PNEUMATICS. 



3 



7.3. 



fig. I. 



be found that the water will not enter 

 tho inverted receiver, except to a very 

 limited height as E, as will be made 

 :e by the cork floating on the sur- 

 face and seen through the glass. Thus 

 the air in the receiver wliich occupies 

 the space C E excludes the water. That 

 this is the cause of its exclusion will be 

 rendered apparent by opening the stop- 

 cock F, by which the air which hitherto 

 prevented the ascent of the water be- 

 yond the level E will escape, and the 

 water will immediately rise until it as- 

 sumes the same level in both receivers. 

 Again, if the piston described in (2.) 

 be forced against the air confined in the 

 cylinder, it will be found that no force 

 whatever will compel it to reach the 

 bottom ; however strong the apparatus 

 may be, it will burst or break before this 

 can happen. 



(8.) In both of these experiments it 

 misrht seem that the air was partially 

 penetrable, since in the former the water 

 entered the cylinder to the height E, and 

 in the latter, although the piston could 

 not be forced to the bottom, yet it was 

 found to approach it. These effects," 

 however, were not produced by the pe- 

 netration of the air contained in the 

 respective vessels, but by its compres- 

 sion. The air in both cases yielded to 

 the body, which entered the cylinder and 

 contracted itself into a smaller space. 



It would be very easy to multiply 

 proofs of the existence of this property 

 in air, but it is the less necessary, as ad- 

 ditional proofs of it will be perceived in 

 many of the experiments which we shall 

 have to describe for other purposes. 

 The same, indeed, may be said of all 

 the other properties of air which we are 

 about to establish, and such is ever the 

 uature of principles established by philo- 

 sophical investigation. They are con- 

 tinually reappearing and soliciting our 

 notice when we are not seeking them, 

 and we recognise fresh proofs of them 

 in investigations apparently the most 



remote from those in which they first 

 originated. 



The quality in air which we have 

 called impenetrability, is sometimes 

 called solidity, and air is said to be 

 solid. There is, however, an objection 

 to the use of this term in this sense. 

 Solid has already been used in opposi- 

 tion to fluid, and air is of the latter 

 class. The word solid would thus be 

 used in two different senses, which 

 should be avoided. We have, there- 

 fore, in the present instance, thought it 

 better to express that universal property 

 of all species of matter by which it re- 

 fuses admission of other matter to the 

 space it fills until it has deserted it, by 

 the negative term impenetrability. 



(9.) Air is inert and moveable. 



The quality of inertia which is known 

 as an universal property- of matter, is 

 that in virtue of which it" requires a cer- 

 tain effort or force to produce motion in 

 matter if it be at rest, and to destroy or 

 modify any motion which it has if not 

 at rest. 



That air possesses inertia we have 

 numerous and familiar proofs. Wind 

 is nothing but air in motion. Any ob- 

 stacle which opposes this motion sus- 

 tains a considerable pressure, and must 

 exert a proportionate resistance, other- 

 wise it will be carried forward with the 

 body of moving air. Such are the 

 effects of wind on balloons and other 

 bodies floating in the atmosphere. Also 

 on ships and all bodies floating on water 

 which present sufficient surfaces in 

 opposition to the wind, such as sails. 

 Nay, we find, notwithstanding the ex- 

 treme thinness of the atmosphere which 

 surrounds us, that it is capable of ex- 

 erting very powerful degrees o f force 

 when it acquires sufficient velocity: 

 witness the effects of hurricanrs in agi- 

 tating the waters of the ocean, in tearing 

 up by the roots the largest frees, in 

 overthrowing buildings, &c. These 

 effects establish beyond question the 

 B 2 



