BAR 



thus the fecond ohfervation is fiifncieritly explained. Dur- 

 ing the fummer of the northern hemifphere, when it is win- 

 ter in the fouthcrn, the dcnfity of the equatorial air becomes 

 fuperior to that of the fouthern air at a much lower height 

 than that at which it becomes fuperior to the northern, 

 whxh is expanded by the prefence of the fun in the northern 

 tropic; the exuberance will therefore be poured on the fouth- 

 ern regions, and a fraaller quantity will flow over the north- 

 ern ; confequently the variations of the barometer are 

 fmaller with us in the fummer feafon. In winter, on the 

 contrary', the fuperior current is chiefly directed to the 

 northern hemif here, and hence the grcateil mercurial heights 

 are found in this feafon : and thus the third obfervation is 

 illullrated and confirmed. This accumulation takes place 

 where the columns of the inferior air are coldeft and fliorteft, 

 as over that part of Afui beyond latitude 35°, and eaft of the 

 Cafpian fca to the Frozen ocean, and over the continent of 

 North America ; and hence the barometer ufually (lands 

 higher in North America, and varies lefs than with us even 

 in Hudfon's bay, latitude 59°. Accumulations are alfo 

 found in the fouthern parts of the old continent ; and 

 wh'en the rarefaction in the northern parts of Europe is fre- 

 quent and confiderable, the fouthern air flows from thefe 

 trafts to reftore the equilibrium., and while this current lafts, 

 the barometer niuft fall in the intermediate regions ; fo that 

 the defcent of the mercui"y is never the effect of a foutherly 

 wind, but both it and this wind are the concomitant effefts 

 of a rarefaftion in the northern parts. On the other hand, 

 the mercury generally lifes under a northerly or eailerlywind, 

 becaufe the fuperior atmofphere is accumulated chiefly in 

 thofe parts of our hemifphere from whence thefe winds iflue, 

 and this accumulated air pafles with them to the fouthern 

 regions. In the fame manner, when the mercurj- falls before 

 a llorm, both the ftorm and this fall proceed from, a great 

 rarefaction of the air in the quarter towards which the ftorm 

 blows, and this rarefaftion is occafioned by the diminution 

 or deftruftion of the fuperior atmofphere. As the fuperior 

 accumulation is derived to us chiefly from North America, 

 hence it is that the variations of the barometer generally 

 begin to the weftward w-ith us in Europe, and are thence 

 gradually propagated eallwards. In fpring the current of 

 fuperior air begins to flow to the fouth, and in autumn to 

 return from it ; hence the equinoftial florms and frequent 

 variations of the barometer in thofe feafons. The quantity 

 of equatorial air devolved on our hemifphere in different 

 years is variable, and fo is the quantity confuraed in the 

 northern regions ; and hence the mean barometrical height 

 is different in different years. In feme years, the accumu- 

 lation rellinj on the mountainous countries of the fouth of 

 Alia and Europe, and the northern part of Africa, is greater 

 than in other years ; owing perhaps to a greater or earlier 

 fall of fnow : when this is the cafe, the northern air is 

 lighter, and the fouthern colder, than ufual, and fouth winds 

 principally prevail, which in the northern parts mud feem to 

 be comparatively v>'arm ; and hence, when the winter is re- 

 markably fevere in the fouth of Europe and Afia, it is often 

 as remarkably mild in the northern parts, and the barometer 

 low. Although clouds and a difpofition to rain frequently 

 follow the defcent of the mercury, this defcent is not the 

 immediate confequence of either clouas or rain on the 

 contrary, the mercury frequently rifes during rain. But the 

 rarefaftion of the atmofphere, which produces the defcent 

 of the mercury, and which arifes from the removal of the 

 fuperior accumulation, is favourable to the produftion of 

 clouds ; as a heavy atmofphere, though it fuppoits vapours 

 once formed, obflrufts evaporation : who - therefore its 

 weight is diminifhed, and evaporation increaied, it foon be- 

 comes faturatsd in the higher regions, and clouds are formed. 



