operation and should be 
 avoided rather than otherwise. The danger lies in shifting the posi- 
 tion of the scale, and if polishing is absolutely necessary, it should, 
 therefore, be done with very great care. 
 
 The upper portion of the glass tube should also be cleansed on the 
 outside with the aid of a damp cloth if necessary. 
 
 129. Air in barometer tubes. How air can gam entrance to the 
 vacuum of a barometer otherwise in good condition, which is sup- 
 posed to have been hanging quietly and undisturbed upon its sup- 
 ports, is a matter that is very difficult both to imagine and to explain. 
 No case of this sort has ever occurred among the hundreds of 
 barometers handled at this office, so that when such a defect is dis- 
 covered in an instrument in use at a station the observer in investi- 
 gating the cause and reporting the matter should make sure that 
 the barometer has not been tampered with or roughly handled by 
 unauthorized persons, as, if uninjured in other respects, misusage is 
 the most probable explanation of the defect. 
 
72 BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 
 
 130. If an appreciable quantity of air is in the tube at the time of 
 cleaning, it can be seen more or less conspicuously in the shape of a 
 small bubble or bubbles adhering closely to the walls of the tube. 
 If these bubbles appear no larger than good-sized pinheads, and 
 especially if they are not more than halfway up the tube, then it is 
 certain that the condition of the vacuum is more likely to be greatly 
 impaired than improved by attempts to remove them. 
 
 131. Sometimes the barometers that observers may be called upon 
 by private parties or friends to inspect or repair, seem to have numer- 
 ous rather flat-shaped air bubbles firmly lodged against the sides of 
 the tube. Generally these are not air bubbles at all, but are parti- 
 cles of moisture, the presence of which is due to carelessness in the 
 original preparation and filling of the tube. The edges of an air 
 bubble are sharp and the mercury generally remains bright and 
 makes well-defined contact at a steep angle with the glass. If some 
 moisture is present, either alone or with the air, the edges are less 
 clearly defined, the mercury is oxidized, and the angle of contact is 
 less steep, the bubble itself being very flat. 
 
 It is impossible, without entire cleansing, drying, and refilling, to 
 do anything with a barometer that contains moisture. 
 
 132. If a bubble or so of air is present in a tube, the plan that 
 should first be tried to remove it is as follows : 
 
 First method. Incline the tube 45 or thereabouts, with the open 
 end up, and tap it gently in the vicinity of the bubble, revolving the 
 tube a little at the same time so as to encourage the bubble to creep 
 along the inclined surface of the glass. If the inclination is too 
 great the bubble will be greatly compressed by the weight of mercury 
 above it; if too small the bubble will not tend to move. 
 
 If the treatment is successful and the bubble is removed, the result 
 will probably be beneficial; but at best the operation is generally very 
 tedious, and often the bubble seems to grow smaller and finally dis- 
 appears, being separated into almost imperceptible portions which 
 remain distributed along the walls of the tube. 
 
 Second method. The following plan is more frequently applied, 
 especially when the quantity of air already in the tube is considerable, 
 is lodged at the top, and must be partially removed at least : 
 
 Empty an inch or two of mercury from the tube. Close the 
 open end tightly with the gloved finger and cause a large bubble of 
 air to glide slowly and regularly along the tube until it unites with 
 all the portions of air it is desired to remove. The large bubble is 
 then as slowly and gradually worked to the open end of the tube 
 again, using every possible precaution to prevent small portions of 
 the bubble from separating and remaining behind. Such a bubble 
 of air may sometimes be successfully passed once into and out of the 
 tube, but even at the best the vacuum in a barometer that has been 
 
BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 73 
 
 treated in this manner is very apt to be greatly impaired and can not 
 be restored. The reason of this is that the glass walls of the tube 
 have very strong hygroscopic properties, and while the air bubble is 
 passing along the tube considerable portions of both moisture and air 
 are invisibly retained upon the walls of the tube. While, therefore, 
 a bubble of air may be successfully passed once into and out of the 
 tube, a repetition will be attended with less good effect, as in the 
 meantime the moisture and gases of the bubble will have acted upon 
 the mercury to produce oxidized films that will probably adhere to 
 the walls of the tube, so that when bubbles are again passed there will 
 presently be a marked tendency to cling to the tube and leave 
 small detached bubbles imprisoned against the walls. When, after- 
 wards, the barometer is set up the walls in the upper portion and 
 near the vacuum, being no longer subjected to the full air pressure as 
 they were while the bubble was passing along the tube, now readily 
 give off both air and moisture, and in many cases numerous little 
 bubbles form against the walls even below the top of the column and 
 probably later work their way into the vacuum. 
 
 133. The removal of air from a barometer tube, therefore, can not 
 be perfectly effected in any such way, and should not be undertaken 
 unless the defect is a very serious one. If the comparative readings 
 taken before cleaning a barometer do not show serious errors, any air 
 the tube may be thought to contain had best be allowed to remain. 
 
 134. One of the most difficult and delicate parts of the process of 
 cleaning is that about the wooden piece, G, and ivory point. The deep 
 and narrow annular space between the glass tube and the boxwood is 
 generally covered with oxide of mercury, which should be thoroughly 
 removed by repeated wiping with clean cloths applied upon the 
 ends of slender sticks or by similar means. Tufts of raw cotton 
 will adhere firmly to, and are readily wrapped about, rough sticks 
 and may serve with advantage in wiping out the narrow spaces. 
 Sometimes, however, the space is so small that it can not be properly 
 cleaned. Care must be observed not only here but in subsequent 
 operations, to blow away or otherwise remove every vestige of lint, 
 dust, shreds of cotton, etc., as, if allowed to remain about the parts 
 of the cistern, they will quickly find their way to the surface of the 
 mercury, upon which they will float about to the detriment of accu- 
 rate adjustments. 
 
 It is obvious that the delicate ivory point should be handled with 
 great care. 
 
 135. The glass cy Under of the cistern should be washed hi soap 
 and water and thoroughly rinsed in copious applications of fresh 
 water. After this it should not be touched with unprotected hands 
 especially upon the inside. Wipe it thoroughly dry with a clean, dry 
 towel or handkerchief^ and polish with clean tissue paper. The 
 
74 BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 
 
 . 
 
 remaining wooden portions of the cistern should also be wiped 
 thoroughly clean and dry without touching the inside with the bare 
 fingers. Shake out of the bag as far as possible every little particle 
 of mercury that tends to remain in hidden corners and crevices. 
 These little particles are very apt to be dirty and impure, and should, 
 therefore, be removed. 
 
 136. The several parts of the barometer should be replaced in the 
 following order: 
 
 First, return the glass tube to its sheath, being careful to place 
 the ivory point in the position in relation to the scale, or front of the 
 barometer, formerly occupied; also to avoid handling the end portion 
 of the barometer tube where it dips into the cistern with the bare 
 fingers, as a slight film of oil may be communicated to the mercury 
 of the cistern by this means. 
 
 The glass cylinder, with its leather washers, one at each end, is 
 next placed in position, followed by the wooden piece, i, and the 
 metal flange ring, R. The three long screws, P, are next to be 
 'inserted and partially screwed up. While these various pieces are 
 still loosely held by the screws, it is well to jostle the parts about 
 a little and twist the ring and boxwood pieces upon each other and 
 the glass cylinder. In other words, try to bring the surfaces in the 
 several joints nicely and uniformly into contact with each other, and 
 adjust the ring, R, so that the screws are not even imperceptibly 
 askew, but, when properly drawn up, produce a direct, uniformly 
 distributed pressure. When the parts are thus adjusted the screws, 
 P, are to be tightened little ~by little, each one a little in turn after the 
 others, until all are drawn down together equally tight. The Observer 
 must judge of this partly by the amount he has turned each 
 screw and partly by the resistance it offers to further turning. It 
 is not necessary that the screws be very tight. A judicious regard 
 for these ideas constitutes in part the skill of the expert and is the 
 secret of perfect joints. To disregard them produces leaky joints 
 and unequal pressures that are apt to break the fragile boxwood 
 flanges or crack the glass. 
 
 Before describing the filling of the cistern, some tests and experi- 
 ments showing the purity and properties of mercury will be men- 
 tioned. 
 
 137. Purity of mercury, Jiow tested. Pure mercury is beautifully 
 briUiant and mobile, and does not exhibit the slightest adhesion to 
 clean, dry glass or porcelain surfaces, whereas the amalgamation of 
 the mercury with the slightest perceptible traces of foreign substances, 
 such as lead, tin, zinc, etc., changes completely the character of this 
 peculiar substance. Each observer should try for himself the fol- 
 lowing instructive and simple experiment : 
 
 . 
 " 
 
BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 75 
 
 Prepare a small, shallow, flat-bottomed porcelain cup or white piece 
 of chinaware, or glass vessel if the others are not to be had. Wash 
 and dry thoroughly without touching the inside with the bare fingers. 
 The vessel may be just a little warm with advantage. Filter into the 
 vessel, through a paper funnel, such as described below, rather less 
 than a teaspoonful of pure mercury. If the mercury has been prop- 
 erly filtered and is of extreme puritjr, the brilliant globule will roll 
 about the cup with the greatest activity, as the latter is moved a 
 little, and will draw out momentarily into slender cylindrical por- 
 tions which, if broken asunder, will quickly separate into smaller 
 portions, which draw themselves up into beautiful little spheres or 
 larger rounded buttons, none of which clings in the slightest degree 
 to the clean surface of the vessel. Under favorable conditions and 
 during the rapid movements of the mercury a scarcely audible but 
 still a very characteristic crackling sound can be heard, due to the 
 development of small sparks of electricity. Such is the character- 
 istic behavior of clean, pure mercury hi a clean porcelain dish. If, 
 however, the mercury contains the most minute trace of lead, tin, 
 zinc, etc., this fact wih 1 be shown by a more or less marked tendency of 
 the little, slender portions of the mercury to draw out into sharply 
 pointed, tapering "tails," the tip ends of which cling to the vessel 
 and remain. If the observer is not in possession of the small quan- 
 tity of extremely pure mercury needed in the above experiment, the 
 most striking part of it will be lost. After watching the beautiful 
 manner in which the pure mercury rolls about the dish, add to it a 
 small flake of lead or solder. The flake should be a very small 
 shaving cut off with a penknife, and should contain not nearly so 
 much material as in the head of the smallest phi. Place this upon 
 the mercury and allow it to remain a moment. It will presently be 
 wholly dissolved. Now repeat the rolling about of the mercury in 
 the cup and observe the wonderful change. 
 
 The former brilliant globule has now a dull surface, with its edges 
 clinging at many points to the surface of the dish. The clean white 
 surface of the dish will now be soiled and discolored when the mer- 
 cury is made to flow over it a few times. The presence of one part 
 of lead hi one hundred thousand parts of mercury is readily shown 
 by this test. 
 
 Only one who has performed this experiment is prepared to appre- 
 ciate fully the importance of absolute cleanliness in barometer cis- 
 terns and the necessity for the avoidance of the slightest metallic 
 contamination of the mercury. 
 
 138. Of course, the mercury used in the above experiment can not 
 be again used until purified. This can be done quite well by washing 
 with dilute nitric acid, about one volume of acid in fifteen volumes 
 of water. The mercury and acid may be placed in a bottle and vi<>- 
 
76 BAEOMETEES AND MEASUBEMENT OF ATMOSPHEEIC PEESSUEE. 
 
 lently shaken, or the acid may be poured over the mercury and 
 allowed to remain several hours. When the acid has thoroughly 
 cleansed the mercury, the latter, upon the contents of the bottle 
 being violently shaken, will break up into very fine globules which, 
 for a moment, do not coalesce. This formation of the mercury 
 into minute globules hi the presence of dilute acid will take place 
 only with quite pure mercury. 
 
 139. Returning now to the processes of restoring the barometer, 
 the next step is to filter the mercury and fill up the cistern. Roll up 
 a small sheet of clean paper into a sharp cone, looking through it to 
 the light to see that the opening is very small. Holding the cone over 
 a clean vessel, partly fill it with mercury. By twisting at the folds of 
 the cone hi a manner that the observer must learn by trial, the open- 
 ing at the point may be regulated to any size'desired, even while the 
 cone contains mercury. Keep the cone well filled with mercury until 
 all has been added, and do not allow the very last portion to pass 
 through the filter. If the observer has only the supply of pure mer- 
 cury taken from the barometer, economy must be exercised, but there 
 is no difficulty whatever in being able to filter and utilize the entire 
 quantity of mercury originally in the barometer, and this is suffi- 
 cient. The purity of some of the filtered mercury may be tested as 
 described above. Another indication of the^ purity of the mercury 
 is the character of the mark left on the paper cone after filtering. 
 To be able to judge by this, observers must filter both pure and 
 impure mercury and compare the marks. 
 
 140. The mercury for the cistern, having been filtered at least once, 
 may next be filtered into the cistern, directing the little stream so as 
 to strike against the glass cistern to avoid inclosing small air bubbles 
 near or upon the barometer tube. The open end of the tube should, 
 in the meantime, be completely filled, and the mercury heaped into 
 a little button on the tip end . This button will unite with the mer- 
 cury of the cistern as it rises around the tube, and the chances of 
 inclosing air in the tip end of the tube are thus greatly lessened. 
 
 In general, the cistern should be filled to the brim of the piece, i. 
 Before fitting the piece, j, the leather bag should be pushed out from 
 the inside and every effort used to detach and remove all dust, shreds, 
 little particles of leather, etc. 
 
 141. In securing the clamp rings the screws should be tightened a 
 little at a tune, and the precautions cited in paragraph 136 observed 
 to insure a closely fitting and uniformly tight joint. 
 
 When the screws are all tightened, the leather bag should be thrust 
 up into the wooden piece, j, and held there firmly by the finger while 
 the barometer is gradually turned right side up, watching to see if 
 any leaks show themselves at any of the joints. The mercurial 
 column should not be lowered under any circumstances at this time. 
 
BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 77 
 
 If a leak occurs, it is probably due to uneven tightening of the joints, 
 and in most cases it is better to loosen the whole joint and shift it 
 a little before tightening again rather than to strain the screws that 
 are already tight, in the hope of making closer contact. 
 
 142. From one to three or more hours after the cleaning operations 
 are completed and the barometer is returned to its support a series of 
 five comparative readings with its companion or standard instrument 
 should be made. 
 
 143. Additional suggestions for cleaning barometers with the Tuch 
 cistern. The parts of the barometers with Tuch cisterns can be 
 removed only by the aid of a special wrench, which will be furnished 
 when any observer is authorized to clean such barometers. 
 
 The special points to be observed are as follows: 
 
 Before fully screwing up and inverting the barometer, close tightly 
 the air vent, d, figure 9. Consult also paragraph 17. 
 
 When about to open the cistern, loosen the screw, W, figure 9, one 
 or two turns. Then unscrew the piece, q, being very careful at this 
 time not to unscrew the part, K. When completely unscrewed, the 
 piece, q, with attached piston, o, may be wholly withdrawn, exposing 
 the mercury. All or nearly all of this may be poured out in the man- 
 ner described in paragraph 127 without danger of starting the mer- 
 cury from the end of the barometer tube. The small portion of mer- 
 cury that is apt to remain should be poured out afterwards, removing 
 first the tube or barrel, K, then the clamp ring, ~h, holding the glass 
 cistern in position by the hand until the last portion of mercury is 
 emptied. 
 
 To remove the barometer tube from the sheath, the cap of the air 
 vent must be first removed. 
 
 It is always best not to disturb any more than necessary the various 
 leather washers and fittings. 
 
 144. In replacing the parts be sure the cap of the air vent is 
 screwed up before introducing the mercury. 
 
 After inserting the glass cistern and washers screw down upon it 
 the ring clamp, h. Then filter in as much mercury as practicable, 
 observing the precautions mentioned hi paragraph 140. Next screw 
 down snugly the tube or barrel, K, and add the remaining mercury. 
 Finally, replace the piston and cap, q. 
 
 If the piston does not fit the barrel snugly enough, the washers 
 
 should be tightened by turning up the screw, j, using the special 
 
 wrench. The parts of the piston should not be taken apart nor the 
 
 washers disturbed, except possibly to tighten the screw mentioned 
 
 i above. 
 
 During the operation of replacing the piston it should not close 
 down upon the mercury, which would be subjected to severe pressure 
 i thereby. At last, tighten the screw, Z. 
 
78 BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 
 
 Before the instrument is placed erect the piston should be screwed 
 up close to the mercury, still leaving a small space, as shown by the 
 air bubble, visible in the cistern when the barometer is held nearly 
 horizontal. 
 
 145. Suggestions about moving and packing barometers. Pre- 
 paratory to moving invert the barometer as described in paragraph 
 111. 
 
 The most approved methods of packing barometers for transporta- 
 tion are to be learned by carefully observing the manner followed in 
 packing instruments sent out by this office. The instrument should 
 be shipped in a horizontal position. The air -bubble space in the 
 cistern should be small, but still sufficient to admit of expansion 
 with temperature changes. 
 
 When carried about by hand, the cistern should be uppermost. 
 
 The barometer should be first wrapped in soft paper, then with a 
 thick layer of cotton sheeting and an outer wrapping of heavy paper. 
 Thus prepared, it is then placed in the middle of a strong wooden box 
 and completely and closely surrounded with good excelsior or cotton 
 or similar elastic packing material. The lid of the box must be 
 screwed down, not nailed, and a strong handle attached to the middle, 
 so that the box may be carried by one hand in a horizontal position. 
 
 146. Leather carrying cases. In using the leather carrying case, 
 supplied when barometers are to be transported by hand, secure the 
 barometer in the hinged wooden sheath, being careful to observe that 
 the latter closes tightly without straining either the milled head for 
 regulating the vernier or the attached thermometer. The wood 
 should be neatly cut away, if necessary, but only sufficiently to receive 
 these projecting parts. Insert the barometer, cistern uppermost, into 
 the leather case. 
 
 147. On steamboats or railroads the barometer, if hung up in any 
 manner, should be secured against striking or pounding the side of 
 the room or car. In wheeled vehicles the barometer should be car- 
 ried by hand, supported by a strap over the shoulder, or held upright 
 between the legs. It should not be allowed to rest on the floor, as a 
 severe jolt may break the tube. On stage routes, when impracti- 
 cable to carry it by hand, hang the barometer on a hook inside the 
 stage and securely fasten the lower end, so that it will not swing 
 when being thus transported. If carried on horseback, it should be 
 strapped over the shoulders of the rider, where it is not likely to be 
 injured. 
 
 148. Change of location. It sometimes becomes necessary to change 
 the location of instruments from one office room to another or to a 
 different point in the same room and making little or no change of 
 elevation. In such cases the barometer box can be moved bodily 
 with the instruments in place. The first step is to prepare the wall 
 
 . 
 
 * 
 
BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 79 
 
 at the new location by setting up the necessary wood strips and the 
 screw at the top on which the hook of the barometer box can be 
 -hung. If no change is to be made in the elevation this screw must 
 be at exactly the same height as the corresponding screw at the old 
 location. After the usual comparative readings have been made 
 and all is prepared the barometer cisterns will be screwed up until 
 full of mercury and the box moved bodily in an upright position and 
 secured at the new location. Comparative readings should not be 
 made after the removal until an interval of two or three hours has 
 elapsed, unless the temperature is practically the same in the two 
 locations. 
 
 149. Shipment of serviceable barometers. Every possible care is 
 taken at the central office to secure the highest attainable precision 
 in the pressure observations at stations and to ascertain the amount of 
 abnormal errors that sometimes develop in the use of instruments. 
 To this end defective instruments are called in to the central office 
 for recomparison, and obviously it is of the highest importance that 
 every precaution be taken in packing such instruments to insure their 
 safe arrival at destination. 
 
 150. In reporting on defective or unserviceable barometers, observ- 
 ers should state clearly the condition of the instrument, and whether 
 or not the mercury can be screwed up to such an extent as to permit 
 of forwarding the instrument " mercury filled." Such barometers 
 are called in by the official in charge, division of supplies, central 
 office, by express, and observers should keep on hand constantly a 
 small supply of the special I 1 Notice," "Glass," and "Very fragile" 
 labels, two or, three of which should be tacked or pasted on outside 
 cover of box in a conspicuous manner. 
 
 151. Shipment of empty instruments. When barometers are so 
 seriously defective that check comparisons can not be made or are 
 valueless, observers will be authorized to forward the barometer to 
 Washington, first carefully emptying all the mercury, which will be 
 preserved in a clean bottle and packed in the box with the barometer. 
 
 (B) CARE AND USE OP BAROGRAPHS. 
 
 152. Exposure of barographs. The general principles of the expo- 
 sure of barometers given in paragraph 89 apply to barographs also, 
 except that the matter of light is not so essential. Every precaution, 
 however, must be observed to prevent the instrument from being 
 exposed to great changes of temperature and to direct influence of 
 sunshine, etc. 
 
 The instructions following apply particularly to the Richard 
 barograph. 
 
 153. Adjustment to standard pressure. When the instrument is first 
 set up at a place, the pen should be made to mark, as nearly as possi- 
 
80 BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 
 
 ble, the corrected pressure (see paragraph 172) given by a standard 
 mercurial barometer at the same place. This adjustment is made by 
 raising or lowering the whole series of aneroid shells by means of a 
 screw reached through a hole in the base of the instrument just under 
 the aneroid shells. This screw is turned by one end of the key sup- 
 plied for winding the clockwork. The adjustment of the barograph 
 to agree with a standard barometer will rarely prove permanent and 
 will require a little alteration from time to time, there being a slight 
 tendency for an aneroid barometer to read too high with age. It 
 is generally necessary to set the barograph to standard at the tune a 
 new sheet is put on, but if the error is small it is better to allow for 
 it than to readjust. 
 
 1 54. Special adjustment for high elevations. The adjustment afforded 
 by the screw underneath the base of the instrument is not sufficient 
 to bring the pen to the proper pressure at stations four thousand feet 
 or more above sea level. In this case, and also to prevent injury to 
 the barometer while in transit over lofty mountain passes, it is some- 
 times necessary at this office to disconnect the system of levers from 
 the aneroid shells and to provide one or more extra holes in the stem 
 projecting at the top. Barographs found disconnected in this man- 
 ner upon arrival at stations need simply that the links be united again, 
 placing the small pin in whichever hole will bring the pen nearest the 
 middle of the record sheet. When shipping barographs, if the instru- 
 ment has to pass over a greater elevation than 3,000 feet, the system 
 of levers should be disconnected, as above. 
 
 155. When the pressure at any particular station is such as not to 
 be included in the rulings on the record sheets furnished with the 
 instrument, observers will change the numbering of the lines by 
 some convenient whole number and adjust the pen of the instrument 
 accordingly. 
 
 156. The sheets should be changed at about noon on the 1st, 8th, 
 15th, 22d, and 29th of each month. 
 
 When a barograph is first put in operation the trace on the sheet 
 should start at the proper date and hour, even if near the end of the 
 sheet. The roman numeral, XII, at the top of the sheet indicates 
 noon, and the letter "M.," midnight. 
 
 If not already done, the lower edge of the sheet should be trimmed 
 accurately parallel to the longitudinal lines and should rest closely 
 against the flange at the bottom of the cylinder and the pen be 
 adjusted to the proper pressure, in accordance with instructions in 
 paragraph 153. 
 
 157. Barograph docks. Every effort should be made to regulate 
 the barograph clock to keep correct time, winding once a week, or 
 oftener if found to give better results. The instrument should be 
 inspected each day by the observer in charge and properly adjusted 
 
BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 81 
 
 by liim when necessary. Whenever a clock is adjusted, a marginal 
 note stating the fact should be connected with the proper hour. If 
 the clockwork goes too fast or slow, it can be regulated in the same 
 manner as the movement of a watch, through an opening in the 
 cylinder marked A (to " accelerate ") and R (to "retard"), correspond- 
 ing to the letters "F" and "S" near the ends of the regulator itself. 
 
 158. Time error of barograph. The clock movements of these baro- 
 graphs often keep but imperfect time, and it is important that the 
 record be checked in tliis respect, so that proper correction may be 
 made in compiling the records of hourly readings. This result is 
 secured by gently touching the lever of the recording pen, so as to 
 deflect it and cause a slight lateral mark to be made on the record 
 sheet across the baromet fie -trace. A mark of this character will be 
 made each day at noon, seventy-fifth meridian time; the record sheet 
 will thus always show how much the barograph clock may be in error. 
 
 In producing these marks great care must be exercised not to strain 
 the lever mechanism in any way; the weight of any ordinary lead 
 pencil is amply sufficient to make the mark, which should not be more 
 than one-eighth inch in length. 
 
 159. Pens. Pens should be kept clean. Only the standard register 
 ink should be used. Care must be observed in cleaning a pen not to 
 bend or deform the points and render it unserviceable. (See Circular 
 A. Instrument Division.) 
 
 160. Corrections. Owing to imperfections in barographs, more or 
 less frequent comparisons should be made with standard instruments, 
 and corrections applied according to the scheme fully described in 
 Circular A, Instrument Division. 
 
 VI. CONCERNING THE ELEVATION OF STATIONS. 
 
 161. In the system of the Weather Bureau the elevation of a station 
 is the height above mean sea level of the zero point that is, the 
 "ivory point" of the barometer scale, and all measurements and 
 levelings for elevations must be made in reference thereto. 
 
 162. Elevation determined by spirit level. Whenever a station is 
 established or an office moved and the elevation of the barometers 
 changed, observers will secure the services of a competent surveyor 
 or city engineer, who will run a line of levels to determine accurately 
 the elevation of the station above or below the "plane of reference." 
 In many instances this survey can be secured without expense 
 through the courtesy of the Government or of the city engineers. 
 In the remaining cases the cost will be included with other items of 
 expense incident to the establishment or removal of the office, 
 authority for the expenditure being procured in the usual manner. 
 
 2293712 6 
 
82 BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 
 
 All heights will be given in feet and hundredths or thousandths of 
 a foot. 
 
 163. Fixed point. The engineer will establish a so-called /'fixed 
 point" or bench mark in a permanent manner on the outer stone- 
 work of the building, from which direct measurements of the height 
 of the barometers can readily be made, or a line of levels conven- 
 iently run to the " plane of reference" at any time. 
 
 164. Plane of reference. The " plane of reference" should, in 
 general, be the top of rail at depot. In case, however, a bench mark 
 of a permanent character and of high precision, such as erecced by 
 the United States Coast Survey, the Lake Survey, the Mississippi 
 River Commission, or the Engineer Corps, has been established, this 
 will be used as "plane of reference." The same plane should be used, 
 as a rule, in all measurements within the same town, and will not be 
 changed except for good -reason. 
 
 165. All levelings must be run by a competent person and the line 
 will be run over a closed circuit which, in the case of a new station, 
 will be from the plane of reference through the " fixed point" to 
 the barometer, and back through the "fixed point" to the plane of 
 reference. In the case of the removal of an office the line will be 
 run from the barometer in the old office to the barometer in the new 
 office, passing through the "fixed points" at both the old and the 
 new office, thence through the "fixed point" at the new office to the 
 "plane of reference," and thence to the point of starting. 
 
 A copy of the field notes, certified to by the surveyor running the 
 levels, will be filed with the report of elevation which will be rendered 
 on Form No. 1058-Met'l. 
 
 The observer will fully inform the surveyor concerning the fore- 
 going provisions for running the levels. 
 
 166. When, for any reason, it is impossible to run the levels pro- 
 vided for in paragraphs 162 to 165, the observer will, in case of the 
 change of location of the barometers, or removal of the office, make 
 a special set of comparative barometer readings; that is to say, the 
 comparative barometer readings always required on changing the 
 location of instruments will in case levels can not be run be made in 
 three sets, as follows: 
 
 First. Set before removal. Second. Set during removal; that is, 
 station barometer in new office, extra barometer in old office. This 
 set of readings will be recorded as usual on Form No. 1027-Met'l, and 
 will be accompanied by two readings of the exposed thermometer, to 
 be noted on the margin, the first taken immediately before and the 
 second immediately after the set of barometer readings. Third. Set 
 after removal; that is, both barometers in new location. 
 
BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 83 
 
 When there is an assistant on station the five readings of the second 
 set must be synchronous, but in case there is no assistant at station, 
 eight readings in all will be made alternately in the two offices, in the 
 following order: No. 1 in new office; Nos. 2 and 3 in old office; Noe, 
 4 and 5 in the new office; Xos. 6 and 7 in the old office, after which 
 the extra barometer will be carried to 'the new office and the eighth 
 reading made on the station barometer. Finally, the third set of 
 comparative readings w r ill be made. 
 
 The interval between readings should be as nearly uniform as pos- 
 sible, and is left to the convenience of the observer, depending upon 
 the distance between stations, etc. 
 
