Barometer, the day-time, the action of the fun's rays is fo violent, that what is gained by the discharge of latent heat from the vapour, is loft by the interpofition of the clouds betwixt the fun and earth, or by the great evaporation which is conftantly going on; and in the night, the cold of the atmosphere is so much increafed, that it abforbs the heat as faft as the vapour difcharges it, fo that no fenfible effect can be produced; for in warm climates, though the day is exceffively hot, the night is obferved to be vaftly colder in proportion than it is with us. This, however, does not prevent the barometer from being affected by other caufes, as well as with us; for Dr Halley obferves, that in the time of hurricanes it finks very low. The cause of this is moft probably a great commotion in the electric fluid, by which the air is internally agitated, and its power of gravitation in part fufpended-A confirmation of the above hypothefis, however, is taken from the different heights at which the mercury arrives in different climates. The barometer-range, for inftance, at the latitude of 45° is the greatest of all; becaufe here the evaporation and condenfation of the vapours are both very confiderable, at the fame time that the latent heat discharged cannot be absorbed fo fuddenly as in the torrid zone, the difference betwixt the length of the days and nights being greater, and confequently the nights warmer in fummer and colder in winter. Farther to the northward the range is lefs, and in the latitude of 60° only two inches, by reason of the greater cold and length of the days and nights; whence the quantity of vapour condenfed, or of latent heat expelled, becomes proportionably lefs.

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Different kinds of

Having thus given an account of the feveral phenomena of the barometer confidered as a weatherbarometers glafs, and likewife endeavoured to account for them in defcribed. the moft fatisfactory manner, we now proceed to give a particular description of the barometers moft com. monly made use of, with various schemes for their improvement.

Plate XCII. Fig. 1. reprefents the common barometer, fuch as was invented by Torricelli, and fuch as we have already given a general defcription of. A B reprefents a tube of glafs, a quarter of an inch in diameter, and 34 inches long, hermetically fealed at A. This tube being fuppofed to be filled with mercury, is then inverted into the bafon CD; upon which the mercury in the tube falls down to GH, fomewhat above 28 inches, while that in the bafon rifes to CF. The lowest station of the mercury in this country is found to be 28 inches, and the higheft 31. From the furface of the mercury CF, therefore, 28 inches are to be mea. fured on the tube AB, which fuppofe to reach to the point K. This point, therefore, is the loweft of the fcale of variation, and in the common barometers is marked formy. In like manner, the higheft point of the scale of variation I, is placed 31 inches above EF; and is marked very dry on one fide for the fummer, and very hard froft on the other for the winter. The next half inch below is marked fet fair on the one fide, and fet froft on the other. At 30 inches from CF is marked the word fair on one fide, and freft on the other. Half an inch below that, is wrote the word changeable, which answers both for fummer and winter. At 29 inches is rain on the one fide, and fnow on the other; and at 28 are the words much rain on the one

fide, and much now on the other. Each of these large Barometer. divifions is ufually fubdivided into ten; and there is a fmall fliding index fitted to the inftrument, by which the afcent or defcent of the mercury to any number of divifions is pointed out. Each of these tenths is fometimes divided into ten more, or hundredths of an inch, by means of a fliding flip of brafs with a vernier scale on it, which fhall be hereafter defcribed and explained. This kind of barometer is the most common, and perhaps the moft ufeful and accurate, of any that has yet been invented, from the following circumftance, that the natural fimplicity of its conftruction, in preference to others hereafter defcribed, does not admit of any kind of refiftance to the free motion of the column of mercury in the tube. The fcale of variation being only three inches, and it being naturally wifhed to difcover more minute variations than can thus be perceived, feveral improvements have been thought of.

The improvement moft generally adopted is the dia. gonal barometer reprefented fig. 2. in which the fcale of variation, inftead of three inches, may be made as many feet, by bending the tube fo as to make the upper part of it the diagonal of a parallelogram of which the fhorteft fide is the three-inches fcale of variation of the common barometer. This, however, has a very great inconvenience: for not only is the friction of the mercury upon the glafs fo much increased that the height doth not vary with every flight change of air; but the column of mercury is apt to break in the tube, and part of it to be left behind, upon any confiderable defcent.

