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Las diagrammes suivants illustrent la m^thoda. 1 2 3 1 6 MICROCOPY RESOLUTION TEST CHART (ANSI and ISO TEST CHART No. 2) A APPLIED INAhGE Inc S^ f twrr» Hiffh ai'^ twt\ Inw t'it]ps. repel? iii at each plate, in twenty-four hours, there would Le but one tide during that time. Indeed the difficulties met with, in the application of the common theory to the reality, drew from the editors of the Encyclopaedia Britannica the following remarks; " The reader will undoubtedly be making some com- parison in his own mind, of the deductions from this theory with the actual state of things. He will find some considerable resemblances ; but he will also find such great differences, as will make him very doubtful of Its justness. In very few places does the high water happen within three quarters of an hour of the moon's southmg, as the theory leads him to expect ; and in no place whatever does the spring tide fall on the day of new and full moon, nor neap tide on the daj of her Jiuadrature. These always happen two or three '" ^he same tune about half the quantity of water only was carried on from the deeper vessel, that was carried oflf from ine other. Two tin vessels of equal capacities, but whose depth, ablve TU r^'' '' ^ *° '* ^'^^ Pl-^-d ^ he n.H /""' "^"^ "^^ ^"' ^^^^ »h- depth of OuJ V "''"^ nevertheless contained an equal quantity of water, was emptied in a decidedly less time. Since, then, by these simple experiments it appears, he degree of heat being the same, evaporation dimini- shes wuh the mcrease of the volume of a liquid, and he more collected its form ; it is concluded that, in such enormously extensive bodies of water as the o- ceans are, evaporation must almost cease. This conclusion is farther supported by facts. It may be generally stated, the more remote from land, the more serene the sky. Mariners going to sea. look for good weather; approaching land, they expect foul. Ihe i'aciiic Ocean, the longest uninterrupted body of water, IS unobscured by a cloud for months together, and from this circumstance derives its name. The atmosphere is generally clear over the Atlantic Ocean, even' d;!" ^.asons -waen the neighbouring countries are enveloped 5 with fogs or clouds. The sun's heat, confined and checked by the solid surfaces which land presents, it spent in dissipating and diffusing the small bodies of water, which, in the form of swamps, lakes, and rivers, may be lodged or running on the face of a country ; and the more frequent such bodies of water occur, the more is the ambient sky loaded with vapour. But, notwithstanding it be admitted evaporation constantly tahs place from ihe oceans which cover the globe, a small part only of the sun's heat is engaged therein, be- cause, 1st. The progress of the sun's heat is so rapid and constant, that the employment of it by evaporation is too slow to prevent the farther action of the sun through- out the whole extent of the water. Thus the heat of a fire acts so powerfully on water placed in a kettle, as to swell the water considerably at the same moment that it is losing much by evaporation. This experiment IS made every day in every kitchen, and is a correct Ihough violent representation of the periodical swells of the oceans caused by the action of the sun. 2d. Because the capacity of the atmosphere is limited, and can receive but a limited quantity of vapour, »nd because the sun's heat is infinite when compared to the capacity of the atmosphere. The infinite remainder of the sun's heat unengaged in evaporation, necessarily perva- des and mingles with the waters of the oceans, and thereby augments their volume. The atmosphere has a limited capacity. It can admit, B2 of water. When charged to its utmost capacity, eva- pcrat,on must cease. It is probable the atmosphere is at all times nearly charged to its utmost capacity, be- cause the extent of water, which is the material, and the sun the agent, are constantly the same, and con- stantly employed in maintaining that utmost charge. The continual changes in the atmosphere, only prove, that an infinitely various distribution of nearly an equal quantity of vapour is taking place, every where and at all times. Some regions of the atmosphere are receiving water, while from other regions it is returning. This IS not made less probable by the long continuance of clear weather, since the atmosphere often acquires a condition which refuses evaporation. The long duration of clear, dry weather, at times, when the same causes of evaporation are in force that loaded the atmosphere with vapour, furnishes a sufficient proof. For two, three, or more weeks, the sky often continues calm and clear, and free from dew, beneath a vertical sun. It follows, that at those periods the atmosphere refuses the vapour con- constantly offered by the action of the sun; or, what seems more probable from the above-recited experi- ments, the sun, in regard to very large bodies of water, is little employed in evaporation, but chiefly in dilating their volume. The gentle evaporation that may take place from the o, ceans does not affect their height. They being co-exten, sive with the atmosphere, whatever quantities of water the atmosphere may acquire in the form of vapour, by the ac- tion of the sun, in some places, are in other places return- ed into the oceans, directly in the form of rain, or indirect- ^\ 4|BWMSH||B!