l66 PHILOSOPHICAL TRANSACTIONS. [^NNOlSSS. 



out it, the greater altitude compensating for the levity of the parts, as when 

 a portion of the sea is covered with a fleet of ships, the under parts are equally 

 pressed, partly by water and partly by ships, though the tops of the ships over 

 some parts be higher than the surface of the water over others. Only in such 

 case the upper part of the atmosphere being fluid, may flow collaterally over 

 the other parts on either side, if lower; and so at leisure reduce itself to an 

 equal height in all parts. 



But though the spring of the air, increased by heat, may thus expand itself 

 upward, yet, because it presses every way, it must act in like manner down- 

 ward also, and thereby press harder on what is under it; and because it will re- 

 quire time to work upward gradually, before the effect reach the top of the 

 atmosphere; and because by such dilatation of its parts more room is left in the 

 intervals to receive what is forced; it is reasonable to believe that in such cases, 

 the pressed vapours, caeteris paribus, may rise more copiously than when the 

 spring of the air, for want of heat, is less strong ; the rather, because the same 

 heat, which thus strengthens the spring of the air, also rarefies the vapours, and 

 makes them lighter, and may also increase the subterraneous heat, or whatever 

 else it is, that drives them upward. Notwithstanding all which, we have more 

 rains in winter: which should argue, that more vapours do then rise to supply 

 them. 



But I suspect that, in this whole business of strengthening the spring, we 

 may be under some mistake ; and what we think to be produced on the open 

 air, is indeed effected upon the quicksilver, or rather on the air latent therein. 

 My meaning is this, we find that in very hot weather, and also in frosty wea- 

 ther, the quicksilver in the tube commonly stands very high, from whence we 

 are apt to conclude, that therefore the outward air presses very hard on the 

 stagnant quicksilver, without the tube. But in this I am not satisfied. For we 

 are apt to consider, that in filling the tube with quicksilver before it is inverted, 

 if great care be not used to cleanse it from air, many aerial particles will remain 

 mixed with it, which, while their spring is weak, are easily pressed by the 

 weight of the quicksilver, so close as hardly to be discerned otherwise than by 

 the effect ; but when, by the external heat, their spring is strengthened, they 

 expand themselves, and cause the quicksilver to swell in bulk, without increasing 

 its weight; and consequently to stand higher though not to press heavier. And 

 the same solution perhaps may serve for its standing so high in frosty weather; 

 for water, we know, though it contract with cold, yet when it freezes it expands 

 itself; which makes ice lighter than water, and to swim on the top of it. Now 

 whether this be purely of itself, or in part at least, from the particles of air 

 lodged in it, may not perhaps be so easy to determine. However, if there be 