 167. When changes in the location of the barometers that do not 
 alter the elevation are authorized, a line of levels need not be run, 
 and only the usual comparative barometer readings before and after 
 removal will be made. 
 
 168. Reduction of observation* to a "station elevation" On January 
 1, 1900, a specific elevation above sea level was adopted for each 
 Weather Bureau station, and for purposes of record and publication 
 all barometric observations will be correlated to this " adopted or 
 station elevation." In case, therefore, an office is moved to new 
 quarters and the elevation of the barometer is thereby changed, a 
 proper correction will be applied to the barometric readings in the 
 new location that will reduce the observed readings to the pressure 
 appropriate to the " station elevation," notwithstanding changes and 
 removals. The pressure thus ascertained will be designated " station 
 pressure." 
 
 169. The " station elevation" for a station in operation January 1, 
 1900, is its elevation above sea level on that date. For stations 
 closed before 1900, or subsequently established, the elevation will be, 
 in general, the elevation above sea level of the zero point of the 
 barometer at the date of closing or opening the respective stations. 
 
 170. Reduction of current observations to a "station elevation" in 
 accordance with the foregoing plan will, therefore, be required only 
 when changes are made hi the elevations of the barometers. In all 
 such cases the Instrument Division of the central office will furnish 
 a new copy of the barometer correction card (Form No. 1059-Met'l), 
 in which a " removal correction," based on the change made in the 
 elevation of the barometers, will be combined with the corrections 
 for local gravity, scale errors, etc. The "sum of corrections" thus 
 determined, together with the "correction for temperature," will be 
 applied to all recorded readings of barometric pressure, and the result 
 will be regarded as the pressure of the air appropriate to the station 
 in question. 
 
84 BABOMETEBS AXD MEASUEEMENT OF ATMOSPHEEIC PBESSUEE. 
 
 171. The following example will elucidate the complete correction 
 of observed barometer readings : 
 
 Observed .barometer reading (attached thermometer, 76.5) 30. 287 
 
 Correction for temperature (Table I) 0. 131 
 
 Sum of corrections, Form No. 1059-Met'l +0. 032 
 
 Total correction . . . - 0. 099 
 
 Station pressure 30. 188 
 
 The "total correction/' as shown above, will be entered on Form 
 No. 1001-Met'l, and applied to the "observed" reading, deriving 
 thereby the pressure of the air appropriate to the adopted elevation 
 of the station; this result will be recorded on Form No. 1001-Met'l, 
 in the columns for "station pressure." 
 
 172. The barograph will be adjusted and corrected to corresponc 
 with the "station pressure" ascertained as shown in the above 
 example. See also paragraph 153. 
 
 173. All pressure observations made at a station and reduced 
 according to the foregoing plan are therefore strictly comparable with 
 each other, all being reduced to the adopted elevation. Furthermore, 
 a change of elevation or removal of office does not necessitate a new 
 table of reductions to sea level; that is, all observations will be reduced 
 to sea level, when required, by one and the same table of reduction, 
 namely, that based on the adopted elevation of the station. 
 
 174. Nomenclature. The following nomenclature, embracing baro- 
 metric terms, will be used, as far as practicable, in the correspondence, 
 records, and publications of the Weather Bureau : 
 
 Actual elevation. The height of the zero points of the barometers 
 of a station above sea level. 
 
 Station elevation. The elevation above sea level adopted for a sta- 
 tion as the basis to which all pressure observations at the station are 
 correlated. 
 
 Observed reading. The direct result of the reading of an instru- 
 ment, uncorrected for any errors. 
 
 Actual pressure. Meaning the actual pressure of the air at a barom- 
 eter, as obtained from the observed reading after applying the neces- 
 sary corrections for temperature, gravity, and instrumental errors. 
 
 Station pressure. A pressure corresponding to an "adopted or sta- 
 tion elevation" which may differ slightly from the actual elevation of 
 the barometer. When the "actual" and "station" elevations are 
 the same at any particular station, the removal correction will be zero 
 and the actual pressure and the station pressure are then numerically 
 equal. 
 
 Reduced pressure. The actual or station pressure reduced to sea 
 level, or to some other specified plane. 
 
BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRKSsrm-:. 85 
 
 Correction for scale errors, capillarity, etc. A mean difference bet ween 
 the readings of a given instrument and those of the standard barom- 
 eter duly corrected. This quantity embraces all outstanding error- 
 in the subdivision of the scale, or its total length: errors in the adjust- 
 ment of the sighting ( ( lge to the zero line of the vernier: error- <>t cap- 
 illarity, imperfect vacuum, etc. 
 
 ( 'orrection for tem/^rature. The correction depending on the tem- 
 perature of the mercury and the metallic scale. 
 ( 'orrection for local gravity: 
 
 (a) Latitude term. The correction based on the variation of the 
 
 force of gravity with latitude. 
 (6) Altitude term. The correction based on the variation of 
 
 gravity with altitude above sea level. 
 
 Removal correction. The correction necessitated by the removal of 
 an office, and based on the difference between the actual elevation of 
 the barometers in the new location and the adopted elevation for the 
 station in question. Strictly speaking, the " removal correction" 
 varies from day to day, or from winter to summer, with large changes 
 of temperature, but this variation, in most cases, can be neglected. 
 When, however, the change in elevation is large, and especially when 
 great differences of temperature, as between win'ter and summer, are 
 considered, the change in the value of the " removal correction" may 
 exceed 0.01 inch or more, and it then becomes desirable to employ a 
 " variable" correction, as more fully explained in paragraph 182 (a). 
 Sum of corrections. A. term embracing all the corrections that are 
 practically constant for. a given instrument in a given location, namely, 
 the correction for scale errors, capillarity and gravity, and the removal 
 correction. This sum is given on the certificate of corrections (Form 
 No. 1059-Met'l) furnished for each instrument. 
 
 Total correction. The correction for temperature, plus the. "sum of 
 corrections" as defined above. 
 
 Reduction to sea level. The quantity which must be added to the 
 "actual" or "station" pressure in order to obtain the ''reduced 
 pressure. 
 
 Reduction for elevation. A quantity which must be added to or sub- 
 tracted from the pressure at a given elevation in order to deduce 
 therefrom the pressure appropriate to some other specified elevation. 
 
 VII. SUMMARY OF SPECIAL INSTRUCTIONS FOR OBSERVERS OF THE 
 
 WEATHER BUREAU. 
 
 175. Station and extra barometers. It is the intention to keep each 
 station supplied as far as possible with two good barometers. The 
 names station and extra apply, respectively, to the one used in tin- 
 regular observations and the one held in reserve. Observers will 
 promptly report, by letter, any defect observed in either in-t ruinent . 
 giving full details as to its nature and probable cause 
 
86 BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 
 
 176. Exposure of barometers. Observers will expose their baro- 
 meters and barographs in acordance with the provisions of paragraphs 
 89 and 152. 
 
 177. Cisterns of extra barometers. When not in use the cistern of 
 the extra barometer may be screwed up so as to raise the level of the 
 mercury about halfway up the ivory point. The cistern should not be 
 filled nor the mercury forced to the top of the tube/ except when the 
 barometer is to be moved. 
 
 178. Elevation of barometers above sea level. At the earliest practi- 
 cable opportunity after establishing a new station or moving into a 
 new office the observer will ascertain and report, upon Form No. 1058- 
 Met'l, the elevation of the barometer above sea level. This measure- 
 ment should refer to the elevation of the lower end of the ivory 
 point of the barometer. Specific instructions in regard thereto are 
 given in paragraphs 161 to 166, and on the back of the form mentioned. 
 
 179. When observations are commenced at a new station and the 
 appropriate table of reductions to sea level has not, as yet, been 
 received from the central office, only the corrections given on Form 
 No. 1059-Met ; l and for temperature will be applied to barometer 
 readings, and the results will be recorded on all forms required and 
 telegraphed in the usual manner until tables of reduction are received. 
 
 180. Regular barometric observations. Settings and readings of the 
 " station" barometer, as required by the instructions providing for 
 the regular observations of the station, will be made in accordance 
 with the provisions of paragraphs 97 to 100, and these readings will be 
 corrected for errors, as illustrated in the example given in para- 
 graph 171. 
 
 181. Reduction of barometric readings to sea level. In reducing 
 barometer readings to sea level the special tables supplied each station 
 according to its elevation will be used. 
 
 The temperature argument will be obtained by taking the mean of 
 the current and the previous 8 a. m. or 8 p. m. observation. For 
 example: Dodge City, Kans., March 1, 1905, temperature, 8 a. m., 
 33.7; 8 p. m., 45.4; the mean of these, 39.6, is the temperature to 
 be used in reducing the 8 p. m. reading to sea level: 
 
 182. Provisional removal correction. When the elevation of the 
 barometer has been changed by removal or otherwise, and the observer 
 has ascertained with reasonable accuracy the new, that is, the " actual 
 elevation" (see paragraph 174), he will immediately report the same 
 to the central office, even if Form No. 1058-Met'l can not be sub- 
 mitted at the same time. Furthermore, pending the receipt of new 
 copies of Form No. 10'59-Met'l, he will reduce his daily barometric 
 observations at the new location by the use of a provisional removal 
 correction deduced in the manner illustrated in paragraph 198. In his 
 
BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 87 
 
 letter the observer will give the value and algebraic sign of the cor- 
 rection he proposes to use. 
 
 182a. Variable removal correction. When extreme changes of 
 temperature, such as occur at different seasons of the year, taken in 
 conjunction with considerable changes of elevation due to removal 
 of office, cause variations in the " removal correction" amounting to, 
 or exceeding 0.010 inch, it then becomes necessary to employ a 
 " variable removal correction," which is taken out for each observa- 
 tion and which depends upon the temperature of the outside air. In 
 such cases this "variable" correction for different temperatures is 
 given in a special table on the back of Form 1059-Met'l, and is used 
 for reducing the observations in the following manner: 
 
 The barometric observations will be reduced practically the same 
 as heretofore (see par. 171), except that the proper "sum of correc- 
 tions" to be used at each observation will be taken from the table 
 mentioned above corresponding to the air temperature observed at 
 the same time. It is important to note that this temperature is the 
 dry-bulb reading in the instrument shelter, not that of the attached 
 thermometer of the barometer. The "Sum of corrections" thus 
 taken from this supplemental table will be added algebraically to 
 the temperature correction in the usual manner, and the resulting 
 total correction applied to the barometer reading, and also entered 
 in the customary spaces on Form 1001, in the column headed "Total 
 correction," on pages 2 and 3. Finally, in entering the data on Form 
 1001-Met'l, a supplementary column will be ruled adjacent to the 
 date column on the left. This column will be headed: "Removal 
 cor.," and therein will be entered, from observation to observation, 
 the particular value of the "Removal correction" from the table 
 given on back of Form 1059-Met'l, corresponding to the "Sum of 
 corrections" employed in reducing the observations. 
 
 Whenever new correction cards (Form 1059-Met'l) are furnished 
 to a station from the central office, the cards previously in use will 
 be destroyed. 
 
 183. Comparative barometer readings. The provisions of paragraphs 
 118 to 121 will be observed in making comparative readings; the 
 results will be tabulated and reported on Form No. 1027-Met'l, and 
 the detailed instructions printed on the back thereof fully complied 
 with. Form No. 1027-Met'l will either be duplicated for the station 
 file, or a letterpress copy retained for the station record. 
 
 Five comparative readings at intervals of hours, half hours, r 
 quarter hours will be made in accordance with the foregoing provi- 
 sions and on the occasions specified as follow- : 
 
 (a) When now barometers are received for the establishment of a 
 station, or, in general, whenever any serviceable Weather Bureau 
 barometer is received at station, by transfer or otherwise. 
 
88 BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 
 
 (6) Regular monthly readings will be made on the 15th of each 
 month. When the 15th falls on a Sunday or a holiday, the readings 
 will be deferred until the next regular working day. 
 
 (c) Before and after removal of office or change in the location of 
 the barometers, also when a line of levels is not run, a set with the 
 station barometer in new office and extra in old office, see para- 
 graph 166. 
 
 (d) Before and after cleaning a barometer. 
 
 184. Except in the case of the regular monthly readings, observers 
 will forward special comparative barometer readings with a letter of 
 transmittal, giving any pertinent remarks relative to the condition 
 of the instruments, the occasion for making the readings and other 
 particulars that may enable the central office to understand more 
 fully the matter under consideration. 
 
 185. Barometers not to be changed. Barometers will not be changed 
 in location nor one instrument substituted for another without author- 
 ity from the central office, unless the circumstances are such that 
 immediate action is necessary. Neither the " station" nor the 
 " extra" instrument will be loaned without authority. 
 
 186. Changes in the location of barometers, especially when the 
 elevation is changed, as in the removal of an office, should be made 
 at the end of the month, if practicable, and the station barometer 
 will be moved in the interval between the last observation of one da}' 
 and the first observation of the following day. The foregoing will 
 also apply, as far as practicable, when a new barometer is substituted 
 for the station instrument. 
 
 187. Authority to dean barometers. Authority to clean barometers 
 will be granted only in special cases, and applications therefor should 
 be made with due consideration of the provisions of paragraph 116. 
 Unless otherwise provided for, only one barometer will be cleaned 
 within any given month, and then just after the regular monthly 
 comparative readings have been made. The set of readings required, 
 after cleaning, may be made the same or the following day, preferably 
 the latter. The specific instructions for cleaning barometers, para- 
 graphs 124 to. 144, will be carefully observed. 
 
 188. Requisition for mercury. Mercury will be furnished to stations 
 only on special requisition, which will be made when the mercury is 
 needed for immediate use in connection with the authorized cleaning 
 and repairing of barometers. 
 
 189. Impure mercury. Mercury of an impure character that has 
 been removed from barometers, or otherwise acquired on station, 
 should be carefully preserved, and in quantities of from a half pound 
 to a pound or more will be forwarded by mail to the central office, 
 in strong bottles or other suitable receptacles, securely wrapped in 
 proper packing material to prevent breakage. The stopper of the 
 
BAROMET1 IIS \XD MEASUREMENT OF ATMOSPHERIC PRESSURE. 89 
 
 bottle should fit tightly and be strongly tied and scaled it' practicable. 
 The mercury is easily reduced to great purity at this office by distil- 
 lation, etc. The package should Remarked: Instrument Division. 
 
 190. Mercury not to be removed. When an unserviceable barometer 
 is returned to this office the mercury will not he removed except as 
 provided for in paragraph 151, and if not returned to Washington 
 with the instrument, will be disposed of as specified in paragraph ls.. 
 
 191. Barographs. The winding and regulating of barograph clock-. 
 the changing of sheets, and the adjustment of pen> to standard pre-- 
 sures, will be attended to in accordance with the provisions of Section 
 (B), pages 80 and 81. 
 
 1!)2. Time error checked. The barograph will be checked for time 
 error at 12 noon, each day, in the manner explained in paragraph 158. 
 
 193. Hourly readings. Detailed instructions for compiling and 
 transcribing records from barographs are given in Circular A, Instru- 
 ment Division. 
 
 194. Comparison of private barometers. Observers will extend even 
 courtesy to shipmasters and others who may apply at the office for 
 information or submit barometers for comparison, adjustment, etc. 
 The mercurial barometers at a station can not as a rule be removed 
 from the office for comparison and adjustment of barometers on board 
 vessels, etc., but in some cases aneroid barometers serve very con- 
 veniently for this purpose and may be supplied when required. 
 
 A memorandum or tag, showing date of comparison, the correction 
 to be applied, and other pertinent facts should accompany an instru- 
 ment that has been compared. 
 
 VIII. TABLES. 
 
 EXPLANATION OF TABLES. 
 
 TABLES I AND II. Correction for ttn,/r<iinn. 
 
 195. Tables I and II give, in English and metric measures, respec- 
 tively, the corrections that should be applied to the observed reading> 
 of a mercurial barometer having a brass scale, in order to eliminate 
 from the barometer reading the effect of temperature in expanding 
 the mercury and the scale. The following example will elucidate the 
 use of either table: 
 
 The attached thermometer of a barometer re*d 
 
 Tin- l.an.iiH-t.-r n-a<U 
 
 In Table I, the pressure nearest 29.415 is 29.5. II.ri/mitally oppo- 
 site 7'J.. r ) in the vertical column under this pre^ure \\e find 0.117. 
 which is the correction required, and we note that it must be sub- 
 tracted. 
 
90 BABOMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 
 
 The reading corrected for temperature is, therefore .................... ..... 29. 415 
 
 .117 
 
 29. 298 
 
 196. Interpolation explained. Suppose the pressure had been 
 29.281, which is about midway between 29 and 29.5. The correc- 
 tion in this case should be about midway between 0.115 and 0.117, 
 or 0.116. The same rules must be followed if the temperature is 
 intermediate between the values of the table. 
 
 TABLES III AND IV. Influence of gravity on barometric observations. 
 
 197. Table III gives the correction required to reduce readings of - 
 the mercurial barometer to standard gravity at latitude 45. 
 
 There is, in addition to the above, another gravity correction 
 required to eliminate the effect of change of gravity with elevation. 
 This is small and is given in Table IV. 
 
 The aneroid barometer does not require any gravity correction. 
 
 Example: For latitude 38 and pressure 29 inches, the gravity 
 correction is minus 0.018 of an inch. For an altitude of 2,490 feet and 
 a pressure of 27.3 inches the gravity correction is 0.004 of an inch. 
 
 TABLE V. Pressure in inches corresponding to changes of 100 feet in elevation. 
 
 198. Table V is employed for computing "removal corrections," 
 paragraph 182, and its use is illustrated by the following example: 
 
 Suppose the " adopted or station elevation" of a given station is 
 1,482.7 feet, and that after removal the "actual elevation" is 1,516.4 
 feet; hence the change in elevation will be 33.7 feet. 
 
 Suppose, also, that the mean annual temperature at the station is 
 56. 
 
 Table V gives Od03 for 1,500 feet and temperature 56. To reduce, 
 therefore, from the "actual elevation" of 1,516.4 feet to the "station 
 elevation" of 1,482.7 feet, the following correction is necessary, viz, 
 
 oo 7 
 
 0.103 = 0.035, which is the "removal correction" required on 
 
 Form No. 1059-Met'l. The correction in this case must be used with 
 the plus sign. 
 
 TABLES VI AND VII. Determination of heights by barometer. 
 
 199. The use of these tables requires that at least two observations 
 of the temperature and the pressure of the air be made, simulta- 
 neously if possible, at two stations. The elevation of one of the 
 stations must be known. 
 
 From Table VI we find the first approximate difference of elevation 
 of the two stations. Table VII gives the allowance that must be made 
 on account of the temperature of the air. 
 
BAROMETERS AND MEASU IM..M KNT OF AT.MusPHERIC PRESSURE. 91 
 
 To be strictly accurate, allowance should also be made for three 
 other effects: (a) For the amount of moisture in the air; (6) for 
 the effect of gravity on the weight of the air: (c) for the effect of 
 gravity on the weight of the mercury (not required when the aneroid 
 is used or when the readings of a mercurial barometer are separately 
 corrected for gravity). By neglecting these effects in computing a 
 high elevation, say 10,000 feet, other conditions being average, an 
 error of fully 100 feet may be made. Greater inaccuracies than this, 
 however, are likely to result if the computations are based upon only 
 a few barometric and temperature readings, -and especially if the 
 readings are not strictly simultaneous. Moreover, the air tempera- 
 ture required is the mean temperature of the whole column between 
 the two stations, but this is only approximately given by the mean 
 of the observed temperatures. 
 
 The two tables following are, therefore, sufficiently accurate for 
 use of tourists who desire a knowledge of the approximate altitude 
 corresponding to the more or less limited and incomplete observations 
 they may make. Example: 
 
 Barometer reading at base station inches. . 27. 58 
 
 Barometer reading at upper station do 21. 47 
 
 Air temperature at base station degrees. . 68 
 
 Air temperature at upper station do 42 
 
 Estimated mean temperature of air column is 
 
 68 + 42 110 ~o_ T 
 . 2 2 ~ 55 - 1 ' 
 
 fa Feet. 
 
 Table W, at 21 .47, gives 9 002 
 
 at 27.58, gives 2,195 
 
 First approximate difference of elevation 6, 807 
 
 Tablet for T = 55 and 6,800 feet, gives: 
 
 JUL Feot 
 
 Allowance for temperature of air 
 
 Hence difference of elevation is 
 
 Suppose the elevation of the base station is * ^ 
 
 Then the elevation of upper station is 
 
92 BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 
 TABLE I. Correction of mercurial barometer for temperature, English measures. 
 
 
 Observed reading of the barometer, in inches. 
 
 F. 
 
 17 
 
 18 
 
 19 
 
 20 
 
 21 
 
 22 
 
 23 
 
 24 25 26 27 28 29 
 
 30 
 
 31 
 
 
 Inches. ADD. Inches. 
 
 -20 
 
 0.075 
 
 0.080 '0.084 
 
 0.089 0.093 
 
 0. 098 JO. 102 
 
 0.107 
 
 0.111 
 
 0.115 
 
 0.120 
 
 0.124 
 
 0.129 
 
 0.133 
 
 0.138 
 
 -19 
 
 .074 
 
 .078 .083 
 
 .087 .091 
 
 . 096 . 100 
 
 .104 
 
 .109 
 
 .113 
 
 .117 
 
 .122 
 
 .126 
 
 .130 
 
 .135 
 
 -18 
 
 .072 
 
 .077 
 
 .081 
 
 .085 .089 
 
 .094 i .098 
 
 .102 
 
 .106 
 
 .111 
 
 .115 
 
 .119 
 
 .123 
 
 .128 
 
 .132 
 
 -17 
 
 .071 
 
 .075 
 
 .079 
 
 .083 .087 
 
 . . 092 .096 
 
 .100 
 
 .104 
 
 .108 
 
 .112 
 
 .117 
 
 .121 
 
 .125 
 
 .129 
 
 -16 
 
 .069 
 
 .073 
 
 .077 
 
 . 081 . 086 
 
 .090 
 
 .094 
 
 .098 
 
 .102 
 
 .106 
 
 .110 
 
 .114 
 
 .118 
 
 .122 
 
 .126 
 
 -15 
 
 .068 
 
 .072 
 
 .076 
 
 .080 .084 
 
 .088 
 
 .092 
 
 .096 
 
 .100 
 
 .103 
 
 .107 
 
 .111 
 
 .115 
 
 .119 
 
 .123 
 
 -14 
 
 .066 
 
 .070 
 
 .074 
 
 . 078 . 082 
 
 .086 
 
 .089 
 
 .093 
 
 .097 
 
 .101 
 
 . 105 . 109 
 
 .113 
 
 .117 
 
 .121 
 
 -13 
 
 .065 
 
 .068 
 
 .072 
 
 .076 .080 
 
 .084 
 
 .087 
 
 .091 
 
 .095 
 
 .099 
 
 . 103 . 106 
 
 .110 
 
 .114 
 
 .118 
 
 -12 
 
 .063 
 
 .067 
 
 .070 
 
 . 074 . 078 
 
 .082 
 
 .085 
 
 .089 
 
 .093 
 
 .096 
 
 . 100 . 104 
 
 .107 
 
 .111 
 
 .115 
 
 -11 
 
 .061 
 
 .065 
 
 .069 
 
 .072 | .076 
 
 .080 
 
 .083 
 
 .087 
 
 .090 
 
 .094 
 
 . 098 . 101 
 
 .105 
 
 .108 
 
 .112 
 
 -10 
 
 .060 
 
 .063 
 
 .067 
 
 .070 .074 
 
 .077 
 
 .081 
 
 .085 
 
 .088 
 
 .092 
 
 .095 
 
 .099 
 
 .102 
 
 .106 
 
 .109 
 
 - 9 
 
 .058 
 
 .062 ! .065 
 
 .069 
 
 .072 
 
 .075 
 
 .079 
 
 .082 
 
 .086 
 
 .089 
 
 .093 
 
 .096 
 
 .099 
 
 .103 
 
 .106 
 
 - 8 
 
 .057 
 
 .060 .063 
 
 . 067 . 070 
 
 .073 
 
 .077 
 
 .080 
 
 .083 
 
 .087 
 
 .090 
 
 .094 
 
 .097 
 
 .100 
 
 .104 
 
 - 7 
 
 .055 
 
 .058 -.062 
 
 .065 . .068 
 
 .071 
 
 .075 
 
 .078 
 
 .081 
 
 .084 
 
 .088 
 
 .091 
 
 .094 
 
 .097 
 
 .101 
 
 - 6 
 
 .054 
 
 .057 .060 
 
 .063 
 
 .066 
 
 .069 
 
 .073 
 
 .076 
 
 .079 
 
 .082 
 
 . 085 . 088 
 
 .092 
 
 .095 
 
 .098 
 
 - 5 
 
 .052 
 
 .055 
 
 .058 
 
 .061 
 
 .064 
 
 .067 
 
 .070 
 
 .074 
 
 .077 
 
 .080 
 
 .083 
 
 .086 
 
 .089 
 
 .092 
 
 .095 
 
 - 4 
 
 .051 
 
 .054 .056 
 
 .059 
 
 .062 
 
 .065 .068 
 
 .071 
 
 .074 
 
 .077 
 
 .080 
 
 .083 
 
 .086 
 
 .089 
 
 .092 
 
 - 3 
 
 .049 
 
 .052 
 
 .055 
 
 .058 
 
 .061 
 
 .063 .066 
 
 .069 
 
 .072 
 
 .075 
 
 .078 
 
 .081 
 
 .084 
 
 .086 
 
 .089 
 
 - 2 
 
 .047 
 
 .050 
 
 .053 
 
 .056 
 
 .059 
 
 .061 .064 
 
 .067 
 
 .070 
 
 .073 
 
 .075 
 
 .078 
 
 .081 
 
 .084 
 
 .087 
 
 - 1 
 
 .046 
 
 . 049 . 051 
 
 .054 .057 
 
 .059 1 .062 
 
 .065 
 
 .067 
 
 .070 
 
 .073 
 
 .076 
 
 .078 
 
 .081 
 
 .084 
 
 
 
