158 PHILOSOPHICAL TRANSACTIONS. [aNNO 1685. 



of the mercury in the barometer, show that the gravity of the atmosphere is 

 not always the same. And now, from these known properties, may be easily 

 deduced a statical account of the rising of vapours, of their floating in the air 

 in clouds, and of their falling down again in rain. 



For if we may be allowed to suppose that when the atmosphere is heaviest, 

 there is some such proportion between its specific gravity, and that of the fluid 

 of vapours, as there is between water and oil ; the vapours, according to the 

 known laws of fluids, must needs ascend; and so long as this proportion con- 

 tinues, they must needs float above in the air : but when the atmosphere's 

 gravity is changed, the vapours must descend again. 



Now let us see whether this accords with observation and experience. And 

 first as to the ascent of vapours, I do not know any determinate instrument 

 that will indicate their ascent, as certainly as the barometer does the change of 

 the air's weight; for the common hygroscopes are not very exact; and besides, 

 their change by moisture shows rather the falling than the rising of vapours ; 

 yet there are two or three observations which seem to be certain indications of 

 their ascent : as first, if the horizon and the remote hills seem smoky and ob- 

 scure, so that nothing can be seen at any distance, even though the heavens be 

 not cloudy, but clear, and though there be no fog, nor any cap of clouds on 

 the hills, which rather indicate the falling of vapours. Again, if on looking 

 to any distant part of the country round about you, it appear all in an undulat- 

 ing motion, this seems to be a sign of the plentiful rising of vapours, for this 

 is only occasioned by looking through an unequal waving medium, which makes 

 frequent inflections of the beams of light; as you see any object seems to have 

 a tremulous motion in all its parts, when viewed through smoke. Another in- 

 dication of the ascent of vapours, seems to be the copious rising of steams 

 above waters, marshy grounds, and fens, which is frequently seen in frosts and 

 in cool nights in summer. To these may be added the redness of the sun, 

 so as to be easily looked on ; and of the moon, a considerable time before setting, 

 or after their rising. Now since I have had occasion to make observations of 

 the barometer, I have always noticed that when the mercury was rising, and 

 consequently in the increase of the atmosphere's gravity, I have then frequently 

 observed all the foregoing phaenomena obscure. But on the contrary, when the 

 mercury has been low, and so the atmosphere's gravity less, I have observed 

 none of these effects ; but the remote hills were clear and distinct, and no waving 

 in the air, nor steams from the waters. I know not whether I may add here a 

 conjecture about the great light and the caprae saltantes, the northern lights, 

 which are some nights to be seen in the north. I have taken notice of them 

 usually when the mercury has been high in the barometer, and then appear- 



