BAR 



is carried off N.E. and S.W. the adjacent air will croud in 

 from the other points, and form a couple of new currents in 

 the direction N.W. and S.E. to fill up the vacancy, and re- 

 ftorc the equilibrium. This is a neceflary confequence from 

 the laws of fluids. 



3dly, If the wind were the fole agent, the alterations in 

 the heiglit of the mercury would only be relative, or to- 

 pical ; there would be ftill the fame quantity fupported at 

 feveral places taken coUeftively : tiius, what a tube at Lon- 

 don loft, another at Paris, or at Pifa, or at Zurich, &c. 

 would at the fame time gain. But we find -the very con- 

 trary true in facl ; for from all the obfervations hitherto 

 made, the barometers in-ftveral parts of the globe rife and 

 fall together ; fo that it mud be fome alteration in the ab- 

 folute weight of the atmofphere that accounts for the rife 

 and fail of the mercury. 



Laftly, Setting afide allobjefiions, thef>: popular pheno- 

 rnena, the mercury's fall before, and rife after rain, feem 

 to be inexplicable on the ground of this hypothefis : for fuD- 

 pofe two contrary winds fweeping the air from over Lon- 

 don, we know that few, if any, of the winds reach above 

 a mile high ; all, therefore, they can do, will be to cut off 

 a certain psrt of the column of air over London : if the con- 

 fequence of this be the fall of the mercury, yet there is no 

 apparent reafon for the rain's following it. The vapours 

 indeed may be let lower, but it will only be till they come 

 into an air of the fame fpecific gravity with thcmfelvco ; and 

 there they will be fufpended as before. 



M. Leibnitz, about the year 1710, in a letter written to 

 the abbe Bignon, endeavoured to fupply the defeats of this 

 hypothefis with a new one of his own. The new princi- 

 ple, upon whic'n Leibnitz's hypothefis is founded, was il- 

 luftrated by IvI. Fontenelle in the Hillory of the Royal Aca- 

 demy of Sciences at Paris for the year 171 1. He affcrts, 

 that a body immerfed in a fluid only weighs with that fluid 

 while it is fuftaincdby it ; fo that when it ceafes to be fuilain- 

 cd, i. e. when let fall, its weight ceafes to make a part of that 

 of the fluid, which by this means becomes lighter. Thus, 

 adds he, the watery vapours, while fuflained in the air, in- 

 creafe its weight ; but when let fall, they ceafe to weigh 

 along with it. Thus the weight of the air is diminiflied ; 

 and thus the mercury falls, and rain enfues. 



But M. Leibnitz's principle, notvvithftanding the experi- 

 ment he brings to confimi it, is falfe, as has been evidently 

 made appear by a counter experiment of Dr. Defaguliers. 

 (See his Courfe of Exp. Philof. vol. i. p. 282, &c.) For a 

 body, whether fpecifically equal or lighter or heavier than a 

 fluid, while it is immerged in it, whether it be at reil, or in 

 motion, adds to the fluid a weight equivalent to that df an 

 equal bulk of the fluid ; as follows from that law in hydro- 

 ftatics, that fluids gravitate according to their perpendicular 

 altitudes. However, were M. Leibnitz's principle true, 

 yet it is defeciive ; and that in the fame refpeft with Dr. 

 Halley's ; nor would it account for the phenomena more 

 than the other. For, fuppofing the vapours by being con- 

 denfed, to be put in a motion downwards, and fo ccafing to 

 gravitate with the atmofphere ; tliey will therefore fall, till 

 they reach a part of the atmofphere of the fame fpecific 

 gravity with themfelves ; and there they will hang as before. 

 If the mercui-y fall, it will only be during the time of that 

 defcent; for thefe once fixed, the former gravity is retrieved ; 

 or, were it not retrieved, yet no rain would fuccced the fall 

 of the mercury. 



The hypothefis, propofed by Mr. Chambers, is fome- 

 what fimiliar to that of Leibnitz, and hable to the fame 

 objeftion. It is as follows : fuppofe any number of 

 watery veucles floating in any part of tJie atmofphere. 