| ly by the channels of rivers and rivulets. And this routine of evaporation and rain, it is certain, keeps up a steady ba- lance of accounts, since the high and low water matks, through successive centuries, remain the same in all places : 80 that, from the natural tendency of liquids to gain a state of equilibrium, :ind the various oceans communicating one with the other, it follows, the waters which cover the globe would remain in a state of undeviating height, were they not otherwise influenced by the expanding power of heat. It IS not to be supposed, the action of the sun on the wa- ters in high latitudes produces the changes there, e. g. the English Channel. The sun's heat may be said to act as a percussive force, and to give a periodical stroke to the great bosom of the ocean over which the sun passes verti- cally. The changes of the water along the shores are the effects of the impulsion given to the central regions of the oceans ; and the farther the shore, to a certain extent, the greater the change. Impulsion is adopted instead of swell, to render the idea more distinct. Each swell moves off in a wave ; and, the greater range it takes, the higher the un- dular.on. This will appear by dipping the hand in an ob- long tub, full of water, nearer to one end than the other ; the rise of the water will be considerably greater at the end most distant from the stroke. And this may account for the greater height of the tides in middle latitudes than near the line. The undulation is exhausted in its progress tO" wards the poles. EXPLANATION OF THE PHENOMENA OF THE TIDES. The sun emits a perpetual flood of heat in all dircc- \4 8 tions, and mingles with and swelk the waters which covei" the globe. The sun's heat pervades one half of the surface of the globe at all times j and by the periodical visitation of the ▼arious ocean? by the sun, in consequence of the diurnal rotation of the globe, periodical swellings are produced in those oceans, which exhibit all the phenomena of the tides. The waters contained in any given tract of the globe, for instance, the Atlantic Ocean, are exposed to, and receive the sun's heat once in twenty-four hours, in consequence of the diurnal motion of the earth. Let the sun be on the line : the nights and days will be equal, and the sun will rise at six o'clock. The moment the sun rises, its heat be- gins to ^ink into the ocean ; as the sun ascends, the heat be- comes greater, its intrusive, pervading, and expansive force greater, until it reaches the zenith, and the gradual augmentation of the water by the immersion of heat during th:s t,me, will exhibit a Eood tide along the edges of the shores m contact with the ocean. It will be noon, and six hours will have elapsed. A. the sun gradually declines westward, there will be also a gradual secession and escape of the heat from the ocean. The ocean, thus insensibly abandoned by the heat, will consequently shrink, and ex- hibit along the shores an ebb tide. At six o'clock in the evening, the sun having set, and the heat altogether vanish- ed, low wattr will take place. Dunng this first solar day of twelve hours, there will be one flood tide and high wa^ ter, and one ebb tide and low water : one rise and one fall oj the water. MMIMM This rise and fall, having disturbed the edges of the o- cean, or where the ocean is in contact with land, a re-acthn will ensue; but, in consequence of the gravity and viscidi- • ty of water, it will be performed in rather a longer time than the first swell, which is the cause. The first re-action would be followed by others; and were the sun, the original agent, to disappear altogether, nevertheless a vibratory tide might endure for ages. This will appear extremely probable, by the effect produced by dippmg a hand in a bason of water. The first rise and fall will be followed by a similar rise and fall, and a vibration will ensue for some time along the edge of the vessel. A stone thrown into a pond of water, will occasion the shore to be lashed for a considerable time by successive equal re-actions of the water, although the water received origi- nally but a single impulse from the stone, 'ihe longest calms are scarce sufficient for the ocean to subside to a state of rest. When it most approaches to. or for a while real, ly acquires, a state of quiescence, a ceaseless rise and retro- cession of its waters continue along the shores which con- Une Jt, commonly called surf. In consequence of the re-action which follows the origl. nal rise and fall cf the waters of the first day. occasioned by the sun the first night will exhibit a repetition of the tide, of the first day. From six o'clock in the evening, being sun.set, the waters will rise till midnight. At midnight vor a httle after, in consequence, as above stated, of the gravity and viscidity of water) there will be high water. iTom niidnight until sometime past six o'clock in the morn- ing, ebb tide will take place; and at past six o'clock in the morning there will again be low water. 