BAR 



again in prfiporlion to their bulks. Thus, as the vcfic'.fi 

 defcend, the bulks continually increafing, the fridlion, and 

 therefore the prcfTure on the earth, and lalUy the height 

 of the mercury, will continually decreafe, during the whole 

 time of the fall. — Hence we fee, both why the veficles, 

 when once beginning to fall, perfevere ; why the mercury 

 begins to fall at the fame time ; and why it continues and 

 ceafes to fall together with them ; which were the great dc- 

 fidcrata in the philofophy of the barometer. 



There is one objeftion that evidently lies againll this the- 

 ory, viz. that the veficles being put in motion, and llriking 

 againft. the particles of the medium and one another with 

 fome moment, will meet with a confiderable refiftance from 

 the I'/V /Vrt/.c thereof ; by which means their defcent will 

 be retarded, and the prclTure of the atmofphere retrieved ; 

 the impetus of the moving veficles being fuppofed to com- 

 penfate for their lofs of furface. Thus a heavy body fuf- 

 tained in a fluid by a hair, and moved up and down therein, 

 prelTes more on the bottom than when held at reft ; which 

 additional prcfTure will be the greater as the velocity of the 

 falling veficles is greater ; a greater impulfe being required 

 to break through the -vis hurl'u of the contiguous particles 

 'in a lefs time than in a larger. 



But it is alleged, that we have both reafon and experi- 

 ment again 11 this objeAion ; for the velocity of the veficles, 

 in thefe circumftances, muft necefiarily be very fmall, and 

 their impulfe inconfiderable ; befides, the -vis inertia of the 

 air muft be exceedingly weak, by reafon of its extreme fub- 

 tility ; and it muft be a very improper vehicle to convey an 

 impulfe to a diftance by reafon of its elafticity ; we alio 

 find that a piece of lead, v/hich is a ponderous body, falling 

 with great moment,, gravitates coufiderably lefs, in its de- 

 fcent through watsr, which is a grofs unelaftic medium, 

 than when fiftained at reft therein ; i(i which the feveral ex- 

 periments of Reaumur, Ramazzini, andDefaguliers, all agree. 

 M. dc Luc (Recberches, 6cc. vol. i. p. 138.) fuppofes that 

 the changes obferved in the weight of the atmofphere are 

 principally produced by the prtfence or abfence of vapours, 

 floating in it. Others have attributed the effect to vapours, 

 but have given a different explication of it. It is his opinion, 

 that vapours diminiih the fpeciiic gravity, and confcquently 

 the abfolute weight of thofe columns of the atmofphere into 

 which they are received, which, notwithftanding this ad- 

 mixture, remain of an equal height with the adjoining co- 

 lumns that confift of pure or dry air. He afterwards more 

 largely explains and vindicates this theory, and applies it to 

 the folntiun of the principal phenomena of the barometer, 

 connefted with or produced by the varying denfity and. 

 weight of the atmofphere. 



Dr. Jaiaes Hutton, in his "Theory of Rain" (fee Rain), 

 printed io the Edinburgh Tranfaftions, vol. i. p. 41, Sec. 

 fuggefts feveral plaufible reafons in favour of his opinion, 

 that the diminution of the weight of the atmofphere by the. 

 fall of rain is not the caufe of the fall of the barometer ; 

 but that the principal, if not the only caufe, is to be fought 

 for ia the commotions of the atmofphere that are chieily 

 produced by fudden changes of heat and cold in the air, 

 « The barometer," he fays (p. 78.), is an iuftruraent ne- 

 *efiarily connefted with motions in the atmofphere ; but it 

 is not equally affefted with every motion in th.at fluid body. 

 The barometer is chiefly affeaed by thofe motions by which 

 there are produced accumulations and abftraftions'of this 

 fluid, in places or regions of fufficient extent to afl^eft the 

 preffure of the atmoiphere upon the furface of the earth. 

