THE ATMOSPHERE. 



^N^VX~*ta^-^ 



195 / 



the vessel be detached from the syringe. Let it then be placed in the dish of 

 a well-constructed balance and accurately counterpoised by weights in the op- 

 posite s,cale. The weight which is thus counterpoised is that of the vessel, 

 and the small portion of air which remains in it, if the latter have any weight. 

 Let the stop-cock be now opened and the external air will be immediately heard 

 rushing into the vessel. 



When a small quantity has been thus admitted let the stop-cock be again 

 closed. It will be found that the copper vessel is now heavier, in a small de- 

 gree, than it was before the air was admitted, for the arm of the balance from 

 which it is suspended will be observed to preponderate. Let such additional 

 weights be placed in the opposite scale as will restore equilibrium, the stop- 

 cock being now once more opened, the air will be observed to rush in as be- 

 fore, and will continue to do so until as much has passed into the vessel as it 

 contained before the exhausting syringe was applied. The weight of the ves- 

 sel will now be observed to be further increased, the end of the beam from 

 which it is suspended preponderating. 



These facts are, perhaps, sufficient proofs that air has weight ; but the ex- 

 periment may be carried further. Let the condensing syringe be now attached 

 to the neck of the vessel, and let the stop-cock in the neck be opened so as to 

 leave a free communication between the vessel and the bottom of the syringe. 

 The construction of this instrument is such that by working it an increased 

 quantity of air may be forced into the vessel to any extent which the strength 

 of the vessel is capable of bearing. A considerably increased quantity of air 

 being thus deposited in the vessel, let the stop-cock be closed so as to pre- 

 vent its escape. The vessel being detached from the syringe, is restored to 

 the dish of the balance : the weights which counterpoised it before the in- 

 creased quantity of air was forced in still remaining unchanged in the opposite 

 scale. The vessel will now no longer remain counterpoised, but will prepon- 

 derate, and will require an increased weight in the opposite scale to restore 

 it to equilibrium. 



In this experiment, we see that every increase which is given to the quan- 

 tity of air contained in a vessel produces a corresponding increase in its 

 weight, and that every diminution of the quantity of air it contains produces a 

 corresponding diminution in its weight. Hence we infer that the air which is 

 introduced into or withdrawn from the vessel has weight, and that it is by the 

 amount of its weight that the weight of the vessel is increased or diminished. 



We shall hereafter have many other instances of the gravitation of atmo- 

 spheric air, but we shall for the present assume the principle that air has 

 weight, founded on the experimental proof just given. 



INERTIA OF AIR. 



That air, in common with all other bodies, possesses the qualify of inertia, 

 numerous familiar effects make manifest. Among the effects which betray this 

 quality in solid bodies, is the fact that when one solid body puts another in 

 motion, the former loses as much force as the latter receives. This loss of 

 force is called resistance, and is attributed to the quality of inertia, or inability 

 in either the striking or struck body to call into existence more force in a given \ 

 direction than previously existed. When the atmosphere is calm and free from 

 wind, the particles of air maintain their position, and are in a state of rest. If 

 a solid body, presenting a broad surface, be moved through the air in this 

 state, it must, as it moves, drive before it and put in motion those parts of the 

 air which lie in the space through which it passes. Now, if the air had no 

 inertia, it would require no force to impart this motion to them, and to drive 



