l60 PHILOSOPHICAL TRANSACTIONS. [aNNO 1685. 



atmosphere, whose weight is not thus lessened, run thither in a current, till the 

 atmosphere thereabouts be reduced to an equilibrium again ; and according to 

 the portion of air thus changed, and the lessened or acquired degrees of the 

 - weight and spring, the currents and winds are strong or weak, and of a long or 

 short continuance. 



Now observation and experience agree with this, the mercury being found to 

 subside for the most part in the barometer at the rising of winds ; at least it is 

 observed to be in motion, and either rising or falling, and consequently there 

 is a change in the atmosphere's pressure at that time. And thus we have an 

 intelligible and aerostatical account, of the ascent of vapours, of their falling 

 down again into rain, and of the currents and impetuosity of winds, from this 

 known property, the variation of the atmosphere's gravity. 



But I acknowledge the great difficulty still remains, how to account for the 

 different changes of the specific gravity of the atmosphere; and of this there 

 can hardly be expected a satisfactory account, till we come to know the cause 

 of gravity in general, and that of the air's weight in particular ; and therefore I 

 shall only here offer 2 or 3 hints, which perhaps may incite others to consider 

 it more narrowly. And 1st, it is now almost generally acknowledged, that 

 there must needs be a fluid much more subtile than common air, and of a far 

 greater pressure than it, which is the cause of the continuity and cohesion of all 

 terrestrial bodies, and in which the air seems as it were to float, and to have the 

 same relation to it, as the vapours have to the air; and therefore if we could 

 reach its nature and properties, it might be considered what influence this may 

 have on the change of the air's gravity. Or 2d, seeing the infusion of one 

 liquor into another, in chemical preparations, will alter its specific gravity, so 

 that the bodies which were formerly sustained in it, will fall down and be pre- 

 cipitated, as the particles of gold floating in aqua regis will be precipitated by 

 the infusion of another chemical liquor, it may be considered whether plenty of 

 nitrous steams, or some such mixture, may not alter the air's specific gravity. 

 Or 3dly, we may possibly come to a nearer solution of this, by considering the 

 influence which the heat and cold have upon the spring of the air. The air has 

 this peculiar property^ that it is endued with elasticity, as well as gravity ; and 

 therefore we are to consider what influence the change of its spring may have 

 on that of its weight ; and it seems evident that the increase of its spring di- 

 minishes its weight, and the lessening of its spring increases it: for on the in- 

 crease of the air's spring, the air is rarefied, and so a less portion of it presses 

 on the subjacent fluid ; but when it is lessened, the air is condensed, and so a 

 greater portion of it presses on the subjacent body. For example, let us sup- 

 pose the springy particles of air to be like to the springy hairs of wool, or the 



