VOL. XVI.}rA i PHILOSOPHICAL TRANSACTIONS. 305 



Helena, make very little or no variation of the height of the mercury, in all 

 weathers. 



I conceive then, that the principal cause of the rise and fall of the mercury, 

 is from the variable winds which are found in the temperate zones. A second 

 cause is the uncertain rising and falling of the vapours lodging in the air, 

 whence it comes to be at one time much more crowded than at another, and 

 consequently heavier ; but this latter in a great measure depends on the former. 

 Now from these principles, I shall endeavour to explain the several phsenomena 

 of the barometer. As, 



1 . Why in calm weather, the air being inclined to rain, the mercury is com- 

 monly low ? I answer that, the mercury's being low, inclines it to rain ; for the 

 air being light, the vapours are no longer supported by it, becoming specifically 

 heavier; so that they descend towards the earth, and in their fall meeting with 

 other aqueous particles, they incorporate together, and form little drops of 

 rain ; but the mercury's being at one time lower than at another, is the effect of 

 two contrary winds, blowing from the place where the barometer stands ; by 

 which the air of that place is carried both ways from it, and consequently the 

 incumbent cylinder of air is diminished, and accordingly the mercury sinks. As 

 for instance, if in the German Ocean it should blow a gale of westerly wind, 

 and at the same time an easterly wind in the Irish sea ; or if in France it should 

 blow a southerly wind, and in Scotland a northern ; it must be granted, that 

 that part of the atmosphere over England, would thereby be attenuated, and 

 the mercury would subside; and the vapours, which before floated in those parts 

 of the air, of equal gravity with themselves, would sink to the earth. 

 Ir 2. Why in serene settled weather, the mercury is generally high ? To this I 

 answer, that the greater height of the barometer is occasioned by two contrary 

 winds blowing towards the place of observation, by which the air of other places 

 is brought thither and accumulated ; so that the incumbent cylinder of air 

 being increased both in height and weight, the mercury pressed by it must ne- 

 cessarily rise ; and then the air being specifically heavier, the vapours are better 

 kept suspended. 



3. Why upon very great winds or storms, though accompanied with no rain, 

 the mercury sinks lowest of all, with relation to the point of the compass, on 

 which the wind blows? This is caused by the very rapid motion of the air in 

 these storms ; for the tract of the earth's surface where these winds rage not 

 extending all round the globe, that stagnant air which is left behind, as also 

 that on the sides, cannot come in so fast as to supply the evacuation made by 

 so swift a current ; so that the air must necessarily be attenuated, more or less, 

 according to the violence of those winds. To which add, that the horizontal 



vox.. III. R R 