 But as every commotion in the atmofphere may, under 

 proper conditions, be a caufe for rain, and as the want of 

 cemmotien in tlie atmofphere is naturally a caufe of fair 



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weather, this inftrument may be made of great importance 

 for the purpofc of meteorological obfervations, although 

 not in the certain and more fimple manner in which it hae 

 been, with the increafc of fcieuce, fo fucccfsfuUy apphed 

 to the meafuringof heights." 



In the " Encyclopedia Britannica," art. Barometer, 

 we have another' theory of changes in tlie barometer, as de- 

 pending on the heat in the atmofphere, not as producing 

 commotions in it, but as altering the fpecific gravity of the 

 air by the variations of heat and cold. The prthminaries to 

 this hypothefis are : ift. That vapour is formed by an in- 

 timate union between the elements of fire and water, in con. 

 fequence of which the fire or heat is fo totally enveloped, 

 and its aftion fo entirely fufpended by the watery particles, 

 that it not only lofes its properties of burning and of giving 

 light, but becomes incapable of aff"eaing the moft fenliblc 

 barometer, in which cafe, it is faid by Dr. Black, the au- 

 thor of this theory, to be in a latent ftate. 2dly. If the 

 atmofphere be aft'efted by any unufual degree ot heat, it 

 thence becomes incapable of fupporting fo long a column of 

 mercuiy as before, for which reafon the barometer (inks. 



From thefe primary principles or axioms it is inferred, 

 that as vapour is formed by an union of fire with water, 

 whether by an eleftive attraiStion or a folution of the water 

 in the fire, the vapour cannot be condenfed till this union, 

 attradlion, or folution be at an end. Hence it follows that 

 the commencement of the condenfatiou of the vapour, or 

 the firft figns of approaching rain, muft be the feparation cf 

 the fire which is latent in the vapour. This may at firft be 

 flow and partial, or it may be fudden and violent ; in the firit 

 cafe the rain will come on flowly, and after a confiderable in- 

 terval ; in the other it will come on very quickly and in a great 

 quantity. But Dr. Black has proved, that when fire quits 

 its latent ftate, hov/ever long it may have lain dormant and 

 infenfible, it always re-affumes its proper qualities, and af- 

 fe£ls the thermomete:- as if it had never been abforbed. The 

 confequence of this muft be, that in proportion as the la- 

 tent heat is difcharged from the vapour, thofe parts of the 

 atmofphere into which it is difcharged, will be fenfibly af- 

 fefted by it ; and in proportion to the heat communicated 

 to thofe parts, they will become fpecifically lighter, and o£ 

 courfe the mercuiy will fink. When the feparation between 

 the fire and water is gradual and flow, the barometer may 

 indicate rain for a confiderable time before it happens ; or 

 if the fenfible heat fhould be abforbed by the colder parts 

 of the atmofphere, or by any means be prevented from af- 

 fetting tlie fpecific gravity of the air, the barometer will 

 not be aftedted J and'yettlie water, deprived of the heat 

 that is neceflaiy for fuftaining it, muft defcend in rain ; and 

 hence it happens, that the indications of the barometer are 

 not always juft. Hence it alfo appears, that though the 

 fpecific gravity of the air is diminillied, unlefs this diminu- 

 tion proceeds from a difcharge of the latent heat contained 

 in the vapours, no rain will follow ;. and. thus the finking 

 of the barometer may prognofticate wind as well as rain, or 

 fometimes no change at all. The great defcent of the 

 mercury in the barometer between the tropics in the time 

 of hurricanes, noticed by Dr. Halley, is afcribed, as to its 

 probable caufe, to a great commotion in the eleftric fluid, 

 by which the air is inteinaily agitated, and its gravitation iu 

 part fufpended. 



In the fourth volume of the " Memoirs of the Literary 

 Society of Manchefter," we have a curious paper on this 

 fubjeft, viz. " Meteorological Obfervations made on differ- 

 ent parts of the Weftern Coaft of Great Britain ;" ar- 

 ranged by T. Garnett, M. D. The materials of. this pa. 

 per were furiuihcd b;^ feveral obfervers ; but thofe of M-. 



Copland;, 