Fig. 3. is the rectangular barometer; where AC reprefents a pretty wide cylinder of glass, from which proceeds the tube CDF bent into a right angle at D. Suppofe now the cylinder AC to be four times larger than the tube CD, fo that every inch of the cylinder from C to A fhould be equal in capacity to four inches of the tube CD. The whole being then filled with mercury, and inverted, the mercury will fubfide from A to B, at the fame time that it cannot run out at the open orifice F, because the air presses in that way. If any alteration then happens in the weight of the air, fuppofe fuch as would be fufficient to raise the mer cury an inch from B towards A, it is evident that this could not be done without the mercury in the horizontal leg retiring four inches from E towards D ; and thus the scale of variation counted on the horizontal leg would be 12 inches. But the inconveniences of friction are much greater here than in the diagonal barometer; and befides, by the leaft accident the mercury is apt to be driven out at the open orifice F.

The pendant barometer (fig. 4.) confifts of a fingle tube, fufpended by a ftring faftened to the end A. This tube is of a conical or tapering figure, the end A being fomewhat lefs than the end B. It is hermetically fealed at A, and filled with mercury: then will the mercury fink to its common station, and admit of a length of altitude CD, equal to that in the common barometers. But from the conical bore of the tube, the mercury will defcend as the air grows lighter, till it reaches its lowest altitude, when the mercury will stand from the lower part of the tube B to E, fo that BE will be equal to 28 inches: confequently the mercury will, in fuch a tube, move from A to E, or 32 inches, if the tube be five feet, or 60 ches; and therefore

th

Barometer, the fcale AE is here above ten times greater than in the common barometer: but the fault of this barometer is, that the tube being of a very small bore, the friction will be confiderable, and prevent its moving freely; and if the tube is made of a wider bore, the mercury will be apt to fall out.

Fig. 5. is an invention of Mr Rowning, by which the scale of variation may be increased to any length, or even become infinite. ABC is a compound tube hermetically fealed at A, and open at C, empty from A to D, filled with mercury from thence to B, and from thence to E with water. Let GBH be a horizontal line; then it is plain from the nature of the fiphon, that all the compound fluid contained in the part from H to G, will be always in æquilibrio with tfelf, be the weight of the air what it will, because the preffure at H and G must be equal. Whence it is evident, that the column of mercury DH is in equilibrio with the column of water GE, and a column of air taken conjointly, and will therefore vary with the fum of the variations of thefe. That the variation in this barometer may be infinite, will appear from the following computation. Let the proportion between the bores of the tube AF and FC be such, that when HD, the difference of the legs wherein the mercury is contained, is augmented one inch, GE, the difference of the legs wherein the water is contained, fhall be diminished 14: then, as much as the preffure of the mercury is augmented, that of the water will be diminished, and fo the preffure of both taken together will remain as it was; and confequently, after it has begun to rife, it will have the fame tendency to rife on, without ever coming to an equilibrium with the air.

Fig. 6. reprefents Dr Hook's wheel-barometer. Here ACDG is a glafs tube, having a large round head at A, and turned up at the lower end F. Upon the furface of the mercury in the bent leg is an iron ball G, with a ftring going over a pulley CD. To the other end of the ftring is faftened a smaller ball H, which as the mercury rifes in the leg FG, turns the index KL from N towards M, on the graduated circle MNOP; as it rifes in the other leg, the index is carried the contrary way by the defcent of the heavier ball G, along with the mercury. The friction of this machine, how ever, unless it is made with very great accuracy, renders it useless.

Fig. 7. is another barometer, invented by Mr Rowning, in which also the scale may be infinite. ABCD is a cylindrical veffel, filled with a fluid to the height W, in which is immerged the barometer SP confifting of the following parts: The principal one is the glafs tube TP (reprefented feparately at tp), whofe upper end T is hermetically fealed: this end does not appear to the eye, being received into the lower end of a tin pipe GH, which in its other end G receives a cylindric rod or tube ST, and thus fixes it to the tube TP. This rod ST may be taken off, in order to put in its ftead a larger or a leffer as occafion requires. S is a ftar at the top of the rod ST; and ferves as an index by pointing to the graduated feale LA, which is fixed to the cover of the veffel ABCD. MN is a large cylindrical tube made of tin (reprefented feparately at m), which receives in its cavity the fmaller part of the tube TP, and is well cemented to it at both ends, that VOL. III. Part 1.

