1% FERGUSON'S LECTURES. 



LECT. That air is a body, is evident from its excluding all 

 **~s"^, other bodies out of the space it possesses : for, if a 

 glass jar be plunged with its mouth downward into a 

 vessel of water, there will but very little water get into 

 the jar, because the air of which it is full keeps the 

 water out. 



As air is a body, it must needs have gravity or 

 weight : and that it is weighty, is demonstrated by ex- 

 periment. For, let the air be taken out of a vessel by 

 means of the air-pump ; then, having weighed the ves- 

 sel, let in the air again, and, upon weighing it when 

 re-filled with air, it will be found considerably hea- 

 vier. Thus, a bottle that holds a wine quart, being 

 emptied of air and weighed, is found to be about 16 

 grains lighter than when the air is let into into it again ; 

 which shews that a quart of air weighs 17 grains. But 

 a quart of water weighs 14621 grains ; this divided by 

 16, quotes 914 in round numbers; which shews, that 

 water is 914 times as heavy as air near the surface of 

 the earth. 



As the air rises above the earth's surface, it grows 

 rarer, and consequently lighter, bulk for bulk. For, 

 because it is of an elastic or springy nature, and its 

 lowermost parts are pressed with the weight of all (hat 

 is above them, it is plain that the air must be mare 

 dense or compact at the earth's surface, than at any 

 height above it ; and gradually rarer the higher up. 

 For, the density of the air is always as the force that 

 compresseth it ; and therefore, the air towards the upper 

 parts of the atmosphere being less pressed than that 

 which is near the earth, it will expand itself, and there- 

 by become thinner than at the earth's surface. 



Dr. Cotes has demonstrated, that if altitudes in the 

 air be taken in arithmetical proportion, the rarity 

 of the air will be in geometrical proportion. For in- 

 stance. 



