BAR 



ftorms appears to Mr. Kirwnn to be unfatisfaflory. " The 

 region of the carlli," lays Halley, " wherein thofe winds 

 rage, not extending round the globe, the llagnant air left 

 behind and that on the fides cannot ruth in fa'.l enough to 

 reftore the evacuation made by fo fwift a current, fo that 

 the air muft be attenuated where the faid winds continue to 

 blow." — " Add that the horizontal motion being fo quick 

 may take off fome part of the perpendicular prelTure." 

 This lall reafon f^emed to acquire fotr.e confirmation from an 

 experiment made by Mr. Hawkfhce ; for having paiTed a 

 ftream of air through a box in which the lower (hank of a 

 barometer was inferted, he obferved the mercury to fall 

 while the cuiTent pafTed through the i>ox ; as alfo in ano- 

 ther barometer which communicated with the bos, over 

 vhich the current of air did not flow. Allowing this, the 

 phenomenon is not fufficiently explained ; for Mr. Kirwan 

 obferves, that not only during the ftorm, but feveral hours, 

 if not days, before it, tlie mercuiy defcends confiderably, as 

 Halley himfclf, and all who recommend the marine baro- 

 meter, attell ; otherwifc this inftrument would be ufelefs. 

 Mr. Cafwcll obferves (Phil. Tranf. Abr. vol. viii. p. 458.), 

 that two days before the great ftorm of Januaiy i734-5> 

 the mercury fell -^ of an inch below 28 inches. But if 

 the fall were concomitant with the ftorm. Dr. Hailey's rea- 

 fons would not prove their connexion. In order to a bo- 

 dy's moving through air with fuch a velocity as to leave a 

 vacuum behind it, there is a neceffity that it ftiould move 

 at the rate of 1 1 or 1200 feet per fecond, as appears by 

 the obfen'ations of Mr. Brice and many others. (See Phil. 

 Tranf. for 1 766, p. 266.) The infufficiency of the fecond 

 reafon alleged by Dr. Halley has been clearly ftiewn by M. 

 de Luc ; nor is the experiment of Mr. Hawkfljee conclu- 

 five, as it appears that part of the air already confined in 

 the boxes was forced out by the blaft of air ; and befides, 

 Mr.Derhara obferved that during the greateft vehemence of 

 ftorm, the mercury rifes inftead of falling lower. (Phil. 

 Tranf. Abr. vol. iv. pt. 2. p. 77-) Mr. Kirwan made the 

 fame obfervation on the 2yth of February 1785, in Lon- 

 don. 



The third hypothefis is that of thofe who afcribe the va- 

 riations of the barometer to the prefence or abfence of va- 

 pours in the atmofphere ; but Mr. Kirwan infers, from a 

 view of the nature of vapours, and the change they pro- 

 duce in the weight and elafticity of the atmofphere, that 

 this theory does not fully account for the phenomena. 

 If we fuppofe, fays he, the atmofphere perfeftly dry, the 

 barometer at 30 inches, and the thermometer at 65, and 

 then a column of it to be faturated v.'ith moiilure, its elaf- 

 ticity being increafed —j-, which, according to his computa- 

 tion, would be the cale, it will contain -;!j- of its volume 

 lefs air than before faturation, fmce the increafe of its elaf- 

 ticity arifes from the introduftion of a new elaftic fluid 

 amounting to -Jj- of its bulk : and lince the weight of tiie 

 whole volume was at firft equal to that of 30 inches of 

 mercur}', its weight will now be lefTened by -Jj. of 30 inches, 

 that is nearly 0.59 of an inch. But on the other hand, it 

 gained 3V °f ''^ volume of vapour, and therefore its real 

 lofs of weight will be the diiference of the weight of -f^ of 

 air, and -^ of vapour ; but the weight of air is to that of 

 vapour as 12 to 10, therefore the gain here is 0.49 of an 

 inch, which dedufted from 0.59 the lofs, leaves the lofs 

 ^5 of an inch. Accordingly, this is the variation which 

 the baromder fliould undergo by the paflage of a column 

 of air from abfolute di-yuefs to complete fitaration ; a cir- 

 cumftance which never takes place, as the atmofphere is 

 never abfolutely dry ; and yet previoufiy to heavy rains, we 

 often obfervc the barometer to fall 3, 4, or j tenths of au 



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inch ; a fall which, from the above calculation, cannot on« 

 ginate from the faturation of the atmofphere with vapour. 