 .044 
 
 .047 
 
 .050 
 
 .052 .055 
 
 .057 .060 
 
 .063 
 
 .065 
 
 .068 
 
 .070 
 
 .073 
 
 .076 
 
 .078 
 
 .081 
 
 1 
 
 .043 
 
 .045 
 
 .048 
 
 .050 
 
 .053 
 
 .055 
 
 .058 
 
 .060 
 
 .063 
 
 .065 
 
 .068 
 
 .070 
 
 .073 
 
 .076 
 
 .078 
 
 2 
 
 .041 
 
 .044 
 
 .046 
 
 .049 
 
 .051 
 
 .053 
 
 .056 
 
 .058 
 
 .061 
 
 .063 
 
 .065 
 
 .068 
 
 .070 
 
 .073 
 
 .075 
 
 3 
 
 .040 
 
 .042 
 
 .044 
 
 .047 
 
 .049 
 
 .051 
 
 .054 
 
 .056 
 
 .058 
 
 .061 
 
 .063 
 
 .065 
 
 .068 
 
 .070 
 
 .072 
 
 4 
 
 .038 
 
 .040 
 
 .043 
 
 .045 
 
 .047 
 
 .049 
 
 .052 
 
 .054 
 
 .056 
 
 .058 
 
 .061 
 
 .063 
 
 .065 
 
 .067 
 
 .070 
 
 5 
 
 .037 
 
 .039 
 
 .041 
 
 .043 
 
 .045 
 
 .047 
 
 .049 
 
 .052 
 
 .054 
 
 .056 
 
 .058 
 
 .060 
 
 .062 
 
 .065 
 
 .067 
 
 6 
 
 .035 
 
 .037 
 
 .039 
 
 .041 
 
 .043 
 
 .045 
 
 .047 
 
 .049 
 
 .052 
 
 .054 
 
 .056 
 
 .058 
 
 .060 
 
 .062 
 
 .064 
 
 7 
 
 .033 
 
 .035 
 
 .037 
 
 .039 
 
 .041 
 
 .043 
 
 .045 
 
 .047 
 
 .049 
 
 .051 
 
 .053 
 
 .055 
 
 .057 
 
 .059 
 
 .061 
 
 8 
 
 .032 
 
 .034 
 
 .036 
 
 .038 
 
 .039 
 
 .041 
 
 .043 
 
 .045 
 
 .047 ! .049 
 
 .051 
 
 .053 
 
 .054 .056 
 
 .058 
 
 9 
 
 .030 
 
 .032 
 
 .034 
 
 .036 
 
 .038 
 
 .039 
 
 .041 
 
 .043 
 
 .045 . 046 
 
 .048 
 
 .050 
 
 .052 .054 
 
 .055 
 
 10 
 
 .029 
 
 .031 
 
 .032 
 
 .034 
 
 .036 
 
 .037 
 
 .039 j .041 
 
 .042 .044 
 
 .046 
 
 .047 
 
 .049 .051 
 
 .053 
 
 11 
 
 .027 
 
 .029 
 
 .030 
 
 .032 
 
 .034 
 
 .035 
 
 .037 
 
 .039 
 
 .040 .042 
 
 .043 
 
 .045 
 
 .047 .048 
 
 .050 
 
 12 
 
 .026 
 
 .027 
 
 .029 
 
 .030 
 
 .032 
 
 .033 
 
 .035 
 
 .036 
 
 .038 
 
 .039 
 
 .041 
 
 .042 
 
 .044 .045 
 
 .047 
 
 13 
 
 .024 
 
 .026 
 
 .027 
 
 .028 
 
 .030 
 
 .031 
 
 . 033 . 034 
 
 '.036 
 
 .037 
 
 .038 
 
 .040 
 
 .041 .043 
 
 .044 
 
 14 
 
 .023 
 
 .024 
 
 .025 
 
 .027 
 
 .028 
 
 .029 
 
 .031 1 .032 
 
 .033 
 
 .035 
 
 .036 
 
 .037 
 
 .039 .040 
 
 .041 
 
 15 
 
 .021 
 
 .022 
 
 .024 
 
 .025 
 
 .026 
 
 .027 
 
 .029 
 
 .030 
 
 .031 
 
 .032 
 
 .033 
 
 .035 
 
 .036 .037 
 
 .038 
 
 16 
 
 .020 
 
 .021 
 
 .022 
 
 .023 
 
 .024 
 
 .025 
 
 .026 
 
 .028 
 
 .029 .030 
 
 .031 
 
 .032 
 
 .033 .034 
 
 .036 
 
 17 
 
 .018 
 
 .019 
 
 .020 
 
 .021 
 
 .022 
 
 .023 
 
 .024 1 .025 
 
 . 026 . 027 
 
 .029 
 
 .030 
 
 . 031 . 032 
 
 .033 
 
 18 
 
 .016 
 
 .017 
 
 .018 
 
 .019 
 
 .020 
 
 .021 
 
 .022 ! .023 
 
 .024 
 
 .025 
 
 .026 
 
 .027 
 
 . 028 . 029 
 
 .030 
 
 19 
 
 .015 
 
 .016 
 
 .017 
 
 .018 
 
 .018 
 
 .019 
 
 .020 
 
 .021 
 
 .022 
 
 .023 
 
 .024 
 
 .025 
 
 .025 .026 
 
 .027 
 
 20 
 
 .013 
 
 .014 
 
 .015 
 
 .016 
 
 .016 
 
 .017 
 
 .018 
 
 i .019 
 
 . 020 . 020 
 
 .021 
 
 .022 
 
 . 023 . 024 
 
 .024 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 21 
 
 .012 
 
 .012 
 
 .013 
 
 .014 
 
 .015 
 
 .015 
 
 .016 
 
 .017 
 
 .017 
 
 .018 
 
 .019 
 
 .019 
 
 .020 .021 
 
 .022 
 
 22 
 
 .010 
 
 .011 
 
 .011 
 
 .012 
 
 .013 
 
 .013 
 
 .014 
 
 .014 
 
 .015 
 
 .016 
 
 .016 
 
 .017 
 
 .017 .018 
 
 .019 
 
 23 
 
 .009 
 
 .009 
 
 .010 
 
 .010 
 
 .011 
 
 .011 
 
 .012 
 
 .012 
 
 .013 .013 
 
 .014 
 
 .014 
 
 .015 .015 
 
 .016 
 
 24 
 
 .007 
 
 .008 
 
 .008 
 
 .008 
 
 .009 
 
 .009 
 
 .010 
 
 .010 
 
 .011 ! .011 
 
 .011 
 
 ..012 
 
 .012 .013 
 
 .013 
 
 25 
 
 .006 
 
 .006 
 
 .006 
 
 .007 
 
 .007 
 
 .007 
 
 .008 
 
 .008 
 
 .008 .009 
 
 .009 
 
 .009 
 
 . 010 | . 010 
 
 .010 
 
 26 
 
 .004 
 
 .004 
 
 .005 
 
 .005 
 
 .005 
 
 .005 
 
 .005 
 
 .006 
 
 .006 .006 
 
 .006 
 
 .007 
 
 .007 ! .007 
 
 .007 
 
 27 
 
 .003 
 
 .003 
 
 .003 
 
 .003 
 
 .003 
 
 .003 
 
 .003 
 
 ! .004 
 
 .004 .004 
 
 .004 
 
 .004 
 
 .004 .004 
 
 .005 
 
 28 
 
 .001 
 
 .001 
 
 .001 
 
 .001 
 
 .001 
 
 .001 
 
 .001 
 
 .001 
 
 .001 
 
 .001 
 
 .002 
 
 .002 
 
 .002 .002 
 
 .002 
 
 
 
 
 
 
 
 
 
 j 
 
 
 
 
 
 1 
 
 
 
 1 
 
 
 SUBTRACT. 
 
 29 
 
 0.001 0.001 
 
 0.001 0.001 0.001 iO.OOl 0.001 
 
 0.001 
 
 0.001 0.001 0.001 0.001 
 
 0.001 
 
 0.001 
 
 0.001 
 
 30 
 
 . 002 . 002 
 
 .002 i .002 .003 
 
 .003 .003 
 
 .003 
 
 .003 , .003 .003 .003 
 
 .004 .004 
 
 .004 
 
 31 
 
 . 004 . 004 
 
 .004 .004 .005 
 
 .005 ' .005 
 
 .005 
 
 .Oa5 .006 i .006 .006 
 
 .006 1 .006 
 
 .007 
 
 32 
 
 .005 .006 
 
 .006 ! .006 .006 
 
 .007 ! .007 
 
 .007 
 
 .008 .008 .008 .009 
 
 .009 .009 .009 
 
 33 
 
 .007 .007 
 
 .008 ; .008 .008 
 
 .009 i .009 
 
 .010 
 
 .010 .010 .011 .011 
 
 .012 .012 
 
 .012 
 
 34 
 
 .008 .009 
 
 .009 .010 .010 
 
 .011 .011 
 
 .012 
 
 .012 
 
 .013 .013 .014 
 
 .014 
 
 .015 
 
 .015 
 
 35 
 
 . 010 . 010 
 
 .011 j .012 .012 
 
 .013 .013 
 
 .014 
 
 .014 
 
 .015 .016 .016 
 
 .017 
 
 .017 
 
 .018 
 
 
 
 
 
 
 
 
 
 
 
 36 
 
 .011 .012 
 
 .013 .013 .014 
 
 . 015 . 015 
 
 .016 
 
 .017 
 
 .017 .018 .019 
 
 .019 .020 .021 
 
 37 
 
 .013 .014 
 
 .014 .015 .016 
 
 .017 .017 
 
 .018 
 
 .019 
 
 . 020 . 021 . 021 
 
 . 022 . 023 . 024 
 
 38 
 
 .014 .015 
 
 .016 : .017 .018 
 
 . 019 . 020 
 
 .020 
 
 .021 
 
 . 022 . 023 . 024 
 
 .025 .026 .026 
 
 39 
 
 .016 .017 
 
 .018 .019 .020 
 
 .021 ; .022 
 
 .023 
 
 .024 
 
 .024 ! .025 .026 
 
 . 027 . 028 . 029 
 
 40 
 
 .018 .019 
 
 .020 .021 ! .022 
 
 . 023 . 024 
 
 .025 
 
 .026 
 
 .027 j .028 .029 
 
 . 030 . 031 . 032 
 
 
 | 
 
 i 
 
 
 
 
 
BAROMKTKRS .\M .M KAST H K M KNT oi ATMOSPHERIC PRESSURE. 93 
 TABLE I. Correction of mercurial barometer for temperature, English measures Con. 
 
 F. 
 
 Observed n-ii.Jinjr of the barometer, in in. 
 
 17 17.5 18 
 
 lx :. 
 
 19 
 
 19.5 20 
 
 20.5 21 
 
 .'I.:, 
 
 22 
 
 22.5 
 
 23 
 
 23.5 
 
 24 
 
 Inches. 
 
 SUBtBACT. 
 
 * IncKtt. 
 
 35 
 35.5 
 
 0.010 o.nio 
 .011 .oil 
 
 t.010 0.011 0.011 
 .011 I .012 .012 
 
 0.011 
 .012 
 
 1.012 
 
 .012 
 
 0.012 
 
 .ou 
 
 0.012 
 .013 
 
 0.012 
 .013 
 
 0.013 
 .014 
 
 0.013 
 .014 
 
 0.013 0.014 
 .014 .015 
 
 0.014 
 015 
 
 36 
 36.5 
 
 .011 .OIL' 
 
 .012 .012 
 
 .013 
 
 .013 
 
 .013 
 
 .013 
 
 .013 
 .014 
 
 .013 
 .014 
 
 .011 
 
 .014 
 
 .014 
 .015 
 
 .014 
 .015 
 
 .014 
 .015 
 
 .015 
 .016 
 
 .015 
 .016 
 
 .015 .016 
 . olt, .017 
 
 .016 
 017 
 
 :<7 
 
 .013 .013 
 
 .014 
 
 .014 
 
 .014 
 
 .015 
 
 .015 
 
 .016 
 
 .016 
 
 .016 
 
 .017 
 
 .017 
 
 .017 .018 
 
 .018 
 
 37.5 
 
 .014 .014 
 
 .014 
 
 .015 
 
 .015 
 
 .016 
 
 .016 
 
 .017 
 
 .017 
 
 .017 
 
 .018 
 
 .018 
 
 . 019 . 019 
 
 .019 
 
 38 
 
 . 014 . 015 
 
 .015 
 
 .016 
 
 .016 
 
 .017 
 
 .017 
 
 .017 
 
 .018 
 
 .016 
 
 .019 
 
 .019 
 
 .020 .020 
 
 .020 
 
 38.5 
 
 .015 .016 
 
 .016 
 
 .016 
 
 .017 
 
 .017 
 
 .018 
 
 .018 
 
 .019 
 
 .019 
 
 .020 
 
 .020 
 
 .021 .021 
 
 .021 
 
 39 .016 .OH, 
 
 .017 
 
 .017 
 
 .018 
 
 .018 
 
 .019 
 
 .019 
 
 .020 
 
 .020 
 
 .021 
 
 .021 
 
 .022 .022 
 
 .023 
 
 .<'.. :, . 017 
 
 .017 
 
 .018 
 
 .018 
 
 .019 
 
 .019 
 
 .080 
 
 .020 
 
 .021 
 
 .021 
 
 .022 
 
 .022 
 
 .023 .023 
 
 .024 
 
 40 1 .018 .(Ms 
 
 .019 .019 
 
 .020 
 
 .020 
 
 .021 
 
 .021 
 
 .022 
 
 .022 
 
 .023 
 
 .023 
 
 .024 
 
 .024 
 
 .025 
 
 40.5 .01* .01!) 
 
 .019 .020 
 
 .020 
 
 .021 
 
 .022 
 
 .022 
 
 .023 
 
 .023 
 
 .024 
 
 .024 
 
 .025 
 
 .026 
 
 41 / .019 .(tin 
 
 .020 
 
 .021 
 
 .021 
 
 .022 
 
 .022 
 
 .023 
 
 .024 
 
 .024 
 
 .006 
 
 .025 
 
 .trji, .IL',, 
 
 .027 
 
 4175 
 
 .020 .020 
 
 .021 
 
 .022 
 
 .022 
 
 .023 
 
 .023 
 
 .024 
 
 .025 
 
 .025 
 
 .026 
 
 .026 
 
 .027 
 
 .027 
 
 UN 
 
 42 
 
 .021 .021 
 
 .022 
 
 .022 
 
 .023 
 
 .024 
 
 .024 
 
 .025 
 
 .025 
 
 .026 
 
 .027 
 
 .027 
 
 .028 .029 
 
 .029 
 
 42.5 .021 .022 
 
 .023 
 
 .023 
 
 .024 .025 
 
 .025 
 
 .026 
 
 .026 
 
 .027 
 
 .028 .028 
 
 .029 
 
 .030 
 
 .030 
 
 43 .022 
 
 .023 
 
 .023 
 
 .024 
 
 .025 .025 
 
 .026 
 
 .027 
 
 .027 
 
 .028 
 
 .029 I .029 
 
 .030 
 
 .031 
 
 .031 
 
 43. o . 023 
 
 .024 
 
 .024 
 
 .025 
 
 .02ii .026 
 
 .027 
 
 .028 
 
 .028 
 
 .029 
 
 .030 .030 
 
 .031 
 
 .032 
 
 .032 
 
 44 .024 
 
 .024 
 
 .025 
 
 .026 
 
 .026 .027 
 
 .028 
 
 .029 
 
 .029 
 
 .030 
 
 .031 
 
 .031 
 
 .032 
 
 .033 
 
 .033 
 
 44. 5 . 024 
 
 .025 
 
 .026 
 
 .027 
 
 .027 .028 
 
 .088 
 
 .030 
 
 .030 
 
 .031 
 
 .032 
 
 .032 
 
 .033 
 
 .034 
 
 .035 
 
 45 .025 
 
 .026 
 
 .027 
 
 .027 
 
 .028 ! .029 
 
 .030 
 
 .030 
 
 .031 
 
 .032 
 
 .033 
 
 .033 
 
 .034 
 
 .035 
 
 .036 
 
 45.5 .026 
 
 .027 
 
 .028 
 
 .028 
 
 .029 .030 
 
 .031 
 
 .031 
 
 .032 
 
 .033 
 
 .034 
 
 .034 
 
 .035 
 
 .036 .037 
 
 4J .027 
 
 .028 
 
 .028 
 
 .029 
 
 .030 
 
 .031 
 
 .031 
 
 .032 
 
 .033 
 
 .034 
 
 .035 
 
 .035 
 
 .036 
 
 .037 .038 
 
 4.,.:, 
 
 .028 
 
 .028 
 
 .029 
 
 .030 
 
 .031 
 
 .032 
 
 .032 
 
 .033 
 
 .034 
 
 .035 
 
 .036 
 
 .036 
 
 .037 
 
 .ix 
 
 .039 
 
 47 
 
 .028 
 
 .029 
 
 .030 
 
 .031 
 
 .032 
 
 .032 
 
 .033 
 
 .034 
 
 .035 
 
 .036 
 
 .037 
 
 .037 
 
 .038 
 
 .039 
 
 .040 
 
 - 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 47.5 .029 
 
 .030 
 
 .031 
 
 .032 
 
 .(Ktt .033 
 
 .034 
 
 .035 
 
 .036 
 
 .037 
 
 .038 
 
 .038 
 
 .039 
 
 .040 
 
 .041 
 
 .030 
 
 .031 
 
 .032 
 
 .032 
 
 .033 .034 
 
 .035 
 
 .036 
 
 .037 
 
 .038 
 
 .039 
 
 .040 
 
 .040 
 
 .041 
 
 .042 
 
 48. 5 .031 
 
 .032 
 
 .032 
 
 .033 
 
 .(4 .035 
 
 .036 
 
 .037 
 
 .038 
 
 .039 
 
 .040 
 
 .041 
 
 .041 
 
 .042 
 
 .043 
 
 49 .031 
 
 .032 
 
 .033 
 
 .034 
 
 . 035 . 036 
 
 .037 
 
 .038 
 
 .039 
 
 .040 
 
 .041 
 
 .042 
 
 .042 
 
 .043 
 
 .044 
 
 49.5 .032 
 
 .033 
 
 .034 
 
 .035 
 
 .i6 .037 
 
 .038 
 
 .039 
 
 .040 
 
 .041 
 
 .042 
 
 .043 
 
 .044 
 
 .044 
 
 .045 
 
 .033 
 
 .034 
 
 .035 
 
 .036 
 
 .037 
 
 .038 
 
 .039 
 
 .040 
 
 .041 
 
 .042 
 
 .643 
 
 .044 
 
 .045 
 
 .046 
 
 .046 
 
 50. 5 .034 
 
 .035 
 
 .036 
 
 .037 
 
 .038 
 
 .039 
 
 .040 
 
 .041 
 
 .042 
 
 .043 
 
 .044 
 
 .045 
 
 .046 
 
 .047 
 
 .048 
 
 51 .034 
 
 .035 
 
 .036 
 
 .038 
 
 .039 
 
 .040 
 
 .041 
 
 .042 
 
 .043 
 
 .044 
 
 .045 
 
 .046 
 
 .047 
 
 .048 
 
 .049 
 
 51.5 .035 
 
 .036 
 
 .037 
 
 .038 
 
 .039 
 
 .040 
 
 .041 
 
 .042 
 
 .044 
 
 .045 
 
 .046 
 
 .047 
 
 .048 
 
 .049 
 
 .050 
 
 52 .036 
 
 .037 
 
 .038 
 
 .039 
 
 .040 
 
 .041 
 
 .042 
 
 .043 
 
 .044 
 
 .046 
 
 .047 
 
 .048 
 
 .049 
 
 .050 
 
 .051 
 
 52.5 .037 
 
 .038 
 
 .039 
 
 .040 
 
 .041 .042 
 
 .043 
 
 .044 
 
 .045 
 
 .047 
 
 .04* 
 
 .049 
 
 .050 
 
 .051 
 
 .052 
 
 53- .038 
 
 .039 
 
 .040 
 
 .041 
 
 .042 .043 
 
 .044 
 
 .045 
 
 .046 
 
 .047 
 
 .049 
 
 .050 
 
 .051 
 
 .052 
 
 .053 
 
 53.5 .038 
 
 .039 
 
 .041 
 
 .042 
 
 .043 .044 
 
 .045 
 
 .046 
 
 .047 
 
 .048 
 
 .050 
 
 .051 
 
 .052 
 
 .053 
 
 .054 
 
 54 .039 
 
 .040 
 
 .041 
 
 .043 
 
 .044 .045 
 
 .046 
 
 .047 
 
 .048 
 
 .049 
 
 .051 
 
 .052 
 
 .053 
 
 .054 
 
 .055 
 
 54.5 .040 
 
 .041 
 
 .042 
 
 .043 
 
 .045 .046 
 
 .047 
 
 .048 
 
 .049 
 
 .050 
 
 .052 .053 
 
 .054 
 
 .055 
 
 .056 
 
 .55 .041 
 
 .042 
 
 .043 
 
 .044 
 
 .045 .047 
 
 .048 
 
 .049 
 
 .050 
 
 .051 
 
 .053 
 
 .054 
 
 .055 
 
 .056 
 
 .057 
 
 55.5 .041 
 
 .043 
 
 .044 
 
 .045 
 
 .046 i .047 
 
 .049 
 
 .050 
 
 .051 
 
 .052 
 
 .054 
 
 .055 
 
 .056 
 
 .057 
 
 ,068 
 
 56 
 
 .042 
 
 .043 
 
 .045 
 
 .046 
 
 .047 .048 
 
 .050 
 
 .051 
 
 .052 
 
 .053 
 
 .055 
 
 .056 
 
 .057 
 
 .058 
 
 .060 
 
 56.5 
 
 .043 
 
 .044 
 
 .045 
 
 .047 
 
 .048 .049 
 
 .050 
 
 .052 
 
 .053 
 
 .054 
 
 .056 
 
 .057 
 
 .058 
 
 .059 
 
 .081 
 
 57 
 
 .044 
 
 .045 
 
 .046 
 
 .048 
 
 .049 .050 
 
 .051 
 
 .053 
 
 .054 
 
 .055 
 
 .057 
 
 .058 
 
 .059 
 
 ,008 
 
 .062 
 
 57.5 
 
 .044 
 
 .046 
 
 .047 
 
 .048 
 
 .050 .051 
 
 .052 
 
 .054 
 
 .055 
 
 .056 
 
 .058 
 
 .059 
 
 .OJOI 
 
 .061 
 
 .063 
 
 58 
 
 .045 
 
 .047 
 
 .048 
 
 .049 
 
 .051 
 
 .052 
 
 .053 
 
 .055 
 
 .056 
 
 .057 
 
 .aw 
 
 .060 
 
 .061 
 
 .(m .064 
 
 
 .046 
 
 .048 
 
 .049 
 
 .050 
 
 .051 .053 
 
 .054 
 
 .055 
 
 .057 
 
 .058 
 
 .060 
 
 .061 
 
 .062 
 
 .(M .065 
 
 59 
 
 .047 
 
 .048 
 
 .050 
 
 .051 
 
 .052 .054 
 
 .055 
 
 .056 
 
 .058 
 
 .059 
 
 .061 
 
 .062 
 
 .063 
 
 .(H,:, 
 
 ,606 
 
 59.5 
 
 .048 
 
 .049 
 
 .050 
 
 .052 
 
 .053 .055 
 
 .056 
 
 .057 
 
 .059 
 
 .060 
 
 .001 
 
 .083 
 
 .084 
 
 .60 
 
 .087 
 
 60 
 
 .048, 
 
 .050 .1151 
 
 .053 
 
 .054 .055 
 
 .057 
 
 .058 
 
 .060 
 
 .061 
 
 .062 
 
 .064 
 
 .065 
 
 .087 
 
 .068 
 
 60.5 
 
 ..14'. 
 
 .050 ! .052 
 
 .053 
 
 .055 .056 
 
 .058 
 
 .059 
 
 .061 
 
 .062 
 
 .063 
 
 .065 
 
 .066 
 
 .068 
 
 ,008 
 
 til 
 
 .050 
 
 .051 .053 
 
 .054 
 
 .056 .057 
 
 .059 
 
 .060 
 
 .062 
 
 .063 
 
 .OM 
 
 .066 
 
 .067 
 
 ,000 
 
 .070 
 
 ',!..-, 
 
 .051 
 
 .052 
 
 .054 
 
 .055 
 
 .057 ; .058 
 
 .060 
 
 .061 
 
 .062 
 
 .004 
 
 .065 
 
 .067 
 
 .068 
 
 .070 
 
 .071 
 
 62 
 
 .051 
 
 .053 
 
 .054 
 
 .056 
 
 .057 .059 
 
 .060 
 
 .008 
 
 .063 
 
 .065 
 
 .066 
 
 .068 
 
 .069 
 
 .071 
 
 .073 
 
 62.5 
 
 .052 
 
 .054 
 
 .055 
 
 .057 
 
 .058 
 
 .060 
 
 .061 
 
 .063 
 
 .064 
 
 .066 
 
 .067 
 
 .069 
 
 .071 
 
 .072 
 
 .074 
 
 63 
 
 .053 
 
 .OM 
 
 . o5; 
 
 .058 
 
 .059 
 
 .061 
 
 .062 
 
 .064 
 
 .065 
 
 .067 
 
 ,068 
 
 .070 
 
 .072 
 
 .073 
 
 .075 
 
 63.5 
 
 .054 
 
 .055 
 
 .057 
 
 .058 
 
 .060 
 
 .062 
 
 .063 
 
 .065 
 
 .066 
 
 .068 
 
 ,068 
 
 .071 
 
 .073 
 
 .074 
 
 .076 
 
 t.4 
 
 .054 
 
 .056 
 
 .on 
 
 .059 
 
 .061 
 
 .062 
 
 .064 
 
 .006 
 
 .067 
 
 .069 
 
 .070 
 
 .072 
 
 .074 
 
 .075 
 
 .077 
 
 64.5 
 
 .055 
 
 .057 
 
 .068 
 
 .060 
 
 .062 
 
 .063 
 
 .065 
 
 .067 
 
 .068 
 
 .070 
 
 .071 
 
 .073 
 
 .075 
 
 .076 
 
 .078 
 
 10 
 
 .056 
 
 .058 
 
 .059 
 
 .061 
 
 .063 
 
 .064 
 
 .066 
 
 .067 
 
 .069 
 
 .071 
 
 .072 
 
 .074 
 
 .076 
 
 .077 
 
 .079 
 
 1)5.5 
 
 .057 
 
 .058 
 
 .060 
 
 .062 
 
 .063 
 
 .065 
 
 .067 
 
 .068 
 
 .070 
 
 .072 
 
 .073 
 
 .075 
 
 .077 
 
 .078 
 
 .060 
 
 66 
 
 .057 
 
 .059 
 
 .061 
 
 .063 
 
 .064 
 
 .008 
 
 .068 
 
 .068 
 
 .071 
 
 .073 
 
 .074 
 
 .076 
 
 ,ora 
 
 .079 
 
 .081 
 
 66.5 
 
 .058 
 
 .060 
 
 ,008 
 
 .001 
 
 .065 .067 
 
 .069 
 
 .070 
 
 .072 
 
 .074 
 
 .075 
 
 .077 
 
 .079 
 
 .081 
 
 .082 
 
 1,7 
 
 .059 
 
 ..061 
 
 .062 
 
 .064 
 
 .066 .068 
 
 .069 
 
 .071 
 
 .073 
 
 .075 
 
 .076 
 
 .078 
 
 ,080 
 
 ,088 
 
 .MX;.; 
 
94 BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. ' 
 TABLE I.- Correction of mercurial barometer for temperature, English measures Con. 
 
 
 Observed reading of the barometer, in inches. 
 
 F. 
 
 24 
 
 24. 5 I 25 25.5 | 26 
 
 26.5 
 
 27 
 
 27.5 
 
 28 
 
 28.5 
 
 29 
 
 29. 5 30 
 
 30.5 
 
 31 
 
 
 Inches. . SUBTRACT. Inches. 
 
 35 
 
 0.014 
 
 0.014 
 
 0.014 
 
 0.015 0.015 0.015 
 
 0.016 
 
 0.016 
 
 0.016 
 
 0.016 
 
 0.017 
 
 0.017 
 
 0.017 0.018 0.018 
 
 35.5 
 
 .015 
 
 .015 
 
 .016 
 
 .016 .016 .017 
 
 .017 
 
 .017 
 
 .017 
 
 .018 
 
 .018 
 
 .018 
 
 .019 i 
 
 .019 .019 
 
 36 
 
 .016 
 
 .016 
 
 .017 
 
 .017 .017 .018 
 
 .018 
 
 .018 
 
 .019 
 
 .019 
 
 .019 
 
 .020 
 
 .020 
 
 . 020 . 021 
 
 36.5 
 
 .017 
 
 .017 
 
 .018 
 
 .018 ' .019 ; .019 
 
 .019 
 
 .020 
 
 .020 
 
 .020 
 
 .021 
 
 .021 
 
 .021 i 
 
 . 022 . 022 
 
 37 
 
 .018 
 
 .019 
 
 .019 
 
 .019 ; .020 
 
 .020 
 
 .021 
 
 .021 
 
 .021 
 
 .022 
 
 .022 
 
 .022 
 
 .023 
 
 . 023 . 024 . 
 
 37.5 
 
 .019 
 
 .020 
 
 .020 
 
 .021 ; .021 
 
 .021 
 
 .022 
 
 .022 
 
 .023 
 
 .023 
 
 .023 
 
 .024 
 
 .024 
 
 .025 .025 
 
 38 
 
 .020 
 
 .021 
 
 .021 
 
 .022 i .022 
 
 .023 
 
 .023 
 
 .023 
 
 .024 
 
 .024 
 
 .025 
 
 .025 
 
 .026 
 
 .026 : .026 
 
 38.5 
 
 .021 
 
 .022 
 
 .022 
 
 .023 ; .023 i .024 
 
 .024 
 
 .025 
 
 .025 
 
 .026 
 
 .026 
 
 .026 
 
 .027 
 
 .027 1 .028 
 
 39 
 
 .023 
 
 .023 
 
 .024 
 
 . 024 J . 024 | . 025 
 
 .025 
 
 .026 
 
 .026 
 
 .027 
 
 .027 
 
 .028 
 
 .028 
 
 .029 .029 
 
 -39.5 
 
 .024 
 
 .024 
 
 .025 
 
 .025 i .026 
 
 .026 
 
 .027 
 
 .027 
 
 .028 
 
 .028 
 
 .029 
 
 .029 
 
 .030 
 
 .030 .031 
 
 40 
 
 , .025 
 
 .025 
 
 .026 
 
 .026 .027 
 
 .027 
 
 .028 
 
 .028 
 
 .029 
 
 .029 
 
 .030 
 
 .030 
 
 .031 
 
 .031 .032 
 
 40.5 
 
 .026 
 
 .026 
 
 .027 
 
 .027 .028 .029 
 
 .029 
 
 .030 
 
 .030 
 
 .031 
 
 .031 
 
 .032 
 
 .032 
 
 .033 .033 
 
 41 
 
 .027 
 
 .027 
 
 .028 
 
 .029 .029 .030 
 
 .030 
 
 .031 
 
 .031 
 
 .032 
 
 .033 
 
 .033 
 
 .034 
 
 .034 .035 
 
 41.5 
 
 .028 
 
 .029 
 
 .029 
 
 .030 .030 ! .031 
 
 .032 
 
 .032 
 
 .033 
 
 .033 
 
 .034 
 
 .034- 
 
 .035 
 
 . 036 . 036 
 
 42 
 
 .029 
 
 .030 
 
 .030 
 
 .031 .032 
 
 .032 
 
 .033 
 
 .033 
 
 .034 
 
 .035 
 
 .035 
 
 .036 
 
 .036 
 
 .037, .038 
 
 42.5 
 
 .030 
 
 .031 
 
 .031 
 
 .032 
 
 . 033 . 033 
 
 .034 
 
 .035 
 
 .035 
 
 .036 
 
 .036 
 
 .037 
 
 .038 
 
 .038 
 
 .039 
 
 43 
 
 .031 
 
 .032 
 
 .033 
 
 .033 
 
 .034 
 
 .035 
 
 .035 
 
 .036 
 
 .0?6 
 
 .037 
 
 .038 
 
 .038 
 
 .039 
 
 .040 
 
 .040 
 
 43.5 
 
 .032 
 
 .033 
 
 .034 
 
 .034 i .035 
 
 .036 
 
 .036 
 
 .037 
 
 .038 
 
 .038 
 
 .039 
 
 .040 
 
 .040 
 
 .041 
 
 .042 
 
 44 
 
 .033 
 
 .034 
 
 .035 
 
 .036 .036 
 
 .037 
 
 .038 
 
 .038 
 
 .039 
 
 .040 
 
 .040 
 
 .041 
 
 .042 
 
 .042. 
 
