16 



PNEUMATICS. 



the entire apparatus. Upon the first 

 depression of the piston this fourth part 

 will be expelled, and three-fourths of 

 the original quantity will remain. One- 

 fourth of this will in like manner be 



consequently there remains in the appa- 

 ratus nine-sixteenths of the original 

 quantity. Calculating in this way, that 

 one-fourth of what is contained in the 

 apparatus is expelled at every descent 



expelled upon the second depression of of the piston, the following Table will 

 the piston, which is equivalent to three- be easily computed. 



sixteenths of the original quantity, and 



No. of 

 Strokes. 



1. 



Air expelled at each 

 Stroke. 



ning 

 and 



Barrel. 



The method by which the computa . 

 tion might be continued is obvious. 

 The air expelled at each stroke