 Nor is there any proportion between the alcent of mercury 

 after heavy rains, and the weight of vapour condenfed ; for 

 in fuch cafes, the mercury frequently nfes 3 or 4 tenths of 

 an inch ; and yet the heavlell rain feldom produces one 

 cubic inch of water, and the weight of a cubic inch of 

 water is not equal to that of even one tenth of a cubic inch 

 of mercury. 



Mr. Kirwan, having examined the caufes to which the 

 variable weight of the atmofphere and height of the baro- 

 meter have been ufually referred, and controverted their 

 fufficiency, proceeds to explain that which alone feems to 

 him adequate to the eftedls produced. This, in his opinion, 

 is the accumulation of air over thofe parts of the globe it» 

 which the mercury exceeds its mean height ; that is, the 

 height fuited to its fituation ; and the diminution or fub- 

 traclion of the natural quantity of air over thofe regions in 

 \vhich the mercuiy falls beneath its mean height. In order 

 to trace the origin of this accumulation and diminution, this 

 ingenious author confiders what may be called the natural 

 ftate of the atmofphere, and how that ftate may be difturbed. 

 The natural ftate of the atmofphere is that in which the ba- 

 rometer on the level of the lea would ftand at 30 inches in 

 fcrene weather, conformably to the fifth obfervation above 

 mentioned. For producing this ftate, the weight of the 

 atmofphere muft be every where equal at the furface of the 

 fea ; and as its weight proceeds from its denfity and height, 

 in order to obtain this equality of weight, it fhould be 

 loweft where its denfity is greateft, and higheft where its 

 denfity is leaft ; and thcfc extremes of denfity take place in 

 the equatorial and polar regions. Hence it follows, that if 

 the height of the mercury be 30 inches under the equator 

 and under the poles, the atmofphere muft be higheft under 

 the equator and loweft under the pole?, with feveral interme- 

 diate gradations. (See Figure of t/ie Aryiosf HERE.) But 

 though the equatorial air be lefs denle to a certain height 

 than the polar, yet at certain greater heights it muft be 

 more denfe ; for the mercurial heights at the level of the 

 fea being equal, the m.alfes of the correfponding atm.ofpheric 

 columns muft be equal. The fame obfervation applies to 

 the extratropical columns with reipeft to each other, where 

 great differences of heat prevail. Hence it follows, that in 

 the higher regions of the atmofpliere, the denfer equatorial 

 air, not being fupported by the collateral extratropical co- 

 lumns, gradually flows to the north and fouth. If the af- 

 fluence of the. northern and louthern air to the equator by 

 the trade winds kept pace witli the effluence of the fupe- 

 rior air, fome degi ee of equilibrium might ftill be maintained. 

 But the trade winds move only at the rate of about 8 miles 

 an hour ; whereas without the tropics, or at leaft beyond 

 latitude 30°, the currents of the upper atmofphere are in- 

 comparably more rapid. The mean heat of thewhole fpace 

 between latitude o" and latitude 30^ being only feven de- 

 grees lefs than the mean heat under the equator, the dif- 

 ference of denfity is not fo great as to caufe any rapid col- 

 lapfion of the fuperior columns within that fpace ; but from 

 latitude 30° to latitude 60'^, a much fmailcr fpace, the mean 

 annual heat over the ocean differs from that of latitude 30° 

 by neariy fourteen degrees; and,therefore,the rapidity of the 

 uppsrr current towards the polar regions is nu ;h greater,and 

 will occafion frequent interruptions, during which the weight 

 of the air will be diminifhed. Hence, notwithftanding the 

 high winds that frequently prevail between the tropics, 

 the barometer inconfiderably and but feldom varies; whereas, 

 without them, the variations are frequent and confiderable, 

 nearly in proportion to the diftance from the equator : and 



thus 