 .043 
 
 44.5 
 
 .035 
 
 .035 
 
 .036 
 
 .037 .037 
 
 .038 
 
 .039 
 
 .040 
 
 .040 
 
 .041 
 
 .042 
 
 .042 
 
 .043 
 
 .044 
 
 .045 
 
 45 
 
 .036 
 
 .036 
 
 .037 
 
 .038 
 
 .039 
 
 .039 
 
 .040 
 
 .041 
 
 .042 
 
 .042 
 
 .043 
 
 .044 
 
 .045 
 
 .045 
 
 .046 
 
 45.5 
 
 .037 
 
 .037 
 
 .038 
 
 .039 
 
 .040 
 
 .041 
 
 .041 
 
 .042 
 
 .043 
 
 .044 
 
 .044 
 
 .045 
 
 .046 
 
 .047 
 
 .047 
 
 46 
 
 .038 
 
 .039 
 
 .039 
 
 .040 
 
 .041 
 
 .042 
 
 .043 
 
 .043 
 
 .044 
 
 .045 
 
 .046 
 
 .046 
 
 .047 
 
 .048 
 
 .049 
 
 46.5 
 
 .039 
 
 .040 
 
 .041 
 
 .041 
 
 .042 
 
 .043 
 
 .044 
 
 .045 
 
 .045 
 
 .046 
 
 .047 
 
 .048 
 
 .049 
 
 .049 
 
 .050 
 
 47 
 
 .040 
 
 .041 
 
 .042 
 
 .042 
 
 .043 
 
 .044 
 
 .045 
 
 .046 
 
 .047 
 
 .047 
 
 .048 
 
 .049 
 
 .050 
 
 .051 
 
 .052 
 
 47.5 
 
 .041 
 
 .042 
 
 .043 
 
 .044 
 
 .045 
 
 .045 
 
 .046 
 
 .047 
 
 .048 
 
 .049 
 
 .050 
 
 .050 
 
 .051 
 
 .052 
 
 .053 
 
 48 
 
 .042 
 
 .043 
 
 .044 
 
 .045 
 
 .046 
 
 .047 
 
 .047 
 
 .048 
 
 .049 
 
 .050 
 
 .051 
 
 .052 
 
 .053 
 
 .054 
 
 .054 
 
 48.5 
 
 .043 
 
 .044 
 
 .045 
 
 . 046 . 047 
 
 .048 
 
 .049 
 
 .050 
 
 .050 
 
 .051 
 
 .052 
 
 .053 
 
 .054 
 
 .055 
 
 .056 
 
 49 
 
 .044 
 
 .045 
 
 .046 
 
 .047 ! .048 
 
 .049 
 
 .050 
 
 .051 
 
 .052 
 
 .053 
 
 .054 
 
 .054 
 
 .055 
 
 .056 
 
 .057 
 
 49.5 
 
 .045 
 
 .046 
 
 .047 
 
 .048 
 
 .049 
 
 .050 
 
 .051 
 
 .052 
 
 .053 
 
 .054 
 
 .055 
 
 .056 
 
 .057 
 
 .058 
 
 .059 
 
 -50 
 
 .046 
 
 .047 
 
 .048 
 
 .049 .050 
 
 .051 
 
 .052 
 
 .053 
 
 .054 
 
 .055 
 
 .056 
 
 .057 
 
 058 
 
 .059 
 
 .060 
 
 50.5 
 
 .048 
 
 .049 
 
 .050 
 
 .051 .052 
 
 .053 
 
 .054 
 
 .055 
 
 .055 
 
 .056 
 
 .057 
 
 .058 
 
 ! 059 
 
 .060 
 
 .061' 
 
 51 
 
 .049 
 
 .050 
 
 .051 
 
 .052 .053 
 
 .054 
 
 .055 
 
 .056 
 
 .057 
 
 .058 
 
 .059 
 
 .060 
 
 .061 
 
 .062 
 
 .063 
 
 51.5 
 
 .050 
 
 .051 
 
 .052 
 
 .053 .054 .055 
 
 .056 
 
 .057 
 
 .058 
 
 .059 
 
 .060 
 
 .061 
 
 .062 
 
 .063 
 
 .064 
 
 52 
 
 .051 
 
 .052 
 
 .053 
 
 .054 .055 .056 
 
 .057 
 
 .058 
 
 .059 
 
 .060 
 
 .061 
 
 .062 
 
 .064 
 
 .065 
 
 .066 
 
 52.5 
 
 .052 
 
 .053 
 
 .054 
 
 .055 
 
 .056 
 
 .057 
 
 .058 
 
 .059 
 
 .061 
 
 .062 
 
 .063 
 
 .064 
 
 .065 
 
 .066 
 
 .067 
 
 53 
 
 .053 
 
 .054 
 
 .055 
 
 .056 
 
 .057 
 
 .059 
 
 .060 
 
 r66t- 
 
 -^962. 
 
 .063 
 
 .064 
 
 
 .066 
 
 .067 
 
 .068 
 
 53.5 
 
 .054 
 
 .055 
 
 .056 
 
 .057 .059 
 
 .060 
 
 .061 
 
 .062 
 
 .063 
 
 
 .065 
 
 *066 
 
 .068 
 
 .069 
 
 .070 
 
 54 
 
 .055 
 
 .056 
 
 .057 
 
 .059 
 
 .060 
 
 .061 
 
 .062 
 
 .063 
 
 .064 
 
 !066 
 
 .067 
 
 .068 
 
 .069 
 
 .070 
 
 .071 
 
 54.5 
 
 .056 
 
 .057 
 
 .059 
 
 .060 
 
 .061 
 
 .062 
 
 .063 
 
 .064 
 
 .066 
 
 .067 
 
 .068 
 
 .069 
 
 .070 
 
 .071 
 
 .073 
 
 55 
 
 .057 
 
 .059 
 
 .060 
 
 .061 
 
 .062 
 
 .063 
 
 .064 
 
 .066 
 
 .067 
 
 .068 
 
 .069 
 
 .070 
 
 .072 
 
 .073 
 
 .074 
 
 55.5 
 
 .058 
 
 .060 
 
 .061 
 
 .062 
 
 .063 .065 
 
 .066 
 
 .067 
 
 .068 
 
 .069 
 
 .071 
 
 .072 
 
 .073 
 
 .074 
 
 .075 
 
 56 
 
 .060 
 
 .061 
 
 .062 
 
 .063 
 
 .064 
 
 .066 
 
 .067 
 
 .068 
 
 .069 
 
 .071 
 
 .072 
 
 .073 
 
 .074 
 
 .076 
 
 .077 
 
 56.5 
 
 .061 
 
 .062 
 
 .063 
 
 .064 
 
 .066 
 
 .067 
 
 .068 
 
 .069 
 
 .071 
 
 .072 
 
 .073 
 
 .074 
 
 .076 
 
 .077 
 
 .078. 
 
 57 
 
 .062 
 
 .063 
 
 .064 
 
 .066 
 
 .067 
 
 .068 
 
 .069 
 
 .071 
 
 .072 
 
 .073 
 
 .075 
 
 '.076 
 
 .077 
 
 .078 
 
 .080 
 
 57.5 
 
 .063 
 
 .064 
 
 .065 
 
 .067 
 
 '.068 
 
 .069 
 
 .071 
 
 .072 
 
 .073 
 
 .075 
 
 .076 
 
 .077 
 
 .078 
 
 .080 
 
 .081 
 
 58 
 
 .064 
 
 .065 
 
 .066 
 
 .068 
 
 .069 
 
 .070 
 
 .072 
 
 .073 
 
 .074 
 
 .076 
 
 .077 
 
 
 .080 
 
 .081 
 
 .082 
 
 58.5 
 
 .065 
 
 .066 
 
 .068 
 
 .069 
 
 .070 
 
 .072 
 
 .073 
 
 .074 
 
 .076 
 
 .077 
 
 .078 
 
 !o80 
 
 .081 
 
 .083 
 
 .084 
 
 59 
 
 .066 
 
 .067 
 
 .069 
 
 .070 
 
 .072 
 
 .073 
 
 .074 
 
 .076 
 
 .077 
 
 .078 
 
 .080 
 
 . 081 
 
 .083 
 
 .084 
 
 .085 
 
 59.5 
 
 .067 
 
 .068 
 
 .070 
 
 .071 
 
 .073 
 
 .074 
 
 .075 
 
 .077 
 
 .078 
 
 .080 
 
 .081 
 
 .'08^ 
 
 .084 
 
 .085 
 
 .087 
 
 *> 60 
 
 .068 
 
 .070 
 
 .071 
 
 .072 
 
 .074 
 
 .075 
 
 .077 
 
 .078 
 
 .080 
 
 .081 
 
 .082 
 
 .084 
 
 .085 
 
 .087 
 
 .088 
 
 60.5 
 
 .069 
 
 .071 
 
 .072 
 
 .074 
 
 .075 
 
 .076 
 
 .078 
 
 .079 
 
 .081 .082 
 
 .084 
 
 .085 
 
 .087* 
 
 .088 
 
 .089 
 
 61 
 
 .070 
 
 .072 
 
 .073 
 
 .075 
 
 .076 .078 
 
 .079 
 
 .081 
 
 . 082 . 084 
 
 .085 
 
 .086 
 
 .088 
 
 .089 
 
 .091 
 
 61.5 
 
 .071 
 
 .073 
 
 .074 
 
 .076 
 
 . 077 . 079 
 
 .080 
 
 .082 
 
 .083 .085 
 
 .086 
 
 .088 
 
 .089 
 
 .091 
 
 .092 
 
 62 
 
 .073 
 
 .074 
 
 .076 
 
 .077 .079 j .080 
 
 .082 
 
 .083 
 
 .085 .086 
 
 .088 
 
 .089 
 
 .091 
 
 .092 
 
 .094 
 
 
 
 
 
 1 
 
 
 
 
 
 
 
 
 
 62.5 
 
 .074 
 
 .075 
 
 .077 
 
 .078 
 
 .080 .081 
 
 .083 
 
 .084 
 
 .086 
 
 .087 
 
 .089 
 
 .090 
 
 .092 
 
 .094 
 
 .095 
 
 63 
 
 .075 
 
 .076 
 
 .078 
 
 .079 
 
 . 081 . 082 
 
 .084 
 
 .086 
 
 .087 .089 
 
 .090 
 
 .092 
 
 .093 
 
 .095 
 
 .096 
 
 63.5 
 
 .076 
 
 .077 
 
 .079 
 
 .080 
 
 .082 . .084 
 
 .085 
 
 .087 
 
 .088 .090 
 
 .092 
 
 .093 
 
 .095 
 
 .096 
 
 .098 
 
 64 
 
 .077 
 
 .078 
 
 .080 
 
 .082 .083 .085 
 
 .086 
 
 .088 
 
 .090 
 
 .091 
 
 .093 
 
 .094 
 
 .096 
 
 .098 
 
 .099 
 
 64.5 
 
 .078 
 
 .080 
 
 .081 
 
 . 083 . 084 . 086 
 
 .088 
 
 i .089 
 
 .091 
 
 .093 
 
 .094 
 
 .096 
 
 .097 
 
 .099 
 
 .101 
 
 65 
 
 .079 
 
 .081 
 
 .082 
 
 .084. .086 .087 
 
 .089 
 
 .091 
 
 .092 .094 
 
 .095 
 
 JJU7 
 
 .099 
 
 .100 
 
 .102 
 
 65.5 
 
 080 
 
 .082 
 
 .083 
 
 .085 
 
 .087 ; .088 
 
 .090 
 
 .092 
 
 .093 
 
 .095 
 
 .097 
 
 
 .100 
 
 .102 
 
 .103 
 
 66 
 66.5 
 
 .081 
 .082 
 
 .083 
 .084 
 
 .085 
 .086 
 
 .086 .088 .090 
 . 087 . 089 . 091 
 
 .091 
 .093 
 
 .093 
 .094 
 
 .095 ; .096 
 .096 .098 
 
 .098 
 .099 
 
 .100 
 .101 
 
 .101 
 .103 
 
 .103 
 .105 
 
 .105 
 .100 
 
 67 
 
 .083 
 
 .085 
 
 .087 
 
 .089 
 
 .090 .092 
 
 .094 
 
 .095 
 
 . 097 . 099 
 
 .101 
 
 .102 
 
 .104 
 
 .106 
 
 .108 
 
 ; 
 
 1 
 
 
 
 
 
 
 , 
 
BAROMETERS AND MKAsr UK.MKXT OF ATMOSPHERIC PRESSURE. 95 
 TABLE I. Correction of mercurial barometer for temperature, English measures Con. 
 
 
 Observed reading of the barometer, in inches. 
 
 F. 
 
 17 
 
 17.5 
 
 18 
 
 18.5 
 
 19 
 
 19.5 
 
 20 20.5 
 
 21 
 
 21.5 
 
 22 
 
 22.5 
 
 
 ->.;. o 
 
 24 
 
 
 Inches. 
 
 SUBTRACT. Inches. 
 
 67 
 67.5 
 
 0.059 
 .060 
 
 0.061 
 .062 
 
 0.062 10.064 
 .063 .065 
 
 0.066 0.068 
 .067 .069 
 
 0.069 
 .070 
 
 .071 
 .072 
 
 0.073 '0.075 
 .074 .076 
 
 0.076 0.078 0.080 !o.082 
 .077 .079 .081 ' row 
 
 0.083 
 084 
 
 68 
 68.5 
 
 .061 
 .061 
 
 .062 
 .063 
 
 .064 
 .065 
 
 .066 
 .067 
 
 .068 
 .069 
 
 .069 
 .070 
 
 .071 
 .072 
 
 .073 
 .074 
 
 .075 .077 
 .076. .078 
 
 .078 .080 .082 
 .079 .081 .083 
 
 .084 
 .085 
 
 1085 
 .087 
 
 69 
 
 .062 
 
 .064 
 
 .066 
 
 .068 
 
 .069 
 
 .071 
 
 .073 
 
 .075 
 
 .077 .079 
 
 .080 .082 .084 
 
 .086 
 
 .088 
 
 69.5 
 
 .063 
 
 .065 
 
 .067 
 
 .068 
 
 .070 
 
 .-072 
 
 .074 
 
 .076 
 
 .078 .079 
 
 .081 .083 .085 
 
 .087 
 
 .089 
 
 70 
 
 .064 
 
 .065 
 
 .067 
 
 .069 
 
 .071 
 
 .073 
 
 .075 
 
 .077 
 
 .079 .080 .082 '< .084 !086 
 
 .088 
 
 .090 
 
 70.5 
 
 .064 
 
 .066 
 
 .068 
 
 .070 
 
 .072 
 
 .074 
 
 .076 
 
 .078 
 
 ..080 .081 ; .083 .085 .087 
 
 .089 
 
 .on 
 
 71 
 
 .065 
 
 .067 
 
 .069 
 
 .071 
 
 .073 
 
 .075 
 
 .077 
 
 .079 
 
 .080 .082 .084 .086 .088 
 
 .090 
 
 092 
 
 71.5 
 
 .066 
 
 .068 
 
 .070 
 
 .072 
 
 .074 
 
 .076 
 
 .078 
 
 .079 
 
 .081 .083 
 
 .085 .087 .089 
 
 .091 
 
 .093 
 
 72 
 
 .067 
 
 .069 
 
 .071 
 
 .073 
 
 .075 
 
 .076 
 
 .078 
 
 .080 
 
 .082 .084 
 
 .086 
 
 .088 .090 
 
 .092 
 
 .094 
 
 72.$ 
 
 .067 
 
 .069 
 
 .071 
 
 .073 
 
 .075 
 
 .077 
 
 .079 
 
 .081 
 
 .083 .085 
 
 .087 .089 
 
 .091 
 
 .093 
 
 .095 
 
 73 
 
 .068 
 
 .070 
 
 .072 
 
 .074 
 
 .076 
 
 .078 
 
 .080 
 
 .082 
 
 .084 .086 
 
 .088 .090 .092 
 
 .094 
 
 .096 
 
 73.5 
 
 .069 
 
 .071 
 
 .073 
 
 .075 .077 
 
 .079 
 
 .081 
 
 .083 
 
 .085 i .087 
 
 .089 .091 . OM 
 
 095 
 
 097 
 
 74 
 
 .070 
 
 .072 
 
 .074 
 
 .076 
 
 .078 
 
 .080 
 
 .082 
 
 .084 
 
 .086 .088 
 
 .090 .092 
 
 .094 
 
 .096 
 
 .098 
 
 74.5 
 
 .070 
 
 .073 
 
 .075 
 
 .077 
 
 .079 
 
 .081 
 
 .083 
 
 .085 
 
 .087 .089 
 
 .091 .093 
 
 .095 
 
 .097 
 
 .100 
 
 75 
 
 .071 
 
 .073 
 
 .075 
 
 .078 
 
 .080 
 
 .082 
 
 .084 
 
 .086 
 
 .088 .090 
 
 .092 .094 
 
 .096 
 
 .099 
 
 .101 
 
 75.5 
 
 .072 
 
 .074 
 
 .076 
 
 .078 
 
 .081 
 
 .083 
 
 .085 
 
 .087 
 
 .089 .091 
 
 .093 .095 
 
 .097 
 
 .100 
 
 .102 
 
 76 
 
 .073 
 
 .075 
 
 .077 
 
 .079 
 
 .081 
 
 .084 
 
 .086 
 
 .088 
 
 .090 .092 
 
 .094 .096 
 
 .098 
 
 .101 
 
 .103 
 
 76.5 
 
 .074 
 
 .076 
 
 .078 
 
 .080 
 
 .082 
 
 .084 
 
 .087 
 
 .089 
 
 .091 .093 
 
 .095 
 
 .097 
 
 .100 
 
 .102 
 
 .104 
 
 77 
 
 .074 
 
 .077 
 
 .079 
 
 .081 
 
 .083 
 
 .085 
 
 .087 
 
 .090 
 
 .092 
 
 .094 
 
 .096 
 
 .098 
 
 .101 
 
 .103 
 
 .105 
 
 77.5 
 
 .075 
 
 .077 
 
 .080 
 
 .082 
 
 .084 
 
 .086 
 
 .088 
 
 .091 
 
 .093 .095 ! .097 
 
 .099 
 
 .102 
 
 .104 
 
 .106 
 
 78 
 
 .076 
 
 .078 
 
 .080 
 
 .083 
 
 .085 
 
 .087 
 
 .089 
 
 .091 
 
 .094 .096 j .098 
 
 .100 
 
 .103 
 
 .105 
 
 .107 
 
 78.5 
 
 .077 
 
 .079 
 
 .081 
 
 .083 
 
 .086 
 
 .088 
 
 .090 
 
 .092 
 
 .095 .097 | .099 
 
 .101 
 
 .104 
 
 .106 
 
 .108 
 
 79 
 
 .077 
 
 .080 
 
 .082 
 
 .084 
 
 .086 
 
 .089 
 
 .091 
 
 .093 
 
 .096 .098 
 
 .100 .102 
 
 .105 
 
 ..107 
 
 .109 
 
 79.5 
 
 .078 
 
 .080 
 
 .083 
 
 .085 
 
 .087 
 
 .090 
 
 .092 
 
 .094 
 
 .097 .099 
 
 .101 
 
 .103 
 
 .106 
 
 .108 
 
 .110 
 
 80 
 
 .079 
 
 .081 
 
 .084 
 
 .086 
 
 .088 
 
 .091 
 
 .093 
 
 .095 
 
 .097 .100 
 
 .102 
 
 .104 
 
 .107 
 
 .109 
 
 .111 
 
 80.5, 
 
 .080 
 
 .082 
 
 ..084 
 
 .087 
 
 .089 
 
 .091 
 
 .094 
 
 .096 
 
 .098 .101 
 
 .103 
 
 .105 
 
 .108 
 
 .110 
 
 .112 
 
 81 
 
 .080 
 
 .083 
 
 .085 
 
 .088 
 
 .090 
 
 .092 
 
 .095 
 
 .097 
 
 .099 .102 
 
 .104 
 
 .106 
 
 .109 
 
 .111 
 
 .114 
 
 8-1.5 
 
 .081 
 
 .084 
 
 .086 
 
 .088 
 
 .091 
 
 .093 
 
 .096 
 
 .098 
 
 . 100 . 103 
 
 .105 
 
 .107 
 
 .110 
 
 .112 
 
 .115 
 
 82 
 
 .082 
 
 .084 
 
 .087 
 
 .089 
 
 .092 
 
 .094 
 
 .096 : 
 
 .099 
 
 .101 
 
 .104 
 
 .106 
 
 .108 
 
 .111 
 
 .113 
 
 .116 
 
 82.5 
 
 .083 
 
 .085 
 
 .088 
 
 .090 
 
 .092 
 
 .095 
 
 .097 
 
 .100 
 
 .102 
 
 .105 
 
 .107 
 
 .109 
 
 .112 
 
 .114 
 
 .117 
 
 83 
 
 .083 
 
 .086 
 
 .088 
 
 '.091 
 
 .093 
 
 .096 
 
 .098 
 
 .101 
 
 .103 
 
 .106 
 
 .108 
 
 .111 
 
 .113 
 
 .115 
 
 .118 
 
 83.5 
 
 .084 
 
 .087 
 
 .089 
 
 .092 
 
 .094 
 
 .097 
 
 .099 
 
 .102 
 
 .104 
 
 .107 
 
 .109 
 
 .112 
 
 .114 
 
 .117 
 
 .119 
 
 84 
 
 .085 
 
 .088 
 
 .090 
 
 .093 
 
 .095 
 
 .098 
 
 .100 
 
 .103 
 
 .105 
 
 .108 
 
 .110 
 
 .113 
 
 .115 
 
 .118 
 
 .120 
 
 84.5 
 
 .086 
 
 .088 
 
 .091 
 
 .093 
 
 .096 
 
 .098 
 
 .101 
 
 .103 
 
 .106 
 
 .108 
 
 .111 
 
 .114 
 
 .116 
 
 .119 
 
 .121 
 
 85 
 
 .087 
 
 .089 
 
 .092 
 
 .094 
 
 .097 
 
 .099 
 
 .102 
 
 .104 
 
 .107 
 
 .109 
 
 .112 
 
 .115 
 
 .117 
 
 .120 
 
 .122 
 
 85.5 
 
 .087 
 
 .090 
 
 .092 
 
 .095 
 
 .098 
 
 .100 
 
 .103 
 
 .105 
 
 .108 
 
 .110 
 
 .113 
 
 .116 
 
 .118 
 
 .121 
 
 .123 
 
 86 
 
 .088 
 
 .091 
 
 .093 
 
 .096 
 
 .098 
 
 .101 
 
 .104 
 
 .106 
 
 .109 
 
 .111 
 
 .114 
 
 .117 
 
 .119 
 
 .122 
 
 .124 
 
 86.5 
 
 .089 
 
 .091 
 
 .094 
 
 .097 
 
 .099 
 
 .102 
 
 .105 
 
 .107 
 
 .110 
 
 .112 
 
 .115 
 
 .118 
 
 .120 
 
 .123 
 
 .125 
 
 87 
 
 .090 
 
 .092 
 
 .095 
 
 .098 
 
 .100 
 
 .103 
 
 .105 
 
 .108 
 
 .111.! .113 
 
 .116 
 
 .119 
 
 .121 
 
 .124 
 
 .126 
 
 87.5 
 
 .090 
 
 .093 
 
 .096 
 
 .098 
 
 .101 
 
 .104 
 
 .106 
 
 .109 
 
 .112 .114 
 
 .117 
 
 .120 
 
 .122 
 
 .125 
 
 .128 
 
 88 
 
 .091 
 
 .094 
 
 .096 
 
 .099 
 
 .102 
 
 .105 
 
 .107 
 
 .110 
 
 .113 .115 
 
 .118 
 
 .121 
 
 .123 
 
 .126 
 
 .129 
 
 88.5 
 
 .092 
 
 .095 
 
 .097 
 
 .100 
 
 .103 
 
 .105 
 
 .108 
 
 .111 
 
 .114 
 
 .116 
 
 .119 
 
 .122 
 
 .124 
 
 .127 
 
 .130 
 
 89 
 
 .093 
 
 .095 
 
 .098 
 
 .101 
 
 .104 
 
 .106 
 
 .109 
 
 .112 
 
 .114 
 
 .117 
 
 .120 
 
 .123 
 
 .125 
 
 .128 
 
 .131 
 
 89.5 
 
 .093 
 
 .096 
 
 .099 
 
 .102 
 
 .104 
 
 .107 
 
 .110 
 
 .113 
 
 .115 .118 
 
 .121 
 
 .124 
 
 .126 
 
 .129 
 
 .132 
 
 90 
 
 .094 
 
 .097 
 
 .100 
 
 .102 
 
 .105 
 
 .108 
 
 .111 
 
 .114 
 
 .116 .119 
 
 .122 
 
 .125 
 
 .127 
 
 .130 
 
 .133 
 
 90.5 
 
 .095 
 
 .098 
 
 .101 
 
 .103 
 
 .106 
 
 .109 
 
 .112 
 
 .114 
 
 . 117 . 120 
 
 .123 
 
 .128 
 
 .131 
 
 .134 
 
 91 
 
 .096 
 
 .099 
 
 .101 
 
 .104 
 
 .107 
 
 .110 
 
 .113 
 
 .115 
 
 M18 .121 
 
 .124 ,181 
 
 .129 
 
 .132 
 
 .135 
 
 91.5 
 
 .096 
 
 .099 
 
 .102 
 
 .105 
 
 .108 
 
 .111 
 
 .113 
 
 .116 
 
 . 119 . 122 
 
 .125 
 
 .128 
 
 .131 
 
 .133 
 
 .136 
 
 92 
 
 .097 
 
 .100 
 
 .103 
 
 .106 
 
 .109 
 
 .112 
 
 .114 
 
 .117 
 
 .120 .123 
 
 .126 
 
 .129 
 
 .132 
 
 .134 
 
 .137 
 
 92.5 
 
 .098 
 
 .101 
 
 .104 
 
 .107 
 
 .110 
 
 .112 
 
 .115 
 
 .118 
 
 . 121 . 124 
 
 .127 .130 
 
 .133 
 
 .135 
 
 .138 
 
 93 
 
 .099 
 
 .102 
 
 .105 
 
 .107 
 
 .110 
 
 .113 
 
 .116 
 
 .119 
 
 .122 
 
 .125 
 
 .128 
 
 .131 
 
 . 134 
 
 .137 
 
 .139 
 
 93.5 
 
 .100 
 
 .102 
 
 .105 
 
 .108 
 
 .111 
 
 .114 
 
 .117 
 
 .120 
 
 .123 
 
 .126 
 
 .129 
 
 .132 
 
 .135 
 
 .138 
 
 .140 
 
 M 
 
 .100 
 
 .103 
 
 .106 
 
 .109 
 
 .112 
 
 .115 
 
 .118 
 
 .121 
 
 .124 
 
 .127 
 
 .130 
 
 .133 
 
 .136 
 
 .139 
 
 .142 
 
 94.5 
 
 .101 .104 
 
 .107 
 
 .110 
 
 .113 
 
 .116 ^ .119 
 
 .122 
 
 .125 
 
 .128 
 
 .131 
 
 .134 
 
 .137 
 
 .140 
 
 .143 
 
 95 
 
 .102 .10.-, 
 
 .108 
 
 .111 
 
 .114 
 
 .117 .120 
 
 .123 
 
 .126 
 
 .129 
 
 .132 
 
 .135 
 
 .138 
 
 .141 
 
 .144 
 
 95.5 
 
 .103 .10., 
 
 .109 
 
 .112 
 
 .115 
 
 .118 
 
 .121 
 
 .124 
 
 .127 
 
 .130 
 
 .133 
 
 .136 
 
 .139 
 
 .142 
 
 .145 
 
 96 
 
 . 103 . 106 
 
 .109 
 
 .112 
 
 .115 
 
 .119 
 
 .122 
 
 .125 
 
 .128 
 
 .131 
 
 .134 
 
 .137 
 
 .140 
 
 .143 
 
 .146 
 
 96.5 
 
 .104 .107 
 
 .110 
 
 .113 
 
 .116 
 
 .119 
 
 .122 .126 
 
 .129 
 
 .132 
 
 .135 
 
 .138 
 
 .141 
 
 .144 
 
 .147 
 
 97 
 
 .105 
 
 .108 
 
 .111 
 
 .114 
 
 .117 
 
 .120 
 
 .123 
 
 .126 
 
 .130 
 
 .133 
 
 .136 
 
 .139 
 
 .142 
 
 .145 
 
 .148 
 
 97.5 
 
 .106 
 
 .109 
 
 .112 
 
 .115 
 
 .118. 
 