none of the fluid may get in. The tube TP, with this Barometer. apparatus, being filled with mercury, and plunged into the bafon MP, which hangs by two or more wires upon the lower end of the tube MN, must be so poised as to float in the liquor contained in the vessel ABCD; and then the whole machine rifes when the atmosphere becomes lighter, and vice verfa. Let it now be fuppofed, that the fluid made ufe of is water; that the given variation in the weight of the atmosphere is fuch, that, by preffing upon the furface of the water at W, the furface of the mercury at X may be raised an inch. higher (measuring from its furface at F) than before; and that the breadth of the cavity of the tube at X, and of the bafon at P, are fuch, that by this ascent of the mercury, there may be a cubic inch of it in the cavity X more than before, and confequently in the bafon a cubic inch lefs. Now, upon this fuppofition, there will be a cubic inch of water in the bafon more than there was before; because the water will fucceed the mercury, to fill up its place. Upon this account the whole machine will be rendered heavier than before by the weight of a cubic inch of water; and therefore will fink, according to the laws of hydroftatics, till a cubic inch of that part of the rod WS, which was above the furface of the water at W, comes under it. Then, if we fuppofe this rod fo fmall, that a cubic inch of it fhall be 14 inches in length, the whole machine will fink 14 inches lower into the fluid than before; and confequently the furface of the mercury in the bason will be preffed, more than it was before, by a column of water 14 inches high. But the preffure of 14 inches of water is equivalent to one of mercury; this additional pressure will make the mercury afcend at X as much as the fuppofed variation in the weight of the air did at first. This afcent will give room for a fecond cubic inch of water to enter the bafon; the machine will therefore be again rendered so much heavier, and will fubfide 14 inches farther, and so on in infinitum. If the rod was fo fmall that more than fourteen inches of it were required to make a cubic inch, the variation of this machine would be negative with respect to the common barometer; and inftead of coming nearer to an equilibrium with the air by its afcent or defcent, it would continually recede farther from it: but if less than 14 inches of rod were required to make a cubic inch, the scale of variation would be finite, and might be made in any proportion to the common one. Neither this nor the other infinite barometer have ever been tried, fo that how far they would anfwer the purposes of a barometer is as yet unknown.

Fig. 8. reprefents another contrivance for enlarging the fcale of the barometer to any fize.-AB is the tube of a common barometer open at B and fealed at A, fufpended at the end of the lever which moves on the fulcrum E.-CD is a fixt glafs tube, which ferves in place of the ciftern. This laft tube muft be fo wide as to allow the tube AB to play up and down within it.-AB being filled with mercury, is nearly counterbalanced by the long end of the lever. When the atmosphere becomes lighter, the mercury defcends in the long tube, and the furface of the mercury rifing in the ciftern pushes up the tube AB, which at the fame time becoming lighter, the lever preponderates, and points out the moit minute variations. Here too

D

the

Barometer. the friction occafions inconveniences; but this may be in some measure remedied by a small shake of the apparatus at each inspection.

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In the Philofophical Tranfactions, Mr Cafwell gives the following account of a barometer, which is recommended by Mr Chambers as the most exact hitherto invented. "Let ABCD (Fig. 9.) reprefent a buc. ket of water, in which is the barometer erezosm, which confifts of a body ersm, and a tube ezyo: the body and tube are both concave cylinders communicating with one another, and made of tin: the bottom of the tube zy, has a lead weight to fink it fo that the top of the body may juft fwim even with the furface of the water by the addition of fome grain weights on the top. The water, when the inftrument is forced with its mouth downwards, gets up into the tube to the height y u. There is added on the top a fmall concave cylinder, which I call the pipe, to diftinguish it from the bottom fmall cylinder which I call the tube. This pipe is to fuftain the inftrument from finking to the bottom: md is a wire; ms, de, are two threads oblique to the furface of the water, which threads perform the office of diagonals: for that while the inftrument finks more or lefs by the attraction of the gravity of the air, there where the furface of the water cuts the thread, is formed a small bubble; which bubble afcends up the thread, as the mercury in the common barometer afcends."

