Pneumatics. 63 



tually weighed. If, for instance, a bottle which 

 holds a wine quart is emptied of its air, either by 

 the action of the air pump, or by filling it with 

 quicksilver, and emptying the quicksilver out, 

 taking care that, in corking it, no air is suffered 

 to enter, it will be found to be sixteen grains 

 lighter than it was before it was emptied of its air. 

 A quart of air, therefore, weighs just sixteen 

 grains ; a quart of water weighs fourteen thou- 

 sand six hundred and twenty-one *., which, di- 

 vided by sixteen, gives a result in round numbers 

 of nine hundred and fourteen ; so that water at 

 a medium is nine hundred and fourteen times 

 heavier than air. 



This, however, is only to be understood of air 

 near the surface of the earth ; for, in fact, as air 

 is a body possessed of gravity, that which is near- 

 est the earth sustains a greater pressure, and is 

 consequently more dense or compact ; and it is 

 rarer or more thin and light in the higher regions 

 of the atmosphere, being less pressed with the 

 weight of air which is above. The atmosphere, 

 I observed in my last lecture, is that mass of air 

 which surrounds the globe, and which is gene- 

 rally computed to be about forty-five miles in 

 height. If altitudes in the air are taken in arith- 

 metical proportion, the rarity of the air will be 

 in geometrical proportion ; and therefore sup- 

 posing that the atmosphere extended to the height 



* A quart of water is generally calculated at two pounds, 

 bat it is in fact something less. 