 .121 
 
 .124 
 
 .127 
 
 .130 
 
 .134 
 
 .137 
 
 .140 
 
 .143 
 
 .146 
 
 .149 
 
 98 
 
 .106 .109 
 
 .113 
 
 .116 
 
 .119 
 
 .122 
 
 .125 
 
 .131 
 
 .135 
 
 .138 
 
 .141 
 
 .144 
 
 .147 
 
 ..150 
 
 98.5 
 
 .107 .110 
 
 .113 
 
 .117 
 
 .120 
 
 .123 
 
 .126 .129 
 
 .132 
 
 .135 
 
 .139 
 
 .142 
 
 .145 
 
 .148 
 
 .151 
 
 99 
 
 .108 .111 
 
 .114 
 
 .117 
 
 .121 
 
 .124 
 
 .127 .130 
 
 .133 
 
 ..136 
 
 .140 
 
 .143 
 
 .146 
 
 .149 
 
 . 152 
 
 99.5 
 
 .109 
 
 .112 
 
 .115 
 
 .118 
 
 .121 
 
 .125 
 
 .128 
 
 .131 
 
 .1M4 
 
 .137 
 
 .141 
 
 .144 
 
 .147 
 
 .150. 
 
 
 100 
 
 .109 
 
 .113 
 
 .116 
 
 .119 
 
 ,189 
 
 .1.'., 
 
 . 129 . 132 
 
 .135 
 
 .138 
 
 . n. 1 
 
 . 1 :, 
 
 .148 
 
 .151 
 
 
96 BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 
 TABLE I. Correction of mercurial barometer for temperature, English measures Con' 
 
 
 Observed reading of the barometer, in inches. 
 
 f 
 
 24 
 
 . 
 24.5 
 
 25 
 
 25.5 
 
 26 
 
 26.5 
 
 27 
 
 27.5 
 
 IN 28. 5 
 
 29 29.5 , 30 I 30.5 31 
 
 
 Inches. SUBTRACT. Inches. 
 
 67 
 
 0.083 0.085 0.087 
 
 0.089 
 
 0.090 0.092 0.094 j 
 
 0.095 
 
 0.097 0.099 0.101 0.102 
 
 0.104 
 
 0.106 0.108 
 
 67.5 
 
 .084 
 
 086 
 
 088 
 
 090 
 
 092 093 -095 I 
 
 .097 
 
 . 098 . 100 
 
 . 102 . 104 
 
 .106 
 
 .107 ' .109 
 
 68' 
 
 !085 
 
 '.087 
 
 .089 
 
 .091 
 
 .093 .094 
 
 .096 
 
 .098 
 
 . 100 . 102 
 
 .103 
 
 .105 
 
 .107 
 
 .109 ; .110 
 
 68.5 
 
 087 
 
 088 
 
 090 
 
 092 
 
 094 .096 
 
 .097 
 
 .099 
 
 .101 
 
 .103 
 
 .105 
 
 .106 
 
 .108 
 
 .110 i .112 
 
 69' 
 
 .088 
 
 .089 
 
 .091 
 
 .093 
 
 .095 I .097 
 
 .099 
 
 .100 
 
 .102 
 
 .104 
 
 .106 
 
 .108 
 
 .110 
 
 .111 .113 
 
 69.5 
 
 .089 
 
 091 
 
 092 
 
 094 
 
 096 098 
 
 .100 
 
 .102 
 
 .104 
 
 .105 
 
 .107 
 
 .109 
 
 .111 
 
 .113 i .115 
 
 70' 
 
 
 !092 
 
 .094 
 
 .095 
 
 .097 1 .099 
 
 .101 
 
 .103 
 
 .105 
 
 .107 
 
 .109' 
 
 .110 
 
 .112 
 
 .114 .116 
 
 70.5 
 
 !091 
 
 .093 
 
 .095 
 
 .097 
 
 .098 ! .100 
 
 .102 
 
 .104 
 
 .106 
 
 .108 
 
 .110 
 
 .112 
 
 .114 
 
 .116 .117 
 
 71 
 
 .092 
 
 .094 
 
 .096 
 
 .098 
 
 .100 : .102 
 
 .103 
 
 .105 
 
 .107 
 
 .109 
 
 .111 
 
 .113 
 
 .115 
 
 .117 .119 
 
 71.5 
 
 .093 
 
 .095 
 
 .097 
 
 .099 
 
 .101 .103 
 
 .105 
 
 .107 
 
 .109 
 
 .110 
 
 .112 
 
 .114 
 
 .116 
 
 .118 .120 
 
 72 
 
 .094 
 
 .096 
 
 .098 
 
 .100 
 
 .102 | .104 
 
 .106 
 
 .108 
 
 .110 
 
 .112 
 
 .114 
 
 .116 
 
 .118 
 
 . 120 . 122 
 
 72.5 
 
 .095 
 
 .097 
 
 .099 
 
 .101 
 
 . 103 . 105 
 
 .107 
 
 .109 
 
 .111 
 
 .113 
 
 .115 
 
 .117 
 
 .119 
 
 .121 .123 
 
 73 
 
 .096 
 
 .098 
 
 .100 
 
 .102 
 
 . 104 . 106 
 
 .108 
 
 .110 
 
 .112 
 
 .114 
 
 .T16 
 
 . 118 1 . 120 
 
 .122 .124 
 
 73.5 
 
 .097 
 
 .099 
 
 .101 
 
 .103 
 
 .105 .108 
 
 .110 
 
 .112 
 
 .114 
 
 .116 
 
 .118 
 
 .120 
 
 .122 
 
 . 124 . 126 
 
 74 
 
 .098 
 
 ..101 
 
 .103 
 
 .105 
 
 .107 .109 
 
 .111 
 
 .113 
 
 .115 
 
 .117 
 
 .119 
 
 .121 
 
 .123 
 
 .125 
 
 .127 
 
 74.5 
 
 .100 
 
 .102 
 
 .104 
 
 .106 
 
 . 108 . 110 
 
 .112 
 
 .114 
 
 .116 
 
 .118 
 
 .120 
 
 .122 
 
 .124 
 
 .126 
 
 .129 
 
 75 
 
 .101 
 
 .103 
 
 .105 
 
 .107 
 
 .109 
 
 .111 
 
 .113 
 
 .115 
 
 .117 
 
 .119 
 
 .122 
 
 .124 
 
 .126 
 
 .128 
 
 .130 
 
 75.5 
 
 .102 
 
 .104 
 
 .106 
 
 .108 
 
 .110 
 
 .112 
 
 .114 
 
 .117 
 
 .119 
 
 .121 
 
 .123 
 
 .125 
 
 .127 
 
 .129 
 
 .131 
 
 76 
 
 .103 
 
 .105 
 
 .107 
 
 .109 
 
 .111 
 
 .113 
 
 .116 
 
 .118 
 
 .120 
 
 .122 
 
 .124 
 
 .126 
 
 .128 
 
 .131 
 
 .133 
 
 76.5 
 
 ,104 
 
 .106 
 
 .108 
 
 .111 
 
 .113 
 
 .115 
 
 .117 
 
 .119 
 
 .121 
 
 .123 
 
 .125 
 
 .128 
 
 .130 
 
 .132 
 
 .134 
 
 77 
 
 .105 
 
 .107 
 
 .109 
 
 .112 
 
 .114 
 
 .116 
 
 .118 
 
 .120 
 
 .122 
 
 .125 
 
 .127 
 
 .129 
 
 .131 
 
 .133 
 
 .136 
 
 77.5 
 
 .106 
 
 .108 
 
 .110 
 
 .113 
 
 .115 
 
 .117 
 
 .119 
 
 .121 
 
 .124 
 
 .126 
 
 .128 
 
 .130 
 
 .133 
 
 .135 
 
 .137 
 
 78 
 
 .107 
 
 .109 
 
 .112 
 
 .114 
 
 .116 
 
 .118 
 
 .120 
 
 .123 
 
 .125 
 
 .127 
 
 .129 
 
 .132 
 
 .134 
 
 .136 
 
 .138 
 
 78.5 
 
 .108 
 
 .110 
 
 .113 
 
 .115 
 
 .117 
 
 .119 
 
 .122 
 
 .124 
 
 .126 
 
 .128 
 
 .131 
 
 .133 
 
 .135 
 
 .137 
 
 .140 
 
 79 
 
 .109 
 
 .112 
 
 .114 
 
 .116 
 
 .118 
 
 .121 
 
 .123 
 
 .125 
 
 .127 
 
 .130 
 
 .132 
 
 .134 
 
 .137 
 
 .139 
 
 .141 
 
 79.5 
 
 .110 
 
 .113 
 
 .115 
 
 .117 
 
 .120 
 
 .122 
 
 .124 
 
 .126 
 
 .129 
 
 .131 
 
 .133 
 
 .136' 
 
 .Ig8 
 
 .140 
 
 .143 
 
 80 
 
 .111 
 
 .114 
 
 .116 
 
 .118 
 
 .121 
 
 .123 
 
 .125 
 
 .128 
 
 .130 
 
 .132 
 
 .135 
 
 .137 
 
 .139 
 
 .142 
 
 .144 
 
 80.5 
 
 .112 
 
 .115 
 
 .117 
 
 .120 
 
 .122 
 
 .124 
 
 .127 
 
 .129 
 
 .131 
 
 .134 
 
 .136 
 
 .138 
 
 .141 
 
 .143, 
 
 .145 
 
 81 
 
 .114 
 
 .116 
 
 .118 
 
 .121 
 
 .123 
 
 .125 
 
 .128 
 
 .130 
 
 .132 
 
 .135 
 
 .137 
 
 .140 
 
 .142 
 
 .144' 
 
 .147 
 
 81.5 
 
 .115 
 
 .117 
 
 .119 
 
 .122 
 
 .124 
 
 .127 
 
 .129 
 
 .131 
 
 .134 
 
 . 136 . 139 
 
 .141 
 
 .143 
 
 .146 
 
 .148 
 
 82 
 
 .116 
 
 .118 
 
 .121 
 
 .123 
 
 .125 
 
 .128 
 
 .130 
 
 .133 
 
 .135 
 
 . 137 . 140 
 
 .142 
 
 .145 
 
 .147 
 
 .149 
 
 82.5 
 
 .117 
 
 .119 
 
 .122 
 
 .124 
 
 .127 
 
 .129 
 
 .131 
 
 .134 
 
 .136 
 
 . 139 . 141 
 
 .144 
 
 .146 
 
 .148 
 
 .151 
 
 83 
 
 .118 
 
 .120 
 
 .123 
 
 .125 
 
 .128 
 
 .130 
 
 .133 
 
 .135 
 
 .138 .140 . 142 
 
 .145 
 
 .147 
 
 .150 
 
 .152 
 
 83.5 
 
 .119 
 
 .121 
 
 .124 
 
 .126 
 
 .129 
 
 .131 
 
 .134 
 
 .136 
 
 .139 
 
 . 141 . 144 
 
 .146 
 
 .149 
 
 .151 
 
 .154 
 
 84 
 
 .120 
 
 .123 
 
 .125 
 
 .128 
 
 .130 
 
 .133 
 
 .135 
 
 .138 
 
 .140 
 
 . 143 . 145 
 
 .148 
 
 .150 
 
 .153 
 
 .155 
 
 84.5 
 
 .121 
 
 .124 
 
 .126 
 
 .129 
 
 .131 
 
 .134 
 
 .136 
 
 .139 
 
 .141 
 
 .144 
 
 .146 
 
 .149 
 
 .151 
 
 .154 
 
 .156 
 
 85 
 
 .122 
 
 .125 
 
 .127 
 
 .130 
 
 .132 
 
 .135 
 
 .137 
 
 .140 
 
 .143 
 
 .145 
 
 .148 
 
 .150 
 
 .153 
 
 .155 
 
 158 
 
 85/5 
 
 .123 
 
 .126 
 
 .128 
 
 .131 
 
 .134 
 
 .136 
 
 .139 
 
 .141 
 
 .144 
 
 .146 
 
 .149 
 
 .152 
 
 .154 
 
 .157 
 
 .159 
 
 86 
 
 .124 
 
 .127 
 
 .130 
 
 .132 
 
 . 135 . 137 
 
 .140 
 
 .142 
 
 .145 
 
 .148 
 
 .150 
 
 .153 
 
 .155 
 
 .158 
 
 .161 
 
 86.5 
 
 .125 
 
 .128 
 
 .131 
 
 .133 
 
 . 136 . 138 
 
 .141 
 
 .144 
 
 .146 
 
 .149 
 
 .152 
 
 .154 
 
 .157 
 
 .159 
 
 .162 
 
 87 
 
 .126 
 
 .129 
 
 .132 
 
 .134 
 
 .137 
 
 .140 
 
 .142 
 
 .145 
 
 .148 
 
 .150 
 
 .153 
 
 .155 
 
 .158 
 
 .161 
 
 .163 
 
 87.5 
 
 .128 
 
 .130 
 
 .133 
 
 .136 
 
 .138 
 
 .141 
 
 .144 
 
 .146 
 
 .149 
 
 .151 
 
 .154 
 
 .157 
 
 .159 
 
 .162 
 
 .165 
 
 88 
 
 .129 
 
 .131 
 
 .134 
 
 .137 
 
 .139 
 
 .142 
 
 .145 
 
 .147 
 
 .150 
 
 .153 
 
 .155 
 
 .158 
 
 .161 
 
 .163 
 
 .166 
 
 88.5 
 
 .130 
 
 .132 
 
 .135 
 
 .138 
 
 .141 
 
 .143 
 
 .146 
 
 .149 
 
 .151 
 
 .154 
 
 .157 
 
 .159 
 
 .162 
 
 .165 
 
 .168 
 
 89 
 
 .131 
 
 .134 
 
 .136 
 
 .139 
 
 .142 
 
 .144 
 
 .147 
 
 .150 
 
 .153 ! .155 
 
 .158 
 
 .161 
 
 .164 
 
 .166 
 
 .169 
 
 
 
 
 
 
 
 
 
 
 1 
 
 
 
 
 
 
 89.5 
 
 .132 
 
 .135 
 
 .137 
 
 .140 
 
 .143 
 
 .146 
 
 .148 
 
 .151 
 
 .154 
 
 .157 
 
 .159 
 
 .162 
 
 .165 
 
 .168 
 
 .170 
 
 90 
 
 .133 
 
 .136 
 
 .138 
 
 .141 
 
 .144 
 
 .147 
 
 .150 
 
 .152 
 
 .155- 
 
 .158 
 
 .161 
 
 .163 
 
 .166 
 
 .169 
 
 .172 
 
 90.5 
 
 .134 
 
 .137 
 
 .140 
 
 .142 
 
 .145 j .148 
 
 .151 
 
 .154 
 
 .156 
 
 .159 
 
 .162 
 
 .165 
 
 .168 
 
 .170 
 
 .173 
 
 91 
 
 .135 
 
 .138 
 
 .141 
 
 .144 
 
 .146 
 
 .149 
 
 .152 
 
 .155 
 
 .158 
 
 .160 
 
 .163 
 
 .166 
 
 .169 
 
 .172 
 
 .175 
 
 91.5 
 
 .136 
 
 .139 
 
 .142 
 
 .145 
 
 . 148 . 150 
 
 .153 
 
 .156 
 
 .159 
 
 .162 
 
 .165 
 
 .167 
 
 .170 
 
 .173 
 
 .176 
 
 92 
 
 .137 
 
 .140 
 
 .143 
 
 .146 
 
 . 149 . 152 
 
 .154 
 
 .157 
 
 .160 
 
 .163 
 
 .166 
 
 .169 
 
 .172 
 
 .174 
 
 .177 
 
 92.5 
 
 .138 
 
 .141 
 
 .144 
 
 .147 
 
 .150 
 
 .153 
 
 .156 
 
 .159 
 
 .161 
 
 .164 
 
 .167 
 
 .170 
 
 .173 
 
 .176 
 
 .179 
 
 93 
 
 .139 
 
 .142 
 
 .145 
 
 .148 
 
 . 151 . 154 
 
 .157 
 
 .160 
 
 .163 
 
 .166 
 
 .168 
 
 .171 
 
 .174 
 
 .177 
 
 .180 
 
 93.5 
 
 .140 
 
 .143 
 
 .146 
 
 .149 
 
 .152 
 
 .155 
 
 .158 
 
 .161 
 
 .164 
 
 .167 
 
 .170 
 
 .173 
 
 .176 
 
 .179 
 
 .181 
 
 94 
 
 .142 
 
 .145 
 
 .147 
 
 .150 
 
 .153 
 
 .156 
 
 .159 
 
 .162 
 
 .165 
 
 .168 
 
 .171 
 
 .174 
 
 .177 
 
 .180 
 
 .183 
 
 94.5 
 
 .143 
 
 .146 
 
 .149 
 
 .152 
 
 .155 
 
 .158 
 
 .160 
 
 .163 
 
 .166 
 
 .169 
 
 .172 
 
 .175 
 
 .178 
 
 .181 
 
 .184 
 
 95 
 
 .144 
 
 .147 
 
 .150 
 
 .153 
 
 .156 
 
 .159 
 
 .162 
 
 .165 
 
 .168 
 
 .171 
 
 .174 
 
 .177 
 
 .180 
 
 .183 
 
 .186 
 
 95.5 
 
 .145 
 
 .148 
 
 .151 
 
 .154 
 
 .157 
 
 .160 
 
 .163 
 
 .166 
 
 .169 
 
 .172 
 
 .175 
 
 .178 
 
 .181 
 
 .184 
 
 .187 
 
 96 
 
 .146 
 
 .149 
 
 .152 
 
 .155 
 
 .158 
 
 .161 
 
 .164 
 
 .167 
 
 .170 
 
 .173 
 
 .176 
 
 .179 
 
 .182 
 
 .185 
 
 .188 
 
 96.5 
 
 .147 
 
 .150 
 
 .153 
 
 '.156 
 
 .159 
 
 .162 
 
 .165 
 
 .168 
 
 .171 
 
 .174 
 
 .178 
 
 .181 
 
 .184 
 
 .187 
 
 .190 
 
 97 
 
 .148 
 
 .151 
 
 .154 
 
 .157 
 
 .160 
 
 .163 
 
 .167 
 
 .170 
 
 .173 
 
 .176 
 
 .179 
 
 .182 
 
 .185 
 
 .188 
 
 .191 
 
 97.5 
 
 .149 
 
 .152 
 
 .155 
 
 .158 
 
 .162 
 
 .165 
 
 .168 
 
 .171 
 
 .174 
 
 .177 
 
 .180 
 
 .183 
 
 .186 
 
 .189 
 
 .193 
 
 98 
 
 .150 
 
 .153 
 
 .156 
 
 .160 
 
 .163 
 
 .166 
 
 .169 
 
 .172 
 
 .175 
 
 .178 
 
 .181 
 
 .185 
 
 .188 
 
 .191 
 
 .194 
 
 98.5 
 
 .151 
 
 .154 
 
 .158 
 
 ,161 
 
 .164 
 
 .167 
 
 .170 
 
 .173 
 
 .176 
 
 .180 
 
 .183 
 
 .186 
 
 .189 
 
 .192 
 
 .195 
 
 99 
 
 .152 
 
 .155 
 
 .159 
 
 .162 
 
 .165 
 
 .168 
 
 .171 
 
 .175 
 
 .178 
 
 .181 
 
 .184 
 
 .187 
 
 .190 
 
 .194 
 
 .197 
 
 99.5 
 
 .153 
 
 .157 
 
 .160 
 
 .163 
 
 .166 
 
 .169 
 
 .173 
 
 .176 
 
 .179 
 
 .182 
 
 .185 
 
 .189 
 
 .192 
 
 .195 
 
 .198 
 
 100 
 
 .154 
 
 .158 
 
 .161 
 
 .164 
 
 .167 
 
 .171 
 
 .174 
 
 . 177 . 180 
 
 .183 
 
 .187 
 
 .190 
 
 .193 
 
 .196 
 
 .200 
 
BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSUKK. 
 TABLE II. Correction of mercurial barometers for temperature, metric meatureg. 
 Observed reading of the barometer, in millimeters. 
 
 ,ure, 
 C. 
 
 640 
 
 650 
 
 660 
 
 670 
 
 680 690 
 
 700 
 
 7W 
 
 720 730 
 
 740 
 
 750 
 
 760 770 
 
 0.0 
 1.0 
 2.0 
 
 0.00 
 .10 
 
 .21 
 
 0.00 
 11 
 
 .21 
 
 0.00 0.00 
 .11 .11 
 
 0.00 0.00 
 .11 .11 
 .22 .23 
 
 0.00 
 
 : 
 
 0.00 
 .12 
 .23 
 
 0.00 0.00 
 12 .12 
 .24 .24 
 
 0.00 
 .12 
 .24 
 
 0.00 
 .12 
 
 0.00 0.00 
 .12 .13 
 .25 .25 
 
 II. IN' 
 
 .13 
 .25 
 
 3.0 
 
 .31 
 
 .32 
 
 .32 .33 .33 .34 
 
 .34 
 
 .35 
 
 .35 .36 
 
 .36 
 
 .37 
 
 .37 38 
 
 00 
 
 4.0 
 
 .42 
 
 .42 
 
 .43 .44 
 
 .44 .45 
 
 .46 
 
 .46 
 
 .47 .48 
 
 .48 
 
 .49 
 
 .60 .60 
 
 . oo 
 .51 
 
 5. 
 
 .52 
 
 .53 
 
 .54 
 
 .55 
 
 .56 .56 
 
 .57 
 
 .58 
 
 .59 .60 .60 
 
 .01 
 
 .62 .63 
 
 .64 
 
 5.5 
 
 .57 
 
 .58 
 
 .59 
 
 .) .61 .62 . 
 
 .64 
 
 .65 .66 
 
 .66 
 
 .67 
 
 .68 .69 
 
 70 
 
 ii. 
 
 .63 
 
 .64 
 
 .65 
 
 .66 
 
 .67 .68 .69 
 
 .70 
 
 .71 .71 
 
 .72 
 
 .73 
 
 . 74 . 75 
 
 . *u 
 .76 
 
 8.1 
 
 .68 
 
 .69 
 
 .70 .71 
 
 .72 .73 .74 
 
 .75 
 
 .76 
 
 .77 
 
 .79 
 
 .so 
 
 .81 .82 
 
 
 ~- 
 
 .73 
 
 .74 
 
 .75 .77 
 
 .78 .79 
 
 .80 
 
 .81 
 
 .82 
 
 .83 
 
 .85 
 
 .86 
 
 .87 .88 
 
 '.89 
 
 7. ") 
 
 .78 
 
 .80 
 
 .81 .82 
 
 .83 .84 .86 
 
 .87 
 
 .88 
 
 .VI 
 
 .91 
 
 .92 
 
 .93 .94 
 
 .95 
 
 8. 
 
 .84 
 
 .85 
 
 .86 .87 .89 .-90 .91 .93 
 
 .94 
 
 
 .97 
 
 .98 
 
 .99 1.01 
 
 1.02 
 
 8.5 
 
 .89 
 
 .90 
 
 .92 
 
 .93 .94 
 
 .96 
 
 .97 
 
 
 1.00 
 
 i!oi 
 
 1.03 
 
 1.04 
 
 1.05 
 
 1.07 
 
 1.08 
 
 9. 
 