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But rain feems to arife from a fubtraftion of the eleftrical 

 fluid, which, when the air abounds with ^-apours, is eafily 

 condufted to the earth. In ferene and fettled weather the 

 mercury is generally high, becaufe the greateft difturbances 

 of the atmofphere are connefted with its rarefied flate, which 

 is commonly pretty diilant when the fuperior accumulation 

 is CDnfiderable. 



Th?t the variations of the mercurial heights (hould be 

 greater at the level of the feathan at great elevations above 

 that level, is very natural. For fuppofing the mercury at 

 the level of the fea to Hard at 30 inches, and at a certain 

 elevation above that level at 25 inches, then if the weight 

 of the atmofphere be diminifned one hundredth part, the mer- 

 cury at the level of the fea ihould fall one h.undredth part of 

 30 inches = 0.3 of an inch, but that on the elevation ftiould 

 fall one hundredth of 25 inches, = 0.25 of an inch. But 

 it has been obferved, that the variation on high mountains 

 is beyond all proportion fm.aiier than on the level of the fea j 

 and this proceeds from a property which they feem to pof- 

 fefs of condenfing and accumulating the a:r incumbent upon 

 them in a greater degree than the air incumbent over plains is 

 condenfed at equal heights ; and hence, when the barometer 

 on the plains falls, and that on the mountain alfo, it will be 

 found, after allowing for the difference of temperature, that 

 the fall is proportionably greater in the inferior than in the 

 fuperior barometer; and, on the contrar)-, if the mercury 

 afcends in both barometers, the afcent will be proportionably 

 greater in the fuperior than in the inferior. To this purpofe 

 General Roy found, on the ythof Auguft 1775, ^^ 9 o'clock, 

 the correft height of a barometer on Caernarvon quay 

 30.075, and on the peak of Snowdon 26.41 8 inches ; at 12 

 o'clock, that on the quay fell to 30.043, and that on the 

 peak to 26.405 ; the fall of the mercury on the plain was 

 therefore xoVr °f ^^'^ whole, and the fall on the mountain 

 was only o^'j^^ of its original height. On the other hand, 

 at 2 o'clock, the barometer on the quay rofe to 30,045, 

 while that on tlie peak rofe to 26.415 inches correft height j 

 therefore that on the quay afcended only , . j, ,^ ^ of the 

 whole, and that on the peak afcended ^^ '^,^th part of its 

 height. Yet as the defcents of the mercury beneath its mod 

 ufual mean height are much more frequent and confiderable 

 than its afccnts above it, the variations on mountains are 

 upon the whole proportionably fmaller than at the furface 

 of the fea. For a more particular illuilration of the theory 

 of Mr. Kirwan, and the collateral obfervation s wiiich he 

 deduces from it, we m.uft refer to his paper, ubi fupra. Sec 

 Atmospheri, and Aurora iorci7/(j. For other prognof- 

 tics of the weather, befiJes the variations of the barometer, 

 fee Weather. 



Another important purpofe to which the variations of the 

 barometer have been applied, is the " meafurement of alti- 

 tudes." Whilll M. F'afcal and M. Pcnier were profecuting 

 experiments for afc:?rtaining the weigh.t of the air by means 

 of the barometer, as early as the year 1648, they found that 

 the mercurial heights varied according to thefituatioHS, either 

 more elevated or more depreffed, in which the barometer 

 was placed ; and hence they concluded, that this inftrument 

 might ferve to deternii.ie how much one place was higher 

 than another. M. Pafcal was "not unacquainted with the 

 dilatability of the air, and he was therefore apprized of 

 one of the difficulties that have attended experiments of 

 this nature. The firil perfon who ciiimated tiie lieight of 

 the atmofphere on thefe principles was Kepler ; but hav- 

 ing, from ill-condufted experiments, ver;^ erroneous ideas of 

 the proportional fpecific gravities of mercury and air, he 

 ftated it at only two or three Enghfh miles. The Honour- 

 able Mr. Boyle, deducing from experiments the proportion 

 of the fpeci^c gravity of mercury to that of air to be as 1 to 



