WEIGHT OF AIR. 



5. It is a great mistake, however, to imagine that air is desti- 

 tute of weight, that quality which is inseparable from whatever 

 is material. Light it undoubtedly is, but only by comparison. 

 Bulk for bulk, it is lighter than stone, earth, or water, or any. 

 other substance in the solid or liquid state. But light as it is, it 

 has a certain definite weight, and a quantity of it can be assigned 

 which will weigh many tons. The pressure produced by its weight 

 is under certain assignable circumstances quite enormous, and 

 when it is moved with a certain velocity its force is so irresistible 

 that trees are torn by it from their roots, the most solid buildings, 

 overturned and reduced to ruins, and devastation spread over vast 

 tracts of country. 



Nothing can be easier than to show practically that air has 

 weight, and what that weight is. 



6. If a glass flask, having the capacity of a cubic foot, be pro- 

 vided with a proper neck, furnished with a stop-cock, we shall be 

 able, by means of a well-constructed syringe, to extract from it 

 the air which it contains, and by closing the stop-cock, and 

 detaching the syringe, we shall have the flask void of air. Let it 

 be weighed in that state in a good balance. Let the stop-cock be 

 then opened so as to admit the air to fill the flask, and let it then 

 be weighed again. It will be found to weigh 1-291 oz. or 564-8 

 grains more than it did when void of air. 



It follows therefore that a cubic foot of air weighs 564-8 grains. 



Since the weight of a cubic foot of water is 997-125 oz., it 

 follows that, bulk for bulk, water is heavier than air in the pro- 

 portion of 997-125 to 1-291, that is, of 772 to 1. 



Since thirty-six cubic feet of water weighs a ton, it follows that 

 772 times thirty- six cubic feet of air also weighs a ton. 



It appears, therefore, that 27810 cubic feet of air will weigh 

 a ton. 



7. When it is considered that the mass of air which taken 

 collectively is called the ATMOSPHERE, extends above us to the 

 height of more than fifty miles, it will easily be imagined that 

 the weight with which it presses on the surface of every object 

 exposed to it, must be very considerable. If, for example, we 

 take a square inch of level surface, it is clear that that square 

 inch must bear the weight of a column of air extending from 

 that surface to the top of the atmosphere. It has been ascer- 

 tained by experiments, susceptible of the greatest precision, 

 (which we shall explain on another occasion) that this pressure 

 or weight amounts to about 15lbs., and that it is subject, from 

 time to time, to a variation not exceeding three quarters of a 

 pound. 



8. It is a well known property of fluids, that any pressure which 



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