 .94 .95 
 
 .97 
 
 .98 1.00 
 
 1.01 
 
 1.03 
 
 1.04 
 
 1.06 
 
 1.07 
 
 1.09 
 
 1.10 
 
 1.12 
 
 1. 13 
 
 1. 15 
 
 9.5 
 
 .99 1.01 
 
 1.02 
 
 1.04 
 
 1.05 
 
 1.07 1.08 
 
 1.10 
 
 1.12 
 
 1.13 
 
 1.15 
 
 1.16 
 
 1.18 
 
 1.19 
 
 1.21 
 
 10.0 
 
 1.04 1.06 
 
 1.08 
 
 1.09 
 
 1.11 
 
 1.13 
 
 1.14 l.hi 
 
 1.17 
 
 1.19 
 
 1.21 
 
 1.22 
 
 1.24 
 
 1.2', 
 
 1.27 
 
 .2 
 
 .06 .08 
 
 .10 
 
 .11 
 
 .13 
 
 .15 
 
 .H, .18 
 
 .20 
 
 .21 
 
 .23 
 
 
 .26 .28 
 
 .30 
 
 .4 
 
 .09 .10 
 
 .12 
 
 .14 
 
 .15 
 
 .17 
 
 .19 .20 
 
 .22 
 
 . 21 
 
 .26 
 
 .27 
 
 .29 .31 
 
 .32 
 
 .6 
 
 .11 " .12 
 
 .14 
 
 .16 
 
 .18 
 
 .19 
 
 .21 .23 
 
 .24 
 
 
 .28 
 
 .30 
 
 .31 
 
 .33 
 
 .35 
 
 .8 
 
 .13 .14 
 
 .16 
 
 .18 
 
 .20 
 
 .22 
 
 .23 ..26 
 
 .27 
 
 ! ~".i 
 
 .30 
 
 .32 
 
 .34 .36 
 
 .37 
 
 11.0 
 
 1.15 1-17 
 
 1.18 
 
 1.20 
 
 1.22 
 
 1.24 1.26 -1.27 
 
 1.29 
 
 1.31 
 
 1.33 
 
 1.35 
 
 1.36 1.38 
 
 1.42 
 
 .2 
 
 .17 .19 
 
 .21 
 
 .22 
 
 .24 
 
 .2ii .28 
 
 .30 
 
 .31 .33 
 
 .35 
 
 .37 
 
 .39 .41 
 
 .45 
 
 .4 
 
 .19 .21 
 
 .23 
 
 .25 
 
 .26 
 
 .28 .30 
 
 .32 
 
 .34 .36 .38 
 
 .39 
 
 .41 
 
 
 .40 
 
 .6 
 
 . 21 . 23 
 
 .25 
 
 .27 
 
 .29 
 
 .31 .32 
 
 .34 
 
 .36 
 
 .38 
 
 .40 
 
 .42 
 
 .44 
 
 
 48 
 
 .8 
 
 .23 
 
 .25 
 
 .27 
 
 .29 
 
 .31 
 
 .33 
 
 .35 
 
 .37 
 
 .39 
 
 .40 
 
 .42 
 
 .44 
 
 .46 
 
 .48 
 
 .50 
 
 12.0 
 
 1.25 1.27 
 
 1.29 
 
 1.31 
 
 1.33 
 
 1.35 
 
 1.37 
 
 1.39 
 
 1.41 
 
 1.43 
 
 1.45 
 
 1.47 
 
 1.49 
 
 1.51 
 
 1.53 
 
 .2 
 
 .27 ' .29 
 
 .31 
 
 .33 
 
 .35 .37 
 
 .39 .41 
 
 .43 
 
 .45 
 
 .47 
 
 49 
 
 .51 
 
 .63 
 
 .55 
 
 .4 
 
 .29 .31 
 
 .33 
 
 .35 
 
 .37 
 
 .39 
 
 
 .44 
 
 .46 .48 
 
 .50 
 
 .52 
 
 .64 
 
 .66 
 
 .68 
 
 .6 
 
 .31 .34 
 
 .36 
 
 .38 
 
 40 
 
 .42 
 
 .44 
 
 .46 
 
 .48 .50 
 
 .52 
 
 .54 
 
 .66 
 
 
 .60 
 
 & 
 
 .34 
 
 .36 
 
 .38 
 
 .40 
 
 .42 
 
 .44 
 
 .46 
 
 .48 
 
 .50 
 
 .52 
 
 .54 
 
 .56 
 
 .59 
 
 la 
 
 .63 
 
 13.0 
 
 1.36 1.38 
 
 1.40 
 
 1.42 
 
 1.44 
 
 1.46 
 
 1.48 
 
 1.50 
 
 1. 53 1. 55 
 
 1.57 
 
 1.59 
 
 1.61 
 
 LM 
 
 1.65 
 
 .2 
 
 .38 .40 
 
 .42 
 
 .44 
 
 .46 
 
 .48 .51 .53 
 
 .57 .59 
 
 .61 
 
 .64 ; .66 
 
 .68 
 
 .4 
 
 .40 .42 
 
 .44 
 
 .46 
 
 .49 
 
 ,j] .53 .55 
 
 . .-,7 . 59 
 
 .62 
 
 .64 
 
 .66 
 
 .fc 
 
 .70 
 
 .6 
 
 .42 .44 
 
 .46 
 
 .49 
 
 .51 
 
 .53 
 
 .55 
 
 .57 
 
 .60 
 
 .62 
 
 .64 
 
 .66 
 
 .68 
 
 .71 
 
 .73 
 
 .8 
 
 .44 
 
 .46 
 
 .48 
 
 .51 
 
 .53 
 
 .55 
 
 .57 
 
 .60 
 
 .62 
 
 .64 
 
 .66 
 
 .69 
 
 .71 
 
 .73 
 
 .75 
 
 14.0 
 
 1.46 1.48 
 
 1.51 
 
 1.53 
 
 1.55 
 
 1.57 
 
 1.60 
 
 1.62 
 
 1.64 
 
 1.67 
 
 1.69 
 
 1.71 
 
 1.73 
 
 1.76 
 
 1.78 
 
 .2 
 
 .48 .50 
 
 .53 
 
 .55 
 
 .o7 
 
 .60 .62 .64 
 
 .67 
 
 .69 
 
 .71 
 
 .74 
 
 .76 
 
 !78 
 
 .81 
 
 .4 
 
 .50 .53 
 
 .55 
 
 .57 
 
 .60 
 
 .62 .64 ,87 
 
 .69 
 
 .71 
 
 .74 
 
 .76 
 
 .78 
 
 .81 
 
 .83 
 
 . ti 
 
 . 52 
 
 .57 
 
 .59 
 
 .62 
 
 .64 
 
 .67 
 
 .69 
 
 .71 
 
 .74 
 
 .76 
 
 .78 
 
 .81 
 
 .83 
 
 .86 
 
 .8 
 
 .54 
 
 .57 
 
 .59 
 
 .62 
 
 .64 
 
 .66 
 
 .69 
 
 .71 
 
 .74 
 
 .76 
 
 .78 
 
 .81 
 
 .83 
 
 .86 
 
 .88 
 
 15.0 
 
 1.56 1.59 
 
 1.61 
 
 1.64 
 
 1.66 
 
 1.69 
 
 1.71 
 
 1.74 
 
 1.76 
 
 1.78 
 
 1.81 
 
 1.83 
 
 1.86 
 
 1.88 
 
 1.91 
 
 .2 
 
 .59 .61 
 
 .63 
 
 .68 
 
 .71 
 
 .73 
 
 .76 
 
 .78 
 
 .81 
 
 .83 
 
 .86 
 
 .88 
 
 .91 
 
 .93 
 
 .4 
 
 .61 .63 
 
 .66 
 
 .68 .71 
 
 .76 
 
 .78 
 
 .81 
 
 .83 
 
 .86 
 
 .88 
 
 .91 .93 
 
 .96 
 
 .1; 
 
 .63 .65 
 
 .68 
 
 .70 .73 .75 
 
 .78 
 
 .80 
 
 .83 
 
 .86 
 
 .88 
 
 .91 
 
 .93 .96 
 
 .98 
 
 .8 
 
 .65 .67 
 
 .70 
 
 . 72 . 75 
 
 .78 
 
 .80 
 
 .83 
 
 .85 
 
 .88 
 
 .90 
 
 .93 
 
 .96 | .98 
 
 2.01 
 
 !.,.() 
 
 1.67 1.69 
 
 1.72 
 
 1.75 1.77 
 
 1.80 
 
 1.82 
 
 1.85 
 
 1.88 
 
 1.90 
 
 1.93 
 
 1.96 
 
 1.98 2.01 
 
 2.03 
 
 
 .69 .72 
 
 .74 
 
 .77 .79 
 
 .82 
 
 .85 
 
 .87 
 
 .90 .93 
 
 .95 
 
 .98 
 
 2.01 
 
 .03 
 
 .06 
 
 .4 
 
 .71 .74 
 
 .76 
 
 .79 .82 
 
 .84 
 
 .87 .90 
 
 .92 
 
 . 95 
 
 .98 
 
 2.00 
 
 .03 
 
 .06 
 
 .06 
 
 .6 
 
 .73 .76 
 
 .78 
 
 .81 
 
 .84 
 
 .87 
 
 .89 .92 
 
 .95 
 
 .97 
 
 2.00 
 
 .03 
 
 .06 
 
 .06 
 
 .11 
 
 .8 
 
 .75 .78 
 
 .81 
 
 .83 
 
 .86 
 
 .89 
 
 .92 
 
 .94 
 
 .97 
 
 2.00 
 
 .03 
 
 .05 
 
 .08 
 
 .11 
 
 .13 
 
 17.0 
 
 1-.77 1.80 
 
 1.83 
 
 1.86 1.88 
 
 1.91 1.94 f 
 
 1.97 
 
 1.99 2.02 
 
 2.05 
 
 2.08 
 
 2.10 
 
 2.13 
 
 2.16 
 
 9 
 
 .79 .82 
 
 .85 
 
 .88 .91 .93 .96 1! .99 
 
 2.02 .05 
 
 .07 
 
 .10 .13 .16 
 
 .19 
 
 ^4 
 
 .81 .84 
 
 .87 
 
 .90 .93 .96 .98 2.01 
 
 .04 .07 
 
 .10 
 
 .13 .15 .18 
 
 .21 
 
 .6 
 
 .83 .86 
 
 .89 
 
 .92 .95 
 
 .98 2.01 .04 
 
 .06 
 
 .09 
 
 .12 
 
 .15 .18 
 
 .21 
 
 .24 
 
 .8 
 
 .86 .88 
 
 .91 
 
 .94 .97 
 
 2.00 .03 .06 
 
 .09 
 
 .12 
 
 .15 
 
 .17 .20 
 
 .23 
 
 .26 
 
 
 
 
 
 
 
 
 
 
 
 
 18.0 
 
 1.88 
 
 1.91 
 
 1.93 
 
 1.96 1.99 2.02 2.05 2.08 
 
 2.11 
 
 2.14 
 
 2.17 
 
 2.20 
 
 2.23 
 
 2.26 
 
 2.29 
 
 .2 
 
 .90 
 
 .93 
 
 .96 
 
 .99 2.02 ! .05 .07 H) 
 
 .13 
 
 .16 
 
 .19 
 
 .22 .25 
 
 .28 
 
 .31 
 
 .4 
 
 . '.12 
 
 " it:, 
 
 .98 
 
 2.01 .04 .07 .10 .13 
 
 .If, 
 
 .19 
 
 .22 
 
 .25 .28 
 
 .31 
 
 .M 
 
 .6 
 
 .94 .97 
 
 2.00 
 
 .03 .06 .09 12 
 
 .15 
 
 ,u 
 
 .21 
 
 .24 
 
 .27 .30 
 
 
 
 .8 
 
 .96 .99 
 
 .02 
 
 .05 .08 .11 .14 
 
 .17 
 
 
 .23 
 
 .27 
 
 .30 .33 
 
 
 
 19.0 
 
 1.98 2.01 
 
 2.04 
 
 2.07 2.10 2.13 2.17 
 
 2.20 
 
 
 2.29 
 
 2.32 2.35 2.38 
 
 2.41 
 
 .2 
 
 2.00 .03 
 
 .06 
 
 .09 .13 .10 .19 
 
 .28 
 
 .31 
 
 .34 .38 .41 
 
 .44 
 
 .4 
 
 .02 .05 
 
 .08 
 
 .12 .15 .18 .21 .24 
 
 .17 .31 
 
 .34 
 
 .37 .40 .43 
 
 .4f. 
 
 .6 
 
 .04 .07 
 
 .11 
 
 .14 .17 .20 .23 
 
 .27 
 
 .30 
 
 .33 
 
 .36 
 
 .39 
 
 .46 
 
 .49 
 
 .8 
 
 .06 .10 
 
 .13 
 
 .16 .19 .22 .26 
 
 .29 
 
 
 .35 
 
 .39 
 
 .42 .45 
 
 
 .51 
 
 2293712 7 
 
98 BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 
 TABLE II. Correction of mercurial barometers for temperature, metric measures Con. 
 
 Observed reading of the barometer, in millimeters. 
 
 640 
 
 650 
 
 660 
 
 670 
 
 680 
 
 690 
 
 700 
 
 710 
 
 720 
 
 730 
 
 740 
 
 750 
 
 760 
 
 770 
 
 
 
 
 
 
 
 
 
 
 
 
 
 2.06 2.10 
 
 2.13 
 
 2.16 
 
 2.19 
 
 2.22 
 
 2.26 
 
 2.29 
 
 2.32 
 
 2.35 
 
 2.39 
 
 2.42 
 
 2.45 
 
 2.48 
 
 2.08 
 
 2.12 
 
 2.15 
 
 2.18 
 
 2.21 
 
 2.25 
 
 2.28 
 
 2.31 
 
 2.34 
 
 2.38 
 
 2.41 2.44 
 
 2.47 
 
 2.51 
 
 .10 
 
 .14 
 
 .17 
 
 .20 
 
 .24 
 
 .27 
 
 .30 
 
 .33 
 
 .37 
 
 .40 
 
 .43 .47 
 
 .50 .53 
 
 .13 
 
 .16 
 
 .19 
 
 .23 
 
 .26 
 
 .29 
 
 .32 
 
 .36 
 
 .39 .42 .46 .49 
 
 .52 .56 
 
 .15 
 
 .18 
 
 .21 
 
 .25 
 
 .28 
 
 .31 
 
 .35 
 
 .38 
 
 .41 -45 .48 .52 
 
 .55 .58 
 
 .17 
 
 .20 
 
 .23 
 
 .27 
 
 .30 
 
 .34 
 
 .37 
 
 .40 
 
 .44 .47 .51 .54 
 
 .57 .61 
 
 
 
 
 
 
 
 
 
 
 
 2.19 
 
 2.22 
 
 2.26 
 
 2.29 
 
 2.32 
 
 2.36 
 
 2.39 
 
 2.43 
 
 2.46 2.50 2.53 2.56 
 
 2.60 2.63 
 
 .21 
 
 .24 
 
 .28 
 
 .31 
 
 .35 
 
 .38 
 
 .42 
 
 .45 
 
 .48 .52 .55 .59 
 
 .62 .66 
 
 .23 
 
 .26 
 
 .30 
 
 .33 
 
 .37 
 
 .40 
 
 .44 
 
 .47 
 
 .51 .54 .58 .61 .65 .68 
 
 .25 
 
 .29 
 
 .32 
 
 .36 
 
 .39 
 
 .43 
 
 .46 
 
 .50 
 
 .53 .57 .60 .64 
 
 .67 .71 
 
 .27 
 
 .31 
 
 .34 
 
 .38 
 
 .41 
 
 .45 
 
 .48 
 
 .52 
 
 .55 .59 .63 .66 
 
 .70 
 
 .73 
 
 2.29 
 
 2.33 
 
 2.36 
 
 2.40 
 
 2.43 
 
 2.47 
 
 2.51 
 
 2.54 
 
 2.58 2.61 2.65 2.69 
 
 2.72 
 
 2.76 
 
 .31 .35 
 
 .38 
 
 .42 
 
 .46 
 
 .49 .53 
 
 .57 
 
 .60 .64 .67 .71 .75 
 
 .78 
 
 .33 
 
 .37 
 
 .41 
 
 .44 .48 .52 ,55 
 
 .59 
 
 .62 .66 
 
 .70 .73 .77 
 
 .81 
 
 .35 
 
 .39 
 
 .43 
 
 .46 
 
 .50 .54 
 
 .57 
 
 .61 
 
 .65 .68 
 
 .72 .76 .80 
 
 .83 
 
 .37 
 
 .41 
 
 .45 
 
 .49 
 
 .52 
 
 .56 
 
 .60 
 
 .63 
 
 .67 .71 .75 .78 
 
 .82 
 
 .86 
 
 2.40 
 
 2.43 
 
 2.47 
 
 2.51 
 
 2.54 
 
 2. 58 . 
 
 2.62 
 
 2.66 
 
 2.69 2.73 
 
 2. 77 2. 81 2. 84 
 
 2.88 
 
 .42 
 
 .45 
 
 .49 
 
 .53 
 
 .57 
 
 .60 
 
 .64 
 
 .68 
 
 .72 
 
 .76 
 
 .79 
 
 .83 
 
 .87 
 
 .91 
 
 .44 
 
 .47 
 
 .51 
 
 .55 
 
 .59 
 
 .63 
 
 .67 
 
 .70 
 
 .74 
 
 .78 
 
 .82 
 
 .86 .89 
 
 .93 
 
 .46 
 
 .50 
 
 .53 
 
 .57 
 
 .61 
 
 .65 
 
 .69 
 
 .73 
 
 .76 
 
 .80 .84 
 
 .88 ' .92 
 
 .96 
 
 .48 
 
 .52 
 
 .56 
 
 .59 
 
 .63 .67 
 
 .71 
 
 .75 
 
 .79 
 
 .83 .87 .90 ; .94 
 
 .98 
 
 2.50 
 
 2.54 
 
 2.58 
 
 2.62 
 
 2.66 
 
 2.69 
 
 2.73 
 
 2.77 
 
 2.81 
 
 2. 85 2. 89 
 
 2.93 j 2.97 
 
 3.01 
 
 .52 
 
 .56 
 
 .60 
 
 .64 
 
 .68 
 
 .72 
 
 .76 
 
 .80 
 
 .83 
 
 .87 .91 
 
 .95 .99 
 
 .03 
 
 .54 
 
 .58 
 
 .62 
 
 .66 
 
 .70 
 
 .74 
 
 .78 
 
 .82 
 
 .86 
 
 .90 .94 
 
 .98 
 
 3.02 
 
 .06 
 
 .56 
 
 .60 
 
 .64 
 
 .68 
 
 .72 
 
 .76 
 
 .80 
 
 .84 
 
 .88 
 
 .92 .96 
 
 3.00 
 
 .04 
 
 .08 
 
 .58 
 
 .62 
 
 .66 
 
 .70 
 
 .74 
 
 .78 
 
 .82 
 
 .86 
 
 .90 
 
 .94 .99 
 
 .03 
 
 .07 
 
 .11 
 
 2.60 
 
 2.64 
 
 2.68 
 
 2.72 
 
 2.77 
 
 2.81 
 
 2.85 
 
 2.89 
 
 2.93 
 
 2.97 3.01 
 
 3.05 
 
 3.06 
 
 3.13 
 
 .62 
 
 .66 
 
 .71 
 
 .75 
 
 .79 
 
 .83 .87 
 
 .91 
 
 .95 
 
 .99 .03 
 
 .07 
 
 .12 
 
 .16 
 
 .64 
 
 .69 
 
 .73 
 
 .77 
 
 .81 
 
 .85 .89 
 
 .93 
 
 .97 
 
 3. 02 .06 . 10 
 
 .14 
 
 .18 
 
 .66 
 
 .71 
 
 .75 
 
 .79 
 
 .83 
 
 .87 .91 
 
 .96 
 
 3.00 
 
 .04 .08 .12 
 
 .16 
 
 .21 
 
 .69 
 
 .73 
 
 .77 
 
 .81 
 
 .85 
 
 .90 
 
 .94 
 
 .98 
 
 .02 
 
 .06 .11 .15 
 
 .19 
 
 .23 
 
 2.71 
 
 2.75 
 
 2.79 
 
 2.83 
 
 2.88 
 
 2.92 
 
 2.96 
 
 3.00 
 
 3.04 
 
 3.09 
 
 3. 13 3. 17 
 
 3.21 
 
 3.26 
 
 .73 
 
 -77 
 
 .81 
 
 .85 
 
 .90 
 
 .94 
 
 .98 
 
 .03 
 
 .07 .11 
 
 .15 .20 
 
 .24 
 
 .28 
 
 .75 .79 
 
 .83 
 
 .88 
 
 .92 
 
 .96 
 
 3.01 
 
 .05 
 
 .09 
 
 .13 
 
 .18 .22 
 
 .26 
 
 .31 
 
 .77 
 
 .81 
 
 .85 
 
 .90 
 
 .94 
 
 .99 
 
 .03 
 
 .07 
 
 .11 
 
 .16 
 
 .20 .24 
 
 .29 
 
 .33 
 
 .79 
 
 .83 
 
 .88 
 
 .92 
 
 .96 
 
 3.01 
 
 .05 
 
 .09 
 
 .14 
 
 .18 
 
 .22 .27 .31 
 
 .36 
 
 2.81' 
 
 2.85 
 
 2.90 
 
 2.94 
 
 2.99 
 
 3.03 
 
 3.07 
 
 3.12 
 
 3.16 
 
 3.20 
 
 3.25 
 
 3.29 
 
 3.34 
 
 3.38 
 
 .83 
 
 .87 
 
 .92 
 
 .96 
 
 3.01 
 
 .05 
 
 .10 
 
 .14 
 
 .18 .23 
 
 .27 .32 
 
 .36 
 
 .41 
 
 .85 
 
 .90 
 
 .94 
 
 .98 
 
 .03 
 
 .07 
 
 .12 
 
 .16 
 
 .21 .25 .30 .34 .39 
 
 .43 
 
 .87 
 
 .92 
 
 .96 
 
 3.01 
 
 .05 
 
 .10 
 
 .14 
 
 .19 
 
 .23 .28 .32 .37 .41 
 
 .46 
 
 .89 
 
 .94 
 
 .98 
 
 .03 
 
 .07 
 
 .12 
 
 .16 
 
 .21 
 
 .25 -30 .34 .39 .43 
 
 .48 
 
 2.91 
 
 2.96 
 
 3.00' 
 
 3.05 
 
 3.10 
 
 3.14 
 
 3.19 
 
 3.23 
 
 3.28 3.32 3.37 
 
 3.41 
 
 3.46 
 
 3.51 
 
 .93 .98 
 
 .03 
 
 .07 
 
 .12 
 
 .16 
 
 .21 
 
 .25 
 
 .30 .35 .39 
 
 .44 
 
 .48 
 
 .53 
 
 .95 3. 00 
 
 .05 
 
 .09 
 
 .14 
 
 .19 
 
 .23 
 
 .28 
 
 .32 .37 .42 
 
 .46 .51 
 
 .56 
 
 .98 
 
 .02 
 
 .07 
 
 .11 
 
 .16 
 
 .21 
 
 .25 
 
 .30 
 
 .35 ' .39 .44 
 
 .49 
 
 .53 
 
 .58 
 
 3.00 
 
 .04 
 
 .09 
 
 .14 
 
 .18 
 
 .23 
 
 .28 
 
 .32 
 
 .37 .42 .46 
 
 .51 
 
 .56 
 
 .60 
 
 3.02 3.06 
 
 3.11 
 
 3.16 
 
 3.21 
 
 3.25 
 
 3.30 
 
 3.35 
 
 3.39 
 
 3.44 3.49 
 
 3.54 
 
 3.58 
 
 3.63 
 
 .04 .08 
 
 .13 
 
 .18 .23 
 
 .27 
 
 .32 
 
 .37 
 
 .42 .46 .51 
 
 .56 .61 
 
 .65 
 
 .06 .11 
 
 .15 
 
 .20 
 
 .25 
 
 .30 
 
 .34 
 
 .39 
 
 .44 i .49 .54 
 
 .58 .63 
 
 .68 
 
 .08 1 .13 
 
 .18 
 
 .22 
 
 .27 
 
 .32 
 
 .37 
 
 .42 
 
 .46 .51 .56 
 
 .61 .66 
 
 .70 
 
 .10 .15 
 
 .20 
 
 .24 .29 
 
 .34 
 
 .39 
 
 .44 
 
 .49 .54 .58 
 
 .63 .68 
 
 .73 
 
 3.12 
 
 3.17 
 
 3.22 
 
 3.27 3.32 
 
 3.36 
 
 3.41 
 
 3.46 
 
 3.51 
 
 3.56 
 
 3.61 
 
 3. 66 3. 71 
 
 3.75 
 
 .14 
 
 .19 
 
 .24 
 
 .29 .34 
 
 .39 
 
 .44 
 
 .48 
 
 .53 
 
 .58 
 
 .63 
 
 . 68 .73 
 
 .78 
 
 .16 ! .21 
 
 .26 
 
 .31 .36 
 
 .41 
 
 .46 
 
 .51 
 
 .56 
 
 . -61 
 
 .66 
 
 .71 .75 
 
 .80 
 
 .18 
 
 .23 
 
 .28 
 
 .33 .38 
 
 .43 
 
 .48 
 
 '.53 
 
 .58 
 
 .63 
 
 .68 
 
 .73 .78 
 
 .83 
 
 .20 .25 
 
 .30 
 
 .35 .40 
 
 .45 .50 
 
 .55 
 
 .60 
 
 .65 
 
 .70 
 
 .75 .80 
 
 .85 
 
 3.22 
 
 3.27 
 
 3.32 
 
 3.37 3.43 
 
 3.48 3.53 
 
 3.58 
 
 3.63 
 
 3.68 
 
 3.73 
 
 3.78 i 3.83 
 
 3.88 
 
 .28 I .33 
 
 .38 
 
 .43 .48 
 
 .53 .58 
 
 .63 
 
 .68 
 
 .74 
 
 .79 
 
 .84 .89 
 
 .94 
 
 .33 
 
 .38 
 
 .43 
 
 .48 .54 
 
 .59 .64 
 
 .69 
 
 .74 
 
 .79 
 
 .85 
 
 .90 ! .95 
 
 4.00 
 
 .38 
 
 .43 
 
 .48 
 
 .54 .59 
 
 .64 
 
 .70 
 
 .75 
 
 .80 
 
 .85 
 
 .91 
 
 .96 
 
 4.01 
 
 .07 
 
 .43 
 
 .48 
 
 .54 
 
 .59 .64 
 
 .70 
 
 .75 
 
 .81 
 
 .86 
 
 .91 
 
 .97 
 
 4.02 
 
 .07 
 
 .13 
 
 3.48 
 
 3.54 3.59 
 
 3.65 3.70 
 
 3.75 
 
 3.81 
 
 3.86 
 
 3.92 
 
 3.97 
 
 4.03 
 
 4. 08 4. 13 
 
 4.19 
 
 .53 
 
 CQ 
 
 .64 
 
 .70 .75 
 
 .81 
 
 .87 
 
 .92 
 
 .98 
 
 4.03 
 
 .09 
 
 .14 .20 
 
 .25 
 
 .59 
 
 if>4 
 
 .70 
 
 .75 .81 
 
 .87 
 
 .92 
 
 .98 
 
 4.03 
 
 .09 
 
 .15 
 
 . 20 .26 
 
 .31 
 
 .64 
 
 .69 
 
 .75 
 
 .81 .86 
 
 .92 
 
 .98 
 
 4.03 
 
 .09 
 
 .15 
 
 .21 
 
 .26 i .32 
 
 .38 
 
 .69 
 
 .75 
 
 .80 
 
 .86 .92 
 
 .98 
 
 4.03 
 
 .09 
 
 .15 
 
 .21 
 
 .26 
 
 .32 .38 
 
 .44 ' 
 
 780 
 
BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 99 
 
 TABLE III. Influence of gravity on barometric observations, English units. 
 [Abridged from International Tables.] 
 REDUCTION TO LATITUDE 45. 
 
 10 
 11 
 12 
 LI 
 
 14 
 
 15 
 
 16 N 
 17 
 18 
 19 
 
 20 
 
 a 
 
 22 
 2:-! 
 M 
 
 25 
 
 
 27 
 28 
 29 
 
 M 
 
 n 
 
 32 
 
 n 
 
 de. 
 
 Reading of the barometer, in inches. 
 
 
 18 
 
 19 
 
 20 
 
 21 
 
 22 
 
 23 24 
 
 25 26 
 
 27 
 
 28 
 
 N 
 
 90 
 
 0.047 
 
 0.049 
 
 0.052 
 
 0.054 
 
 0.057 
 
 0.060 0.062 
 
 0.065 0.067 
 
 0.070 
 
 0.073 
 
 0.075 0.07S 
 
 85 
 
 .046 
 
 .048 
 
 .051 
 
 .054 
 
 .056 
 
 .059 .061 
 
 .064 .066 
 
 .069 .071 
 
 . 074 . 077 
 
 84 
 
 .046 
 
 .048 .051 
 
 .053 
 
 .056 
 
 .058 .061 
 
 .063 .066 
 
 .068 .071 
 
 .073 .076 
 
 83 
 
 .045 .048 .050 
 
 .053 
 
 .055 
 
 .058 .060 
 
 .063 .065 ' .068 
 
 .070 
 
 . 073 . 075 
 
 82 
 
 .046 .047 .050 
 
 .052 .055 
 
 .057 .060 
 
 .062 .065 .067 
 
 .070 
 
 . 072 . 07 j 
 
 81 
 
 .044 .047 .049 
 
 .052 
 
 .054 
 
 .057 .059 
 
 .062 .064 
 
 .067 .069 
 
 .071 .074 
 
 80 
 
 .044 .046 . 049 
 
 .051 
 
 .054 
 
 .056 .058 
 
 .061 .063 
 
 .066 
 
 .068 
 
 . 071 . 073 
 
 79 
 78 
 
 .043 
 .043 
 
 .046 .048 
 .045 .047 
 
 .050 
 .050 
 
 .053 
 .052 
 
 .055 .058 
 .054 .057 
 
 .060 .062 
 
 . 059 . Ot)2 
 
 .065 .067 
 .064 .066 
 
 .070 .072 
 .069 .071 
 
 77 
 
 .042 
 
 .044 .047 
 
 .049 .051 
 
 .054 .056 
 
 .058 .061 .063 .065 
 
 .068 
 
 76 
 
 . 041 .043 .046 
 
 .048 .050 
 
 .053 .055 
 
 .057 .059 .062 .064 
 
 .066 .069 
 
 75 
 
 .040 .043 .045 
 
 .047 
 
 .049 
 
 .052 .054 
 
 .956 .058 .061 .063^ 
 
 *.065 .067 
 
 74 
 
 .040 .042 .044 
 
 .046 
 
 .048 
 
 .051 .053 
 
 .055 .057 i .059 .06F 
 
 .064 .066 
 
 73 
 
 . 039 . 041 . 043 
 
 .045 .047 
 
 .049 .052 
 
 .054 .056 .058 .060 
 
 .062 . .064 
 
 72 
 
 .038 .040 
 
 .042 
 
 .044 
 
 .046 
 
 .048 .050 
 
 .052 
 
 .054 .057 
 
 .059 
 
 .061 .063 
 
 71 
 
 . 037 . 039 
 
 .041 
 
 .043 
 
 .045 
 
 .047 .049 
 
 .051 
 
 .053 
 
 .055 
 
 .057 
 
 .059 .061 
 
 
 
 
 
 
 
 
 
 
 
 
 70 
 
 . 036 
 
 .038 
 
 .040 
 
 .042 
 
 .044 
 
 .046 .048 
 
 .050 
 
 .052 
 
 .054 
 
 .056 
 
 .058 .060 
 
 69 
 
 .035 
 
 .037 
 
 .038 
 
 .040 
 
 .042 
 
 .044 .046 
 
 .048 
 
 .050 
 
 .052 
 
 .054 
 
 .056 .058 
 
 68 
 
 .034 
 
 .035 
 
 .037 
 
 .039 
 
 .041 
 
 .043 .045 
 
 .047 
 
 .048 
 
 .050 
 
 .052 
 
 .054 .066 
 
 67 
 
 .032 ! .034 
 
 .036 
 
 .038 
 
 .040 
 
 .041 . .043 
 
 .045 
 
 .047 
 
 .049 
 
 .050 
 
 .052 .054 
 
 66 
 
 .031 
 
 .033 
 
 .035 
 
 .036 
 
 .038 
 
 .040 .042 
 
 .043 
 
 .045 
 
 .047 
 
 .049 
 
 .050 .052 
 
 65 
 
 .030 
 
 .032 
 
 .033 
 
 .035 
 
 .037 
 
 .038 .040 
 
 .042 
 
 .043 
 
 .045 
 
 .047 
 
 .048 .050 
 
 64 
 
 .029 
 
 .030 
 
 .032 
 
 .033 
 
 .035 
 
 .037 .038 
 
 .040 
 
 .041 
 
 .043 
 
 .045 
 
 .046 .048 
 
 63 .027 
 
 .029 
 
 .030 
 
 .032 
 
 .033 
 
 .035 .037 
 
 .038 
 
 .040 
 
 .041 
 
 .043 
 
 .044 .046 
 
 62 .026 
 
 .028 
 
 .029 
 
 .030 
 
 .032 
 
 .033 .035 
 
 1008 
 
 .038 
 
 .039 
 
 .041 
 
 .042 .043 
 
 61 .025 
 
 .080 
 
 .027 .029 
 
 .030 
 
 .032 .033 
 
 .034 
 
 .036 
 
 .037 
 
 .038 
 
 .040 .041 
 
 60 .OJH 
 
 .025 
 
 .026 .027 
 
 .028 
 
 .030 .031 
 
 .032 
 
 .034 
 
 .035 
 
 .036 
 
 .038 .039 
 
 59 .022 
 
 .023 
 
 .024 .026 
 
 .027 
 
 .028 .029 
 
 .030 
 
 .032 
 
 .033 
 
 .034 
 
 .035 .036 
 
 58 .020 
 
 .022 
 
 .023 .024 
 
 .025 
 
 . 026 . 027 
 
 .028 
 
 .030 
 
 .031 
 
 .032 
 
 .033 .034 
 
 57 .019 
 
 .020 
 
 .021 .022 
 
 .023 
 
 . 024 . 025 
 
 .026 
 
 .027 
 
 .028 
 
 .029 
 
 .031 .032 
 
 56 . 017 
 
 .018 
 
 . 019 . 020 
 
 .021 
 
 .022 .023 
 
 .024 
 
 .025 
 
 .026 
 
 .027 
 
 .028 .029 
 
 oo . 016 
 
 .017 
 
 .018 .019 
 
 .019 
 
 . 020 . 021 
 
 .022 
 
 .023 
 
 .024 , .025 
 
 .026 .027 
 
 54 . 014 
 
 .01.5 
 
 .OK) .017 
 
 .018 
 
 .018 .019 
 
 .020 
 
 .021 
 
 .022 ' .022 
 
 .023 .024 
 
 53 , .013 
 
 .014 
 
 .014 .015 
 
 .016 
 
 .016 i017 
 
 .018 
 
 .019 
 
 .019, .020 
 
 .021 .021; 
 
 52 . Oil 
 
 .012 
 
 .013 .013 
 
 .014 
 
 .014 .015 
 
 .016 
 
 .016 
 
 .017 .018 
 
 .018' .om 
 
 51 .010 
 
 .010 
 
 .011 .011 
 
 .012 
 
 . 012 . 013 
 
 .013 
 
 .014 
 
 .015 .015 
 
 .016 .016 
 
 50 .008 
 
 .009 
 
 .009 .009 
 
 .010 
 
 .010 .011 
 
 .011 
 
 .012 
 
 .012 
 
 .013 
 
 .013 ' .013 
 
 49 . .006 
 
 .007 
 
 .007 .008 
 
 .008 
 
 .008 .009 
 
 .009 
 
 .009 
 
 .010 
 
 !oio 
 
 .010 .011 
 
 48 .005 
 
 .005 
 
 .005 .006 
 
 .006 
 
 .006 .006 
 
 .007 
 
 .007 
 
 .007 
 
 .008 
 
 .008 .008 
 
 47 . 003 
 
 .003 
 
 .004 .004 
 
 .004 
 
 .004 .004 
 
 .005 
 
 .005 
 
 .005 
 
 .006 
 
 ,008 .001 
 
 46 
 
 .002 
 
 .002 
 
 .002 .002 
 
 .002 
 
 .002 .002 
 
 .002 
 
 .002 
 
 .002 
 
 .003 
 
 .003' .003 
 
 II 
 
 .000 
 
 .000 
 
 .000 .000 
 
 .000 
 
 .000 .000 
 
 .000 
 
 .000 
 
 .000 
 
 .000 
 
 .000 .00)) 
 
 -Ob * 
 
 
100 BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 
 
 TABLE IV. Influence^of gravity on barometric observations, English units. 
 