The dimensions of this inftrument given there are, 21 inches for the circumference of the body, the altitude 4,each bafe having a convexity of 64 inches. The inner cirrumference of the tube is 5.14 inches, and its length 4; fo that the whole body and tube will contain almoft 2 quarts. The circumference of the pipe, that the machine may not go to the bottom on every small alteration of the gravity of the air, is 2.14 inches; according to which dimenfions, he calculates that it will require 44 grains to fink the body to the bottom, allowing it only four inches to defcend; at the fame time that it is evident, that the fewer grains that are required to fink it to this depth, the more nice the barometer will be. He alfo calculates, that when the mercury in the common barometer is 30 inches high, the body with a weight of 44 grains on its top will be kept in aquilibrio with the water; but when the mercury ftands at 28 inches, only 19 grains can be fupported: and laftly, by computing the lengths of the diagonal threads, &c. he finds, that his inftrument is 1200 times more exact than the common barometer. The following are his obfervations on the ufe of it.

Mr Caf"1. While the mercury of the common barometer well's ob- is often known to be stationary 24 hours together, the fervations bubble of the new barometer is rarely found to ftand with his ftill one minute. barometer.

"2. Suppofe the air's gravity increafing, and accordingly the bubble afcending; during the time that it afcends 20 inches, it will have many fhort descents of the quantity of half an inch, one, two, three, or more inches; each of which being over, it will afcend again. These retroceffions are frequent, and of all varieties in quantity and duration; fo that there is no judging of the general courfe of the bubble by a fingle infpection, though you fee it moving, but by waiting a little time.

66 3. A fmall blaft of wind will make the bubble Barometer. defcend; a blast that cannot be heard in a chamber of the town will fenfibly force the bubble downward. The blafts of wind fenfible abroad, cause many of the abovementioned retroceffions or accelerations in the general courfe; as I found by carrying my barometer to a place where the wind was perceptible.

66 4. Clouds make the bubble defcend. A fmall cloud approaching the zenith, works more than a great cloud near the horizon. In cloudy weather, the bubble defcending, a break of the clouds (or clear place) approaching to the zenith, has made the bubble to afcend: and after that break had paffed the zenith a confiderable space, the bubble again defcended.

"5. All clouds (except one) hitherto by me obferved, have made the bubble to defcend. But the other day, the wind being north, and the course of the bubble defcending, I faw to the windward a large thick cloud near the horizon, and the bubble ftill defcended: but as the cloud drew near the zenith, it turned the way of the bubble, making it to afcend; and the bubble continued afcending till the cloud was all paffed, after which it refumed its former descent. It was a cloud that yielded a cold shower of small hail."

Thefe are the most remarkable contrivances for the improvement of the common barometer: and indeed we must agree with Mr Chambers, that the last, on account of its being fo exceedingly fenfible, and likewife eafy of conftruction and portable, feems to deferve attention much more than the others, which are always the more unexact, and the lefs eafily moved, according to the enlargement of their fcale; whereas this is feemingly fubject to no fuch inconvenience. It Marine ba is evident, however, that none of thefe could be ufed rometer by at fea, on account of the unfteady motion of the ship: Mr Hook. for which reafon Dr Hook thought of constructing a barometer upon other principles.

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27

His contrivance was no other than two thermometers. The one was the common fpirit-of-wine thermc. meter, which is affected only by the warmth of the air: the other, which acts by the expanfion of a bubble of air included, is affected not only by the external warmth, but by the various weight of the atmofphere. Therefore, keeping the spirit thermometer as a ftandard, the excefs of the afcent or defcent of the other above it would point out the increase or decrease of the fpecific gravity of the atmofphere. This in- Recomftrument is recommended by Dr Halley, who fpeaks mended of it as follows. "It has been obferved by fome, by Dr Halthat, in long keeping this inftrument, the air in-ey. cluded either finds a means to escape, or depofites fome vapours mixed with it, or elfe for fome other caufe becomes lefs elaftic, whereby in procefs of time it gives the height of the mercury fomewhat greater than it ought: but this, if it fhould happen in fome of them, hinders not the usefulness thereof, for that it may at any time very eafily be corrected by experi ment, and the rifing and falling thereof are the things chiefly remarkable in it, the juft height being barely a curiofity.

"I had one of thefe barometers with me in my late fouthern voyage, and it never failed to prognosticate and give early notice of all the bad weather we had,