11 1 10 Thus, in a little more than twenty-four hours, there will be two flood tides and two ebb tides, twice high and twice low water ; and this corresponds with the reality. But It appears the tides gradually increase for seven or eight days, at the end of which time the highest flood tide, called springtide, takes place; and, for the seven or eight days following, the tides decrease, and terminate in what IS called a neap tide, that is, the lowest tide. When the sun rises the second day, it overtakes the «- i>cthg tide, which is then begining to perform a new flood tide. The sun, acting in conjunction with this tide, in- creases its extent and duration; so that instead of being at noon, as on the first day, it is near one o'clock when high water takes place. In consequence of the increase of the second day's flood tide, the re-action will be greater; that is, on the ensuing night the tide will be greater. On the third day, the sun acting in conjunction with the re- actmg flood tide, increased by the additional impulse of the second day; the flood tide on the third day will be greater than on the second, and high water will take place still later. The flood tide of the ensuing night, being a re- action of the third day's^ flood tide, will also be greater, and take place later than the flood tide of the second night ; and this increase will continue until the highest or spring tide takes place. The successive increase of the extent and duration of the tides, will evidently change the periods of their vicissitudes or changes ; so that, although the sun acted in conjunction with the re-acting tides on the second, third, and fourth -ays, en the seventh or eighth, in consequence of the pro- ti longation of the tides, it will come to act Jn opposition to them, until they are worn down to the lowest measure, or neap tide. As has been stated, after the first day, the sun acts jointly with the re-acting tides occasioned by the first disturbance of the ocean. By thus acting together, the ex- tent and duration of the tides are increased : each successive day, they will turn later and later, until the seventh or JHghth morning, when, atsun-rise, the time of change will have so much altered, that the ebb tide, which, on the se- cond morning, terminated shortly after sun-rise, will, oa those mornings, be so far delayed, that the sun will resist it. Suppose the ebb tide, at sun-rise on the seventh or eighth morn-ng, to be but half spent, the gradual immersion of Aeat will check and shorten, though it cannot destroy it. Something similar to this daily takes place on the sm^e otthe ocean. Any supposed wind, blowing strongly over t^e water, gradually raises a greater and greater swell of the sea. At first a gentle curl will appear; small waves will follow, which will fitially grow i«to long and extensive swells or mountains of water. Let the wind veer round, and blow from the opposite point : the original swell will con- tmuetoroll, although with decreasing dimensions, and se- veral days will elapse before the original sweU will be alto- gether subdued by th« adverse wind. REMARKS. g deviations from these general movements of the tides are occasioned by local circumstances. o- _- b>3„on:, vaiiiiui ukcr me supposed action of the sun on the oceans, as its heat constantly strikes a hemi. spbere of the globe. Whether the sun is considered to b» 1« on one side of the line or the other, it is immaterial ; since tl e oceans, extending on each side of the line to the poles, must continue to receive the sun's heat. Could it be pos- sible that the half of the globe, on which the sun shone at any given time, was masked by clouds, the sun's heat would nevertheless reach the waters thus hidden. He^t is so diffusive, that the most dense bodies check it only for a- while : it therefore cannot be supposed that vapour can ma- terially check its progress. The mercury of a thermome- ter will rise in the shade However, interventions of this kind may cause some of the caprices of the tides. It may be objected, that, during the night, the atmos- phere often continues warm, and that this heat must pre- vent any distinct effect from the sun's presence. But the vrarmth experienced after sun-set, is evidently the heat es- caping from the surface of the globe, accumulated during the day } since the wzrmth graduJ/y diminishes, and towards morning the atmosphere becomes cool. The diminution i? .more rapid over water than land, because of the less resis- tance given to the escape of the heat, in consequence of the less continuity of the particles of fluids than of solid bodies. i'l r,