VOL. XV.] PHILOSOPHICAL TEANSACTIONS. ISQ 



ing in that quarter of the heavens where the sun is at that time below the 

 horizon ; tliis has given me occasion to think that perhaps the steams of vapours 

 may have ascended so far in the atmosphere, as to be beyond the earth's shadow 

 in that part of the horizon, and so, by refracting the beams of light towards 

 us, to occasion that light and those caprae saltantes. It may be considered also 

 whether the red skies in the evening, which denote fair weather, do not proceed 

 from the height of the clouds, occasioned at that time by the increase of the 

 specific gravity of the atmosphere. 



Now as to the falling down again of the vapours, it is visible by their gather- 

 ing into thick and dark clouds, by the falling down of clouds and mists on the 

 tops of hills, and thick fogs in the air, and by their dropping down into rain, 

 snow, &c. and that these do usually happen only when the mercury subsides a 

 little, and consequently when the atmosphere's gravity is less, is the constant 

 observation of those who have had occasion to take notice of the changes of the 

 barometer. 



Against this it may be objected, that it is observable many times that even 

 when the mercury in the barometer is rising, there will be rain, and particularly 

 sometimes in north-east winds. To this I answer, that the vapours will fall 

 down in rain, not only when the specific gravity of the atmosphere is lessened, 

 but also if the clouds have been carried for some while towards one quarter of 

 the heavens by the winds, and then if the winds change suddenly into another 

 quarter, these vapours which were formerly scattered in small particles, and so 

 easily floated, are suddenly driven together into little drops, and so must needs 

 fall down in rain, and therefore the falling of rain while the mercury is rising, 

 is observable only on the sudden change of contrary winds. 



In the next place, let us consider, whether those frequent commotions in the 

 air, which we call winds, may not be accounted for on the same principles. As 

 to fluids in general, that known definition of Archimedes is universally acknow- 

 ledged, viz. that the parts less pressed, give way to those that are more pressed; 

 so that if there be any portion of a fluid of a far less pressure and resistance than 

 the rest, the whole fluid runs in a current thither, till all be reduced to an 

 equilibrium. ISlow as to the fluid of air in particular, it is evident that its 

 pressure is not always the same. And it is very probable that the air's gravity is 

 not alike changed throughout the whole atmosphere in an instant; but that the 

 mercury may have subsided in the barometer, and consequently the air become 

 lighter, at London, for example, when there is no such change observed at 

 Paris or Edinburgh. Now this supposition affords an intelligible account of 

 winds from the known nature of fluids. For when the air becomes specifically 

 lighter in one place, or its pressure is lessened, the neighbouring parts of the 