Vol. III. 



BAH 



over any detemiinate portion of the globe ; if the upper 

 veficles be condenfed by the cold of the fuperior regions, 

 their fpecific gravity will be increafed, and they will de- 

 fcend ; where meeting with other veficles not yet precipi- 

 tated, they will coalcfce, or run into larger veficles, by the 

 known laws of attraction. Or, if we ratlier chcofe to have 

 the wind aft, let it drive either horizontally or obliquely, 

 fome veficles will be driven againft others; by wliich means 

 likewife will the particles coalefce, and form new a- d larger 

 veficles as before; fo that their number, v.hich before v. as, 

 fuppofe a million, will now be reduced, v. gr. to a hundred 

 thoufand. 



But by the fame coalition whereby their number is dimi. 

 niftied, their fpecific gravity, if we may fo call it, is in- 

 creafed, i. e. they come to have more matter in the fame 

 fpace, or under an equal furface ; as may be eafily proved 

 from principles of geometry : lor in augmentirg the mafs 

 of any homogeneous body, the increafe of furface does not 

 keep pace with that of the folidity ; but that of the former 

 is as the fquare of the diameter, and that of the latter as 

 the cube of the fame. 



But fince the fame quantity of m.atter is now in a lefs 

 fpace or under lefs dixenfions, it will lofe lefs of its weight 

 by the refillance of the medium. This is evident ; for a 

 body immerfed in a fluid lofcs nothing of its weight but by 

 the friiElion of its parts againft thofe of the fluid ; but the 

 friftion is evidently as the furface ; therefore, when the fur- 

 face is lefl"ened, the refifl;ance muft be fo too. Confequer.tly, 

 the veficles, whofe gravity before the coalition was equal to 

 the refinance of the medium, now that refiilance is dimi- 

 niflied, will defcend ; and that with a velocity in the ratio 

 of the increafe of the mafs to the increafe of the furface. 



In their defcent, as they arrive at denfer paits of tlic at- 

 mofphere, their mafs and furface again will be increafed by 

 new coalitions ; and thus, by conftant frefli acceffiuns, more 

 than equal to the conftant reCftances, they will be enabled 

 to purfue their journey through all the ftages of the air, 

 till they reach the earth ; their maflcs exceedingly magnified, 

 and in the form of rain. 



Now that the vapours have got down, let us confiderhow 

 the barometer muft have been affeded during their paflage. 



Before any of the veficles began to fubfide, either from 

 the action of the cold, or of the wind, they all floated in a 

 certain portion of the atmofphere, and all gravitated to- 

 wards the centre. Here now, each refpeclively refiding in 

 a part of the medium of the fame fpecific gravity with it- 

 felf, will lofe as much of its weight as is equal to that of 

 a part of the medium of the fame bulk with itfclf, i. e. 

 each will lofe all its weight. But then, whatever weight 

 each lofcs, it communicates to the medium, which now 

 preffes on the furface of the earth with its own weight and 

 that of the veficles conjointly. Suppofe then this united 

 prefl\ire keeps up the mercury in the barometer at thirty 

 inches : by the coalition of the veficles from the caufet 

 aforefaid, their furfaces, and confequently their friftion, are 

 Icflened ; therefore, they will communicate lefs of theif 

 weight to the air, i. c. lefs than the whole ; and confequently 

 they will defcend with the excefs, i. e. with a velocity equal 

 to the rem.ainder, as before obferved. Now, as the veficlei 

 can aft no otherwife on the furface of the earth but by the 

 mediation of the interjacent air, in proportion as their action 

 on the medium is lefs, their aftion on the earth will be lefs. 

 It is alfo evident, that the furface 01 the earth muft be noi* 

 lefi prelTed than befoie ; and that in proportion as the 

 veficles referve more of their weight uncommunicated to 

 the medium, to promote their own defcent, i. e. in pro- 

 portion to the velocity of the falling veficles ; which it 

 4 K. agaia 