 [From International Tables.] 
 
 REDUCTION FOR ALTITUDE. 
 
 To be subtracted. 
 
 Height in 
 feet. 
 
 Reading of the barometer, in inches. 
 
 18 
 
 19 
 
 20 
 
 21 
 
 22 
 
 23 
 
 24 
 
 25 
 
 26 
 
 27 28 29 
 
 30- 
 
 31 
 
 
 500 
 1,000 
 1,500 
 2,000 
 
 2,500 
 3,000 
 3,500 
 4,000 
 4,500 
 
 5,000 
 5,500 
 6,000 
 6,500 
 7,000 
 
 7,500 
 8,000 
 8,500 
 
 9; ooo 
 
 9,500 
 10,000 
 
 
 
 
 
 
 
 
 
 0.000 0.000 0.(XKL 
 . 001 . 001 K 001 
 .002 .(JO'S i .002 
 .002 .003 .003 
 .003 .003 .003 
 
 0.000 
 
 Tooi 
 
 .002 
 .003 
 .004 
 
 0.000 
 .001 
 .002 
 
 
 
 
 
 
 
 
 
 
 l 
 
 
 
 
 
 
 0.002 
 .003 
 
 
 
 
 
 
 
 
 
 
 
 0.004 .004 
 .004 .005 
 .005 .005 
 .006 ! .006 
 .007 i .007 
 
 .004 .004 .004 
 .005 .005 .005 
 .006 .006 
 .006 .007 
 
 
 
 
 
 
 
 
 
 0.005 
 .006 
 .006 
 
 
 
 0.006 
 
 007 
 
 
 
 
 
 
 
 .007 
 
 .008 
 .008 
 009 
 
 .007 
 .008 
 .009 
 009 
 
 .007 
 .008 
 .009 
 .010 
 
 .008 
 .009 
 .009 
 
 .008 
 
 
 
 6 (X)8 
 
 .008 
 009 
 
 
 
 
 
 
 
 
 
 
 .009 
 
 .009 
 .010 
 .011 
 .011 
 .012 
 
 .013 
 
 .009 
 
 .010 
 .011 
 .011 
 .012 
 .012 
 
 .013 
 
 .010 
 
 .010 
 .011 
 .012 
 .012 
 
 .010 
 
 .011 
 .011 
 
 .010 
 
 
 
 o.'oio" 
 
 .011 
 
 o.'oio" 
 
 .010 
 .011 
 
 .011 
 
 0.009 
 .010 
 .010 
 .011 
 .011 
 
 .012 
 
 ------ --- - 
 
 
 ------ 
 
 
 
 
 
 
 
 
 i 
 
 
 TABLE V. Pressure, in inches, corresponding to changes of 100 feet in elevation. 
 
 [BlGELOW.] 
 
 Height, in 
 
 Temperature, Fahrenheit. 
 
 
 feet. 
 
 
 
 
 
 
 
 
 
 i j 
 
 -20 -10 
 
 
 
 10 
 
 20 
 
 30 
 
 40 
 
 50 
 
 60 
 
 70 80 i 90 _ 
 
 
 
 0.128 0.125 
 
 0.122 
 
 0.119 
 
 a. 117 
 
 0.114 
 
 0.112 
 
 0-HP.L 
 
 0.108 
 
 0.106 
 
 0.104 
 
 0.102 
 
 500 
 
 . 125 . 122 
 
 .120 
 
 .117 
 
 .115 
 
 .112 
 
 .110 
 
 .108 
 
 **.106 
 
 .104 
 
 ,102 
 
 .100 
 
 1,000 
 
 . 122 . 120 
 
 .118 
 
 .115 
 
 .113 
 
 .110 
 
 .108 
 
 .106 
 
 .104 
 
 .102 
 
 .100 
 
 .098 
 
 1,500 
 
 . 120 . 117 
 
 .115 
 
 .112 
 
 .110 
 
 .108 
 
 .106 
 
 .104 
 
 .102 
 
 .100 
 
 .098 
 
 .096 
 
 2,000 
 
 .118 .115 
 
 .113 
 
 .110 
 
 .108 
 
 .106 
 
 .104 
 
 .102 
 
 .100 
 
 .098 
 
 .096 
 
 .094 
 
 2,500 
 
 .115 .112 
 
 .110 
 
 .108 
 
 .106 
 
 .104 
 
 .102 
 
 .100 
 
 . .098 
 
 .096 
 
 .094 
 
 .092 
 
 3,000 
 
 .113 .110 
 
 .108 
 
 .106 
 
 .104 
 
 .102 
 
 .100 
 
 .098 
 
 .096 
 
 .094 
 
 .092 
 
 .090 
 
 3,500 
 
 .111 .108 
 
 .106 
 
 .104 
 
 .102 
 
 .100 
 
 .098 .096 
 
 .094 
 
 .092 
 
 .090 
 
 .088 
 
 4,000 
 
 . 108 . 106 
 
 .104 
 
 .102 
 
 .100 
 
 .098 
 
 .096 
 
 .094 
 
 .092 
 
 .090 
 
 .088 
 
 .086 
 
 4,500 
 
 . 106 . 104 
 
 .102 
 
 .100 
 
 .098 
 
 .096 
 
 .094 
 
 .092 
 
 .090 
 
 .088 
 
 .086 
 
 .084 
 
 5,000 
 
 . 104 . 102 
 
 .100 
 
 .098- 
 
 .096 
 
 .094 .092 
 
 .090 
 
 .088 
 
 .086 
 
 .084 
 
 .082 
 
 5,500 
 
 .102 .100 
 
 .098 
 
 .096 
 
 .094 
 
 .092 .090 
 
 .088 
 
 .086 
 
 .084 
 
 .082 
 
 .081 
 
 6,000 
 
 .100 .098 
 
 .096 
 
 .094 
 
 .092 
 
 .090 .088 
 
 .086 
 
 .084 
 
 .082 
 
 .081 
 
 .080 
 
 6,500 
 
 .098 .096 
 
 .094 
 
 .092 
 
 .090 
 
 . 088 . 086 
 
 .085 
 
 .083 
 
 .081 
 
 .080 
 
 .079 
 
 7,000 
 
 . 097 . 095 
 
 .093 
 
 .091 
 
 .089* 
 
 .087 .085 .084 
 
 .082 
 
 .080 
 
 .079 
 
 .078 
 
BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 101 
 
 TABLE VI. Determination of heights by the barometer, English measures. 
 [Abridged from the Smithsonian Tables.] 
 
 29 90 
 Values of 60368 [1+0.0010195X361 log. -^ 
 
 
 
 
 
 
 
 
 
 Barometric 
 
 pressure, B. "" 
 
 .02 
 
 .03 
 
 .04 .05 
 
 .06 
 
 
 .08 
 
 .09 
 
 Inchet. 
 
 Feet. Feet. 
 
 Feet. 
 
 Feet. 
 
 Feet. Feet. 
 
 Feet. 
 
 Feet. Feet. 
 
 Feet. 
 
 17.00 
 
 15,347 
 
 15,331 
 
 15,315 
 
 15,299 
 
 15,283 15,267 
 
 16,261 
 
 15,235 1.VJ19 
 
 16,21 
 
 .10 
 
 15, 187 
 
 15, 172 
 
 15, 156 
 
 15, 140 
 
 15,124 15,108 
 
 16,092 
 
 15.081 
 
 
 .20 
 
 15,029 
 
 15,013 
 
 14.997 
 
 14,982 
 
 14,966 14,950 
 
 11. '<:;. 
 
 14,919 14,903 
 
 14,887 
 
 .30 
 
 14,871 
 
 14,866 
 
 14,840 
 
 14,824 
 
 14,809 14,793 
 
 14,777 
 
 14,746 
 
 
 .40 
 
 14,715 
 
 14,699 
 
 14,684 
 
 14,668 
 
 14,652 14,637 
 
 14,621 
 
 14,606 14,590 
 
 
 17.50 
 
 14,559 
 
 14,544 
 
 14,528 
 
 14,512 
 
 14, 497 14. 481 
 
 14,466 
 
 14.4M 14,435 
 
 14,420 
 
 .60 
 
 14.404 
 
 14,389 
 
 14,373 
 
 14.3.58 14,342 14;327 
 
 14,312 
 
 14,296 14,281 
 
 
 .70 
 
 14,260 
 
 14,235 
 
 14,219 
 
 14.204 14,189 14,173 
 
 14,168 
 
 14,143 14.12X' 
 
 14.11 
 
 .80 
 
 14,097 
 
 14,082 
 
 14,067 
 
 14,051 14,036 14,021 
 
 14,008 
 
 13.990 13,975 
 
 13,960 
 
 .90 
 
 13,945 
 
 13,930 
 
 13,914 
 
 13,899 13,884 13,869 
 
 13,854 
 
 13,839 13.824 
 
 13,808 
 
 18.00 
 
 13,793 
 
 13,778 
 
 13.763 
 
 13,748 13,733 13,718 
 
 13,703 
 
 13,688 13,673 
 
 13,658 
 
 .10 
 
 13,643 13,628 
 
 13,613 
 
 13,598 13,583 13,568 
 
 13,553 13,538 13,523 
 
 18,888 
 
 .20 
 
 13.493 13,478 
 
 13,463 
 
 13,448 ' 13,433 13,418 
 
 13,404 13,389 13,374 
 
 13.359 
 
 .30 
 
 13.344 13,329 
 
 13,314 
 
 13,300 13,285 13,270 
 
 13,255 13,240 1 
 
 13.211 
 
 .40 
 
 13, 196 
 
 13,181 
 
 13, 166 
 
 13,152 13,137 13,122 
 
 13,107 
 
 13,093 13,078 
 
 13,063 
 
 18.50 13,049 
 
 13,034 
 
 13,019 
 
 13,005 12,990 12,975 
 
 12,961 12,946 12,931 
 
 12.917 
 
 .60 12,902 12,888 
 
 12, 8V3 
 
 12,858 12,844 12,829 
 
 12.815 12,800 12,785 
 
 12.771 
 
 .70 12.7/ir. 12.742 12.727 
 
 12,713 12,698 12,684 
 
 12,669 12,655 12,640 
 
 
 .80 12,611 12,597 12,583 12,568 12,554 12,539 
 
 12,525 12,510 12,496 
 
 12,483 
 
 .90 12.407 
 
 12,453 
 
 12,438 12,424 12,410 
 
 12,395 
 
 12,381 12,367 12,352 
 
 
 19.00 12.324 
 
 12.310 12.295 12,281 12.267 
 
 12.252 
 
 12,238 12,224 12.210 
 
 12,19.-) 
 
 .10 12,181 12; 167 12,153 12,138 12,124 12,110 
 
 12,096 12,082 12,068 
 
 12,053 
 
 .20 12,039 12,025 
 
 12,011 11,997 11,983 
 
 11,969 
 
 11,954 11,940 11,926 
 
 11,912 
 
 .30 
 
 11,898 
 
 11,884 
 
 11,870 
 
 11,856 
 
 11,842 
 
 11,828 
 
 11,814 11,800 11,786 
 
 11.77. 
 
 .40 
 
 11,758 
 
 11,744 
 
 11,730 
 
 11,716 
 
 11,702 
 
 11,688 
 
 11,674 11,660 11,646 
 
 11,632 
 
 19.50 
 
 11,618 
 
 11,604 
 
 11,590 11,576 
 
 11,562 11,548 
 
 11,534 11,520 11,507 
 
 11,493 
 
 .60 11.4V9 11.465 11.451 11.437 
 
 11,423 11,410 
 
 11,396 11,382 11,368 
 
 11,354 
 
 .70 
 
 11,340 11,327 11,313 11,299 
 
 11,285 11,272 
 
 11,258 11,244 11,230 
 
 11.217 
 
 .80 
 
 11,203 11.189 
 
 11,175 11.162 
 
 11,148 11,134 
 
 11,121 11,107 11,093 
 
 11,080 
 
 .90 
 
 11,06(3 
 
 11,052 
 
 11,039 
 
 11,025 
 
 11,011 1 10,998 
 
 10,984 10,970 10,957 
 
 10,943 
 
 20.00 
 
 10.930 10,916 
 
 10,903 
 
 10,889 
 
 10,875 10,862 
 
 10,848 10.83ff 10,821 
 
 10,80* 
 
 .10 10.794 10.781 
 
 10.767 10,754 
 
 10,740 10,727 
 
 10,713 10,700 10,686 
 
 lo. ..7, 
 
 .20 
 
 10,659 10,646 
 
 10,632 10,619 
 
 10,605 10,592 
 
 10,579 10,565 10,652 
 
 
 .30 
 .40 
 
 10,525 
 10,391 
 
 10,512 
 10,378 
 
 10,498 10,485 
 10,365 10,352 
 
 10,472 10,458 
 10,338 10,325 
 
 10,4*5X0.431 10,418 
 10,312 10,298 10,285 
 
 10. 405 
 10.272 
 
 20.50 
 
 10,259 10,245 
 
 10,232 10,219 
 
 10,206 10,192 
 
 10,179 10,166 10,153 
 
 
 .60 
 
 10,126 10,113 
 
 10,100 10,087 
 
 10,074 
 
 10,060 
 
 10,047 10,034 
 
 10.021 
 
 10.II- - 
 
 .70 
 
 9,995 9,982 
 
 9,988 
 
 9,966 
 
 9,942 
 
 9,929 
 
 9,916 9,903 
 
 9,890 
 
 
 .80 
 
 9,864 9,851 
 
 9,838 
 
 9,825 
 
 9,812 
 
 9,799 
 
 9,786 ,772 
 
 9,759 
 
 
 .90 
 
 9,733 9,720 
 
 9,707 
 
 9,694 
 
 9,681 
 
 9,668 
 
 9.655 
 
 ,642 
 
 9,629 
 
 9,617 
 
 21.00 
 
 9,604 
 
 9.591 
 
 9,578 
 
 9,565 
 
 9,552 
 
 9,539 
 
 9,526 .513 
 
 9,500 
 
 9,487 
 
 .10 
 
 9, 474 9, 462 
 
 9,449 9,436 
 
 9,423 
 
 9,410 
 
 9,397 ,384 
 
 9,372 
 
 
 .20 
 
 9,346 9,333 
 
 9,320 9.307 
 
 9,295 
 
 9,282 
 
 9,269 ,256 
 
 9.244 
 
 9.231 
 
 .30 
 
 9.218 9.205 
 
 9.193 
 
 9,180 
 
 9,167 
 
 9,154 
 
 9,142 ,129 
 
 8,116 
 
 9,103 
 
 .40 
 
 9.091 
 
 9,078 
 
 9,065 
 
 9,053 
 
 9,040 
 
 9,027 
 
 9,015 
 
 9,002 
 
 8,989 
 
 8,977 
 
 21.50 
 
 8,964 8,951 
 
 8,939 
 
 8,926 
 
 8,913 
 
 8,901 
 
 8,888 8,876 
 
 8,881 
 
 8,850 
 
 . tt). 
 
 .70 
 
 8,838 8,825 
 8, 712 8. 700 
 
 8,813 
 8,687 
 
 8,800 
 8,675 
 
 8,788 
 8,662 
 
 8,775 
 8,650 
 
 8, 762 8. 750 
 8,637 8,625 
 
 8,737 
 8,8U 
 
 8.725 
 3,800 
 
 .80 
 
 .90 
 
 8,587 
 8,463 
 
 8,575 
 8,451 
 
 8,562 
 8,438 
 
 8,550 
 8,426 
 
 8,538 
 8,413 
 
 8,525 
 1 8,401 
 
 8,513 8,500 
 8,389 8,376 
 
 8,488 
 
 8,364 
 
 
 22.00 8,339 
 .10 8.216 
 
 8,327 
 8,204 
 
 8,314 
 8,191 
 
 8,302 
 8.179 
 
 8,290 
 8,167 
 
 8,277 
 8,154 
 
 8,265 8,253 
 8,142 8,130 
 
 V -'" 
 
 8,118 
 
 8.228 
 
 .20 
 
 8,098 
 
 8,061 
 
 8,069 8:056 
 
 8,044 
 
 8,032 
 
 8.020 8,008 
 
 7.995 
 
 7.'.^ 
 
 .30 
 .40 
 
 7,971 
 7,849 
 
 7.959 
 7.837 
 
 7,947 
 7,825 
 
 7,935 
 7,813 
 
 7,922 
 7,801 
 
 7,910 
 7,788 
 
 7.818 
 
 7,777 
 
 7,880 
 7,765 
 
 7.874 
 7,753 
 
 
 22.50 
 .60 
 
 7,728 
 7,608 
 
 7,716 
 
 7,692 
 7,572 
 
 7,680 
 7,560 
 
 7,668 
 7,548 
 
 7,656 
 
 7,644 
 7.524 
 
 7,632 
 
 7.51-; 
 
 7,820 
 
 7 BOO 
 
 .70 
 .80 
 .90 
 
 7,488 
 7,368 
 7,249 
 
 7J 476 
 7,288 
 
 5.452 
 
 7.214 
 
 7.440 
 7,321 
 
 7,428 
 
 7. 4i'. 
 
 7,178 
 
 7,288 
 
 7,166 
 
 TJtn 
 
 7,155 
 
 7J143 
 
102 BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 
 TABLE VI. Determination of heights by the barometer, English measures Con. 
 
 Barometric 
 
 .00 
 
 .01 
 
 .02 
 
 .03 
 
 .04 
 
 .05 
 
 .06 
 
 .07 
 
 r.os 
 
 .09 
 
 pressure, B. 
 
 
 
 
 
 
 
 
 
 
 
 Inches. 
 
 Feet. 
 
 Feet. 
 
 Feet. 
 
 Feet. 
 
 Feet. 
 
 Feet. 
 
 Feet. 
 
 Feet. 
 
 Feet. 
 
 Feet. 
 
