VOL. XV.] PHILOSOPHICAL TRANSACTIONS. ] 65 



when things are in other respects at rest ; but when they are in commotion, it 

 is often otherwise. And in such cases, we must, beside the respective gravity, 

 take into consideration the force, impulse, or impetus, superadded to the re- 

 spective gravity of the parts or matter. Thus, in a jetty of water, the water is 

 thrown up into the air to a great height, not because it so becomes lighter than 

 the air, but on account of the impressed force. And this I take to be the cause 

 of fumes, vapours, and other like matters, which ascend in the air, not that 

 they are lighter than it, but because impelled upward from the bowels of the 

 earth, or from the superficial parts of it by an external force. And that there 

 are such fumes, &c. projected upwards, from the bowels of the earth, and 

 some of them with great violence, is undeniable, from the instance of earth- 

 quakes and other eruptions. And to such causes I principally attribute the ori- 

 gin of winds, and the ascent of most other things into the air. 



There is yet another motion suggested, which is also very considerable as to 

 this affair; which is, the weakening or strengthening the spring of the air. 

 And that this spring of the air is sometimes stronger, and sometimes weaker, 

 I think is undoubted also: and that the spring of the air is strengthened both 

 by compression and by heat, but in a different manner. For, if the same quan- 

 tity of air be compressed into a less room, the spring is certainly stronger; as is 

 plainly seen in the wind-gun, and other compressing engines. Again, the same 

 quantity of air included in a close vessel, will, by application of heat, have its 

 spring strengthened; as is observable in thermometers of all sorts. If the 

 spring be strengthened by compression, it is manifest that the intensive gravity 

 must be thereby increased, because the same quantity of air, and consequently 

 of weight, extensively taken, is now contracted into less room, which therefore 

 must be intensively heavier, as being the same weight in a less bulk; now this 

 may possibly, as a greater pressure or stronger spring, force up the vapours 

 under it with a greater impetus, and so make them fly higher, but not so as to 

 make them lighter, but rather the contrary, as pressing them closer together, 

 much less to make them specifically lighter than the air itself. 



If the spring be strengthened the other way, as by heat, this rather dimi- 

 nishes its intensive gravity by thrusting its parts further asunder, and so pos- 

 sessing a larger space. Now in case this air be, by a close vessel, so confined 

 as not to expand upward, it will certainly press the stronger on the stagnant 

 quicksilver below, and make that in the tube rise higher. But in case it be un- 

 confined, as in the open air, it may as well expand itself upward, by making the 

 atmosphere in this part so much higher. Nor is there any necessity, as to the 

 subjacent parts, that the atmosphere should be every where of the same height. 

 For the laws of statics, as to the subjacent parts, may be equally preserved with- 