 23.00 
 
 7,131 
 
 7,119 
 
 7,107 
 
 7,096 
 
 7,084 
 
 7,072 
 
 7,060 
 
 7,048 
 
 7,037 
 
 7,025 
 
 .10 
 
 7,013 
 
 7,001 
 
 6,990 
 
 6,978, 
 
 - 6,966 
 
 6,954 
 
 6,943 
 
 6,931 
 
 6,919 
 
 6,907 
 
 .20 
 
 6,896 
 
 6,884 
 
 6,872 
 
 6,861 
 
 6,849 
 
 6,837 
 
 6,825 
 
 6,814 
 
 6,802 
 
 6,790 
 
 .30 
 
 6,779 
 
 6,767 
 
 6,755 
 
 6,744 
 
 6,732 
 
 6,721 
 
 6,709 
 
 6,697 
 
 6,686 
 
 6,674 
 
 .40 
 
 6,662 
 
 6,651 
 
 6,639 
 
 6,628 
 
 6,616 
 
 6,604 
 
 6,593 
 
 6,581 
 
 6, 570 
 
 6,558 
 
 23.50 
 
 6,546 
 
 6,535 
 
 6,523 
 
 6,512 
 
 6,500 
 
 6,489 
 
 6,477 
 
 6,466 
 
 6,454 
 
 6,443 
 
 .60 
 
 6,431 
 
 6,420 
 
 6,408 
 
 6,397 
 
 6,385 
 
 6,374 
 
 6,362 
 
 6,351 
 
 6,339 
 
 6,328 
 
 .70 
 
 6,316 
 
 6,305 
 
 6,293 
 
 6,282 
 
 6,270 
 
 6,259 
 
 6,247 
 
 6,236 
 
 6,225 
 
 2,213 
 
 .80 
 
 6,202 
 
 6,190 
 
 6,179 
 
 6,167 
 
 6,156 
 
 6.145 
 
 6,133 
 
 6,122 
 
 6,110 
 
 6,099 
 
 .90 
 
 6,088 
 
 6,076 
 
 6,065 
 
 6,054 
 
 6,042 
 
 6,031 
 
 6,020 
 
 6,008 
 
 5,997 
 
 5,986 
 
 24.00 
 
 5,974 
 
 5,963 
 
 5,952 
 
 5,940 
 
 5,929 
 
 5,918 
 
 5,906 
 
 5,895 
 
 5,884 
 
 5,872 
 
 .10 
 
 5,861 
 
 5,850 
 
 5,839 
 
 5,827 
 
 5,816 
 
 5,805 
 
 5,794 
 
 5,782 
 
 5.771 
 
 5,760 
 
 .20 
 
 5,749 
 
 5,737 
 
 5,726 
 
 5,715 
 
 5,704 
 
 5,693 
 
 5,681 
 
 5,670 
 
 5,659 
 
 5,648 
 
 .30 
 
 5,637 
 
 5,625 
 
 5,614 
 
 5,603 
 
 5,593 
 
 5,581 
 
 5,570 
 
 5,558 
 
 5,547 
 
 5,536 
 
 .40 
 
 5,525 
 
 5,514 
 
 5,503 
 
 5,492 
 
 5,480 
 
 5,469 
 
 5,458 
 
 5,447 
 
 5,436 
 
 5,425 
 
 . 24.50 
 
 5,414 
 
 5,403 
 
 5,392 
 
 5,381 
 
 5,369 
 
 5,358 
 
 5,347 
 
 5,336 
 
 5,325 
 
 5,314 
 
 .60 
 
 5,303 
 
 5,292 
 
 5,281 
 
 5,270 
 
 5,259 
 
 5,248 
 
 5,237 
 
 5,226 
 
 5,215 
 
 5,204 
 
 .70 
 
 5,193 
 
 5,182 
 
 5,171 
 
 5,160 
 
 5,149 
 
 5,138 
 
 5,127 
 
 5,116 
 
 5,105 
 
 5,094 
 
 .80 
 
 5,083 
 
 5,072 
 
 5,061 
 
 5,050 
 
 5,039 
 
 5,028 
 
 5,017 
 
 5,006 
 
 ,995 
 
 4,985 
 
 .90 
 
 4,974 
 
 4,963 
 
 4,952 
 
 4,941 
 
 4,930 
 
 4,919 
 
 4,908 
 
 4,897 
 
 ,886 
 
 4,876 
 
 25.00 
 
 4,865 
 
 4,854 
 
 4,843 
 
 4,832 
 
 4,821 
 
 4,810 
 
 ,800 
 
 4,789 
 
 ,778 
 
 4,767 
 
 .10 
 
 4,756 
 
 4,745 
 
 4,735 
 
 4,724 
 
 4,713 
 
 4,702 
 
 ,691 
 
 4,681 
 
 ,670 
 
 4,659 
 
 .20 
 
 4,648 
 
 4,637 
 
 4,627 
 
 4,616 
 
 4,605 
 
 4,594 
 
 ,584 
 
 4,573 
 
 ,562 
 
 4,551 
 
 .30 
 
 4,540 
 
 4,530 
 
 4,519 
 
 4,508 
 
 4,498 
 
 4,487 
 
 ,476 
 
 4,465 
 
 ,455 
 
 4,444- 
 
 .40 
 
 4,433 
 
 4,423 
 
 4,412 
 
 4,401 
 
 4,391 
 
 4,380 
 
 ,369 
 
 4,358 
 
 ,348 
 
 4,337 
 
 25.50 
 
 4,326 
 
 4,316 
 
 4,305 
 
 4,295 
 
 4,284 
 
 4,273 
 
 ,263 
 
 4,252 
 
 4,241 
 
 4,231 
 
 .60 
 
 4,220 
 
 4,209 
 
 4,199 
 
 4,188 
 
 4,178 
 
 4,167 
 
 ,156 
 
 4,146 
 
 4,135 
 
 4,125 
 
 .70 
 
 4,114 
 
 4,104 
 
 4.093 
 
 4,082 
 
 4,072 
 
 4,061 
 
 ,051 
 
 4,040 
 
 4,030 
 
 4,019 
 
 .80 
 
 4,009 
 
 3,998 
 
 3,988 
 
 3,977 
 
 3,966 
 
 3,956 
 
 3,945 
 
 3,935 
 
 3,924 
 
 3,914 
 
 .90 
 
 3,903 
 
 3,893 
 
 3,882 
 
 3,872 
 
 3,861 
 
 3,851 
 
 3,841 
 
 3,830 
 
 3,820 
 
 3,809 
 
 26.00 
 
 3,799 
 
 3,788 
 
 3,778 
 
 3,767 
 
 3,757 
 
 3,746 
 
 3,736 
 
 3,726 
 
 3,715 
 
 3,705 
 
 .10 
 
 3,694 
 
 3,684 
 
 3,674 
 
 3,663 
 
 3,653 
 
 3,642 
 
 3,632 
 
 3,622 
 
 3,611 
 
 3,601 
 
 .20 
 
 3,590 
 
 3,580 
 
 3,570 
 
 3,559 
 
 3,549 
 
 3,539 
 
 3,528 
 
 3,518 
 
 3,508 
 
 3,497 
 
 .30 
 
 3,487 
 
 3,477 
 
 3,466 
 
 3,456 
 
 3,446 
 
 3,435 
 
 3,425 
 
 3,415 
 
 3,404 
 
 3,394 
 
 .40 
 
 3,384 
 
 3,373 
 
 3,363 
 
 3,353 
 
 3,343 
 
 3,332 
 
 3,322 
 
 3,312 
 
 3,301 
 
 3,291 
 
 26.50 
 
 3,281 
 
 3,270 
 
 3,260 
 
 3,250 
 
 3,240 
 
 3,230 
 
 3,219 
 
 3,209 
 
 3,199 
 
 3,189 
 
 .60 
 
 3,179 
 
 3,168 
 
 3,158 
 
 3,148 
 
 3,138 
 
 3,128 
 
 3,117 
 
 3,107 
 
 3,097 
 
 3,087 
 
 .70 
 
 3,077 
 
 3,066 
 
 3,056 
 
 3,046 
 
 3,036 
 
 3,026 
 
 3,016 
 
 3,005 
 
 2,995 
 
 2,985 
 
 .80 
 
 2,975 
 
 2,965 
 
 2,955 
 
 2,945 
 
 2,934 
 
 2,924 
 
 2,914 
 
 2,904 
 
 2,894 
 
 2,884 
 
 .90 
 
 2,874 
 
 2,864 
 
 2,854 
 
 2,843 
 
 2,833 
 
 2,823 
 
 2,813 
 
 2,803 
 
 2,793 
 
 2,783 
 
 27.00 
 
 2,773 
 
 2,763 
 
 2,753 
 
 2,743 
 
 2,733 
 
 2,723 
 
 2,713 
 
 2,703 
 
 2,692 
 
 2,682 
 
 .10 
 
 2,672 
 
 2,662 
 
 2,652 
 
 2,642 
 
 2,632 
 
 2,622 
 
 2,612 
 
 2,602 
 
 2,592 
 
 2,582 
 
 .20 
 
 2,572 
 
 2,562 
 
 2,552 
 
 2,542 
 
 2,532 
 
 2,522 
 
 2,512 
 
 2,502 
 
 2,493 
 
 2,483 
 
 .30 
 
 2,473 
 
 2,463 
 
 2,453 
 
 2,443 
 
 2,433 
 
 2,423 
 
 2,413 
 
 2,403 
 
 2,393 
 
 2,383 
 
 .40 
 
 2,373 
 
 2,363 
 
 2,353 
 
 2,343 
 
 2,334 
 
 2,324 
 
 2,314 
 
 2, 304 
 
 2,294 
 
 2,284 
 
 27.50 
 
 2,274 
 
 2,264 
 
 2,254 
 
 2,245 
 
 2,235 
 
 2,225 
 
 2,215 
 
 2,205 
 
 2,195 
 
 2,185 
 
 .60 
 
 2,176 
 
 2,166 
 
 2,156 
 
 2,146 
 
 2,136 
 
 2,126 
 
 2,116 
 
 2,107 
 
 2,097 
 
 2,087 
 
 .70 
 
 2,077 
 
 2,067 
 
 2,058 
 
 2,048 
 
 2,038 
 
 2,028 
 
 2,018 
 
 2,009 
 
 1,99 
 
 1,989 
 
 .80 
 
 1,979 
 
 1,970 
 
 1,960 
 
 1,950 
 
 1,940 
 
 1,930 
 
 1,921 
 
 1,911 
 
 1,901 
 
 1,891 
 
 .90 
 
 1,882 
 
 1,872 
 
 1,862 
 
 1,852 
 
 1,843 
 
 1,833 
 
 1,823 
 
 1,814 
 
 1,804 
 
 1,794 
 
 28.00 
 
 1,784 
 
 1,775 
 
 1,765 
 
 1,755 
 
 1,746 
 
 1,736 
 
 1,726 
 
 1,717 
 
 1,707 
 
 1,697 
 
 .10 
 
 1,688 
 
 1,678 
 
 1,668 
 
 1,659 
 
 1,649 
 
 1,639 
 
 1,630 
 
 1,620 
 
 1,610 
 
 1,601 
 
 .20 
 
 1,591 
 
 1,581 
 
 1,572 
 
 1,562 
 
 1,552 
 
 1,543 
 
 1,533 
 
 1,524 
 
 1,514 
 
 1,504 
 
 .30 
 
 1,495 
 
 1,485 
 
 1,476 
 
 1,466 
 
 1,456 
 
 1,447 
 
 1,437 
 
 1,428 
 
 1,418 
 
 1,408 
 
 .40 
 
 1,399 
 
 1,389 
 
 1,380 
 
 1,370 
 
 1,361 
 
 1,351 
 
 1,342 
 
 1,332 
 
 1,322 
 
 1,313 
 
 28.50 
 
 1,303 
 
 1,294 
 
 1,284 
 
 1,275 
 
 1,265 
 
 1,256 
 
 1,246 
 
 1,237 
 
 1,227 
 
 1,218 
 
 .60 
 
 1,208 
 
 1,199 
 
 1,189 
 
 1,180 
 
 1,170 
 
 1,161 
 
 1,151 
 
 , 1,142 
 
 1,132 
 
 1,123 
 
 .70 
 
 1,113 
 
 1,104 
 
 1,094 
 
 1,085 
 
 1,075 
 
 1,066 
 
 1,057^ 
 
 1,047 
 
 1,038 
 
 1,028 
 
 .80 
 
 1,019 
 
 1,009 
 
 1,000 
 
 990 
 
 981 
 
 972 
 
 962 
 
 953 
 
 943 
 
 934 
 
 .90 
 
 925 
 
 915 
 
 906 
 
 896 
 
 887 
 
 878 
 
 868 
 
 859 
 
 849 
 
 840 
 
 29.00 
 
 831 
 
 821 
 
 812 
 
 803 
 
 794 
 
 784 
 
 775 
 
 765 
 
 756 
 
 746 
 
 .10 
 
 737 
 
 728 
 
 718 
 
 709 
 
 700 
 
 690 
 
 681 
 
 672 
 
 663 
 
 653 
 
 .20 
 
 644 
 
 635 
 
 625 
 
 616 
 
 607 
 
 597 
 
 588 
 
 57ft 
 
 . 570 
 
 560 
 
 .30 
 
 551 
 
 542 
 
 532 
 
 523 
 
 514 
 
 505 
 
 495 
 
 486 
 
 477 
 
 468 
 
 .40 
 
 458 
 
 449 
 
 440 
 
 431 
 
 421 
 
 412 
 
 403 
 
 394 
 
 384 
 
 375 
 
BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 103 
 TABLE VI. Determination of heights by the barometer, English measures < 
 
 Barometric 
 pressure, B. 
 
 .00 
 
 .01 
 
 .02 
 
 .03 .04 
 
 .05 .06 
 
 i 
 
 .07 
 
 .08 
 
 Inches. 
 
 Feet. 
 
 Feet. ' 
 
 Feet. 
 
 Feet. Feet. 
 
 Feet. Feet. 
 
 Feet. 
 
 Feet. Feet. 
 
 29.50 
 
 366 
 
 357 348 
 
 338 329 
 
 320 311 
 
 302 
 
 292 
 
 .00 
 
 274 
 
 256 
 
 247 237 
 
 228 219 
 
 201 192 
 
 .70 
 
 182 
 
 173 164 
 
 155 146 
 
 137 128 
 
 118 
 
 109 100 
 
 .80 
 
 + 91 
 
 + 82 +73 
 
 + 64 +55 
 
 + 45 + .36 : + 27 
 
 + 18 +9 
 
 .90 
 
 
 
 - 9 - 18 
 
 - 27 -36 
 
 - 45 - 55 
 
 - 64 
 
 - 73 
 
 30.00 
 
 - 91 
 
 -100 -109 
 
 -118 -127 
 
 -136 -145 -154 
 
 -163 -172 
 
 .10 
 
 -181 
 
 -190 
 
 -199 
 
 -208 -217 
 
 -226 -235 -244 
 
 -253 -262 
 
 .30 
 
 -271 
 
 -280 
 
 -289 
 
 -298 -307 
 
 -316 -325 -334 
 
 -343 -352 
 
 .30 
 
 -361. 
 
 -370 
 
 -379 
 
 -388 -397 
 
 -406 -415 -424 
 
 -433 -442 
 
 .40 
 
 -451 
 
 -460 
 
 -469 
 
 -478 -486 
 
 -495 -504 
 
 -513 
 
 -522 -531 
 
 30.50 
 
 -540 
 
 -549 
 
 -558 
 
 -567 -576 
 
 -585 -593 
 
 -602 
 
 -611 -620 
 
 .60 
 
 -629 
 
 -638 
 
 -647 
 
 -656 -665 
 
 -673 -682 -691 
 
 -700 -709 
 
 .70 
 
 -718 
 
 -727 
 
 -735 
 
 -744 -753 
 
 -762 -771 -780 
 
 -788 -797 
 
 .80 
 
 -806 
 
 -815 
 
 -824 
 
 -833 -841 
 
 -850 -859 
 
 -868 
 
 -877 -885 
 
104 BAROMETERS AND MEASUREMENT OF ATMOSPHERIC PRESSURE. 
 
 TABLE VII. Determination of heights by the barometer, English measures. 
 [Abridged from the Smithsonian Tables.] 
 Term for temperature: 0.002039 (T-50) z. 
 
 For temperatures {$ I ?;} the values are * be 
 
 Mean tempera- 
 ture T. 
 
 Approximate elevations obtained from Table VI. 
 
 1,000 
 
 2,000 i 3,000 
 
 4,000 
 
 5,000 
 
 6,000 
 
 7,000 
 
 8,000 
 
 : 9,000 
 
 10,000 
 
 F. 
 
 F. 
 
 Feet. 
 
 Feet. Feet. 
 
 Feet. 
 
 Feet. 
 
 Feet. 
 
 Feet. 
 
 Feet. 
 
 Feet. 
 
 Feet. 
 
 49 
 
 51 2 
 
 4 6 
 
 8 
 
 10 
 
 12 
 
 14 16 
 
 18 
 
 20 
 
 48 
 
 52 4 
 
 8 12 
 
 16 
 
 20 
 
 24 
 
 29 33 
 
 37 
 
 41 
 
 47 
 
 53 6 
 
 12 18 
 
 24 
 
 31 
 
 37 43 49 
 
 55 
 
 61 
 
 46 
 
 54 8 
 
 16 24 
 
 33 
 
 41 
 
 49 ' 57 05 
 
 73 
 
 82 
 
 45 
 
 55 10 
 
 20 31 41 
 
 51 
 
 61 71 82 
 
 92 
 
 102 
 
 44 
 
 56 12 
 
 24 37 49 
 
 61 
 
 73 86 98 
 
 110 
 
 122 
 
 43 
 
 57 14 
 
 29 43 
 
 57 
 
 71 
 
 86 100 , 114 
 
 128 
 
 143 
 
 42 
 
 58 16 
 
 33 49 
 
 65 
 
 82 
 
 98 | 114 130 
 
 147 
 
 163 
 
 41 
 
 59 18 
 
 37 55 
 
 73 
 
 92 
 
 110 128 ! 147 
 
 165 
 
 184 
 
 40 
 
 60 
 
 ^ 
 
 20 
 
 41 61 
 
 82 
 
 102 
 
 122 
 
 143 
 
 163 
 
 
 204- 
 
 39 
 
 61' 
 
 22 
 
 45 67 
 
 90 
 
 112 
 
 135 
 
 157 
 
 179 
 
 
 224 
 
 38 
 
 62 24 
 
 49 73 
 
 98 
 
 122 
 
 147 171 196 
 
 220 
 
 245 
 
 37 
 
 63 27 
 
 53 80 
 
 106 
 
 133 
 
 159 186 212 
 
 239 
 
 265 
 
 36 
 
 64 
 
 29 
 
 57 86 
 
 114 
 
 143 
 
 171 200 228 
 
 257 
 
 285 
 
 35 
 
 65 
 
 31 
 
 61 92 
 
 122 
 
 153 
 
 184 214 
 
 245 
 
 275 
 
 306 
 
 34 
 
 66 
 
 33 
 
 65 98 
 
 130 163 
 
 196 228 
 
 261 
 
 294 
 
 326 
 
 33 
 
 67 35 
 
 69 104 
 
 139 173 
 
 208 243 277 
 
 312 
 
 347 
 
 32 
 
 68 i 37 
 
 73 
 
 110 
 
 147 184 
 
 220 257 294 
 
 330 
 
 367 
 
 31 
 
 69 39 
 
 77 
 
 116 
 
 155 194 
 
 232 271 310 
 
 349 
 
 387 
 
 30 
 
 70 41 
 
 82 
 
 122 
 
 163 204 
 
 245 285 326 
 
 367 
 
 408 
 
 29 
 
 71 43 
 
 86 
 
 128 
 
 171 214 
 
 257 
 
 300 343 
 
 385 
 
 428 
 
 28 
 
 72 45 
 
 90 
 
 135 
 
 179 224 1 269 314 359 
 
 404 
 
 449 
 
 27 
 
 73 47 
 
 94 
 
 141 
 
 188 234 ! 281 328 375 
 
 422 
 
 469 
 
 26 
 
 74 49 
 
 98 
 
 147 
 
 1% 245 294 I 343 391 
 
 440 
 
 489 
 
 25 
 
 75 
 
 51 
 
 102 
 
 153 
 
 204 255 
 
 306 357 408 
 
 459 
 
 510 
 
 24 
 
 76 
 
 53 
 
 106 
 
 159 
 
 212 265 
 
 318 
 
 371 424 
 
 477 
 
 530 
 
 23 
 
 77 
 
 55 
 
 110 
 
 165 
 
 220 275 
 
 330 
 
 385 440 
 
 495 
 
 551 
 
 22 
 
 78 
 
 57 
 
 114 
 
 171 
 
 228 285 
 
 343 
 
 400 
 
 457 
 
 514 
 
 571 
 
 21 
 
 79 
 
 59 
 
 118 
 
 177 
 
 236 296 
 
 355 
 
 414 
 
 473 
 
 532 
 
 591 
 
 20 
 
 80 
 
 61 
 
 122 
 
 184 
 
 245 306 
 
 367 
 
 428 489 
 
 551 
 
 612 
 
 19 
 
 81 
 
 63 
 
 126 
 
 190 
 
 253 316 
 
 379 442 
 
 506 
 
 569 
 
 632 
 
 18 
 
 82 
 
 65 
 
 130 
 
 196 
 
 261 326 391 457 522 
 
 587 
 
 652 
 
 17 83 
 
 67 
 
 135 
 
 202 
 
 269 336 404 471 538 
 
 606 
 
 673 
 
 16 - 84 
 
 69 
 
 139 
 
 208 
 
 277 347 
 
 416 485 
 
 555 
 
 624 
 
 693 
 
 15 85 
 
 71 
 
 143 
 
 214 
 
 285 357 
 
 428 
 
 500 
 
 571 
 
 642 
 
 714 
 
 
 
 
 
 
 
 
 
 
 14 86 
 
 73 
 
 147 
 
 220 
 
 294 367 
 
 440 514 ; 587 
 
 661 
 
 734 
 
 13 87 
 
 75 
 
 151 
 
 226 
 
 302 377 j 453 528 604 
 
 679 
 
 754 
 
 12 88 
 
 77 
 
 155 
 
 232 
 
 310 387 ., 46? 542 620 
 
 697 
 
 775 
 
 11 89 
 
 80 
 
 159 
 
 239 
 
 318 398 477 557 
 
 636 
 
 716 
 
 795 
 
 10 90 
 
 82 
 
 163 
 
 245 
 
 326 408 
 
 489 \ 571 652 
 
 734 
 
 816 
 
 9 91 
 
 84 
 
 167 
 
 251 
 
 334 ! 418 
 
 502 585 
 
 669 
 
 752 
 
 836 
 
 8 92 
 
 86 
 
 171 
 
 257 
 
 343 428 514 599 
 
 685 
 
 771 
 
 856 
 
 7 93 
 
 88 
 
 175 
 
 263 
 
 351 438 526 614 
 
 701 
 
 789 
 
 877 
 
 6 94 
 
 90 
 
 179 
 
 269 
 
 359 449 
 
 538 
 
 628 
 
 718 
 
 807 
 
 897 
 
 5 95 
 
 92 
 
 184 
 
 275 
 
 367 459 ; 
 
 551 
 
 642 
 
 734 
 
 826 
 
 918 
 
 
 
 
 
 
 
 
 
 
 
 4 96 
 
 94 
 
 188 
 
 281 
 
 375 469 
 
 . 
 563 657 
 
 750 
 
 844 
 
 938 
 
 3 97 
 
 96 
 
 192 
 
 287 
 
 383 479 
 
 575 671 
 
 767 
 
 862 
 
 958 
 
 2 98 
 
 98 
 
 196 
 
 294 
 
 391 489 587 685 783 
 
 881 
 
 979 
 
 1 99 
 
 100 
 
 200 
 
 300 
 
 400 500 599 699 799 
 
 899 
 
 999 ' 
 
 100 
 
 102 
 
 204 
 
 306 
 
 408 510 612 714 
 
 816 
 
 918 
 
 1,020 
 
INDEX. 
 
 Paragraph. 
 
 Actual elevation defined J 7 1 
 
 Adjustment of barographs at high elevations 154 
 
 to station pressure 153, 172 
 
 Attached thermometer 8 
 
 Atmospheric pressure 4 
 
 Authority to clean barometers 187 
 
 Air and moisture in barometer tubes 130, 131 
 
 can not be perfectly removed from barometer tubes 133 
 
 how removed from barometer tubes 133 
 
 in barometer tubes . . - 129 
 
 vent in cistern of barometer Ill 
 
 Tuch cisterns 143, 144 
 
 Aneroid barographs, Richard's 88 
 
 barometer 6, 46-58 
 
 "creeping of" 56 
 
 defects in : 49, 55 
 
 effects of temperature 48, 51 
 
 Goldschmidt's 50 
 
 how adjusted 54 
 
 , in determining heights 105, 106 
 
 reading, how made 52 
 
 test of condition 58 
 
 Barographs 68,69,191 
 
 adjusted for high elevations 154 
 
 to standard pressure 153 
 
 to station pressure 1 " 
 
 aneroid, Richard's 
 
 clocks for, regulated and wound 157 
 
 compensated siphon, Marvin system 72-87 
 
 check observations 87 
 
 cleaning mercury 
 
 dismantling 
 
 magnifying and recording mechanisms 
 
 temperature compensation ... 80, 81 
 
 time checks 
 
 marker 85, 86 
 
 corrections for errors of 
 
 exposure of 89, 152, 176 
 
 Foreman's 
 
 forms for, changed 
 
 refigured 
 
 trimmed 
 
 hourly reading of 
 
 how exposed 
 
 105 
 

 
 106 INDEX. 
 
 
 
 Paragraph. 
 
 Barographs, Marvin's normal. 71 
 
 pens for, cleaned 159 
 
 time error, checked 158, 192 
 
 weighing 69 
 
 Barometer, air vent of cistern Ill 
 
 adjustment of cistern 97 
 
 vernier 98 
 
 aneroid 6 
 
 at elevated stations, how handled 114 
 
 boxes for 91 
 
 old style 93 
 
 marine 96 
 
 care and preservation of 108, 115 
 
 change or substitution of 116 
 
 changes in location of 148 
 
 cisterns, how to tighten screws of 1 136 
 
 not to be strained 16, 111, 114 
 
 comparison of, private 194 
 
 corrections, for capillarity 41 
 
 imperfect vacuum 41 
 
 instrumental or scale error 41 
 
 temperature 41 
 
 correction cards, Form No. 1059 Met'l 42, 170 
 
 diagonal 62 
 
 dial.. 63 
 
 errors of 39, 40 
 
 exposure of 89, 152, 176 
 
 Fortin, Weather Bureau pattern 7 
 
 frequent cleaning of cisterns to be avoided 117 
 
 glass cisterns of, washed 135 
 
 tubes of, to be cleaned 128 
 
 glycerin ' 61 
 
 Green 7 
 
 holosteric 6 
 
 how carried Ill 
 
 disposed on steamboats, etc 147 
 
 inverted 110,111 
 
 set and observed 97 
 
 to open cistern of leaky 126 
 
 packed 145 
 
 unpacked ' 109 
 
 Howson's 65 
 
 instructions for cleaning 123-144, 187 
 
 marine 26-31 
 
 box for 96 
 
 handling of 112 
 
 magnifying 66, 67 
 
 measurement of height by 105, 107 
 
 moving and packing of 145 
 
 normal 45 
 
 not to be changed without authority - 185 
 
 parts of assembled 136 
 
 pumping of 89a 
 
9 
 
 INDEX. 107 
 
 Paragraph. 
 
 Barometer, readings completely corrected 171 
 
 reduction to sea level 103 
 
 standard gravity 102 
 
 report on defective or unserviceable 150 
 
 scale of, cleaned 128 
 
 screwed up for transportation 145 
 
 screws of cistern tightened regularly 136, 141 
 
 shipment of empty 151 
 
 serviceable 145, 149 
 
 siphon 3 
 
 spiral : 62 
 
 standard 45 
 
 "station" and "extra" defined 175 
 
 Torricelli's 2 
 
 tube, air removed from 132 
 
 tubes, filling of 32-37 
 
 air-pump method 36 
 
 boiling method 34 
 
 funnel method 33 
 
 cleaning of large 35 
 
 vacua in 38 
 
 Tuch, cisterns cleaned 143 
 
 verticality of 90, 92, 99 
 
 water , 60 
 
 when to make change in location of 186 
 
 Boxes for barometers 91 
 
 old style 93 
 
 Capacity correction, how found 24 
 
 Capillarity, correction for 174 
 
 Cards, barometer correction, Form No. 1059 Met'l 42, 170 
 
 Care and preservation of barometers 108 
 
 Carrying cases, leather 146 
 
 Change in location of barometer 148 
 
 when made 186 
 
 or substitution of barometers * 116, 185, 186 
 
 Cistern, adjustment of 97 
 
 filtering mercury into 140 
 
 fixed and adjustable . 18 
 
 glass part washed 135 
 
 of barometer 9 
 
 not to be strained Ill, 114 
 
 frequently cleaned 117 
 
 screws of, tightened regularly. . . 136, 141 
 
 ' straining of 16, 111, 114 
 
 Tuch, instructions for cleaning 143 
 
 pattern described 
 
 Cleaning around ivory point , 
 
 barometers, instructions for. 123-144, 187 
 
 pens for barograph 
 
 Clocks for barographs, regulated and wound 
 
 Comparative barometer readings 118-122, 18 
 
 object of H9 
 
 recorded on Form No. 1027 Met'l 166 
 
108 INDEX. 
 
 Paragraph. 
 
 Comparative barometer readings when levels can not be run 166 
 
 readings, interval for, after, cleaning. 121, 142 
 
 order for making, on removal of office 166 
 
 Correction of barometer readings, example 171 
 
 for capacity 18, 24 
 
 density of mercury 44 
 
 error of barograph 160 
 
 imperfect vacuum 41, 174 
 
 scale errors, capillarity, etc 41, 174 
 
 temperat.ure 41, 43, 17' 
 
 tables for 195 
 
 provisional removal 182 ; 198 
 
 variable removal 174, 182a 
 
 Determination of heights by aneroids 104, 105, 107 
 
 tables for 199 
 
 Diagonal barometers 62 
 
 Dial barometer 63 
 
 Elevation determined by competent person 162, 165 
 
 of a station defined 161, 174 
 
 barometer above sea level, determined promptly 178 
 
 actual 174 
 
 station 174 
 
 to be given in feet and decimals 162 
 
 Empty barometers, shipment of 151 
 
 Engineer to be informed concerning levels 165 
 
 Error of barograph clocks checked 158 
 
 barometers as shown by comparative readings 116 
 
 Exposure of barographs 89-152, 176 
 
 barometers 89-152, 176 
 
 Extra barometers, cisterns, lowered before comparative readings 177 
 
 screwed up 177 
 
 Field notes, copy of, furnished 165 
 
 "Fixed point " to be established 163 
 
 Foreman's barograph 70 
 
 Forms for barographs changed .*. 156 
 
 trimmed 156 
 
 refigured for barographs at elevated stations 155 
 
 Form No. 1001 Met'l 171 
 
 1027 Met'l 183 
 
 1058 Met'l 165 
 
 1059 Met'l 42, 170 
 
 Fortin barometers, Weather Bureau pattern 7 
 
 Glycerine barometer 61 
 
 Gravity, influence of, on barometric observations, tables 197 
 
 reduction to standard 102, 197 
 
 altitude term 174 
 
 latitude term 174 
 
 Green barometer 7 
 
 Heights determined by aneroids 105, 107 
 
 barometric readings 104, 107 
 
 tables for, determination of, by barometer 199 
 
 Holosteric barometer 6 
 
INDEX. 109 
 
 Paragraph. 
 
 Howson's barometer : 65 
 
 Impure mercury not to be used , 138 
 
 Index point of barometer 9 
 
 Ink for barograph pens 159 
 
 Instructions for cleaning barometers 123-144 
 
 Interpolation explained 196 
 
 Interval between comparative readings on removal of barometers 148, 166 
 
 Inverting barometers at elevated station 114 
 
 Ivory point, difficulty of cleaning around 134 
 
 of barometer 10 
 
 position not to be changed in cleaning barometer 128, 136 
 
 Leaks in cistern of barometer 141 
 
 Leaky barometer, how to open cistern of 126 
 
 Leather carrying cases 146 
 
 Line of levels, how run 165 
 
 need not be run 167 
 
 must be run 162 
 
 Location of barometers changed 148 
 
 Magnifying barometers, comments on 67 
 
 Marine barometer 26-31 
 
 Marvin's normal barograph 71 
 
 Mercury, how filtered 139 
 
 how purified 138 
 
 impure, disposed of 189 
 
 not to be removed 190 
 
 purity of, indicated 139 
 
 tested 137 
 
 requisition for 188 
 
 "Metallic click" in barometer tubes Ill, 113 
 
 Moving and packing barometer 109, 145 
 
 Nomenclature 174 
 
 Observations of pressure comparable 173 
 
 reduced to station elevation 170 
 
 regular barometric 97,100,171,180 
 
 Observed reading defined 174 
 
 Packing barometers 109, 145 
 
 Pens for barograph cleaned 159 
 
 Plane of reference to be adopted 164 
 
 I'n-ssure actual, defined 174 
 
 of one atmosphere 4 
 
 reduced 174 
 
 station 174 
 
 tables of, corresponding to change in elevation of 100 feet 198 
 
 Private barometers, comparison of 194 
 
 Pumping of barometers 89a 
 
 Purity of mercury, how indicated 139 
 
 tested 137 
 
 Reading of vernier - 12, 13, 14, 15 
 
 Readings of barometer corrected 
 
 Recording barometers or barographs ." 68, 69 
 
 Reduction to sea level defined 
 
 temperature and moisture terms 103 
 
HO INDEX. 
 
 ' 
 
 Paragraph. 
 
 Reduction for elevation denned 174 
 
 Removal correction . 174, 182 
 
 to barometer, provisional 182 
 
 variable 182a 
 
 Scale of barometer 11 
 
 contracted 19 
 
 polished 128 
 
 errors and capillarity, correction for 174 
 
 of elevations on aneroid, how used 106 
 
 feet on aneroid, adjustable, how used 106 
 
 only approximate 105 
 
 Sea level, reduction of barometer to 103, 181 
 
 Serviceable barometers, how shipped 149 
 
 Shipment of empty barometers 151 
 
 serviceable barometers 149 
 
 Siphon barograph, Marvin system 72, 87 
 
 barometer 3 
 
 magnifying 66 
 
 Spiral barometers 62 
 
 Standard gravity, reduction to 102, 1.97 
 
 Station elevation 168, 169, 174 
 
 Straining cistern of barometer Ill, 114 
 
 Sum of corrections denned 174 
 
 Surveyor to be informed concerning levels 165 
 
 Sympiesometer 64 
 
 Temperature, correction for 41, 43, 174, 195 
 
 Thermometer attached 8 
 
 Time error of barographs checked 158, 192 
 
 Torricelli's barometer 2 
 
 "Total correction " denned 171, 174 
 
 entered on Form No. 1001 Met'l 171 
 
 Tube of barometer to be cleaned 128 
 
 Unpacking barometers Ill 
 
 Unserviceable barometers, how shipped '. 151 
 
 Vernier of barometer - 12 
 
 adjustment of 98 
 
 Verticality of barometers 90, 92, 99 
 
 Water barometer 60 
 
 Weighing barographs 69 
 
 Winding clocks for barographs 146 
 
 Wrench required for cleaning Tuch cisterns 143 
 
 . 
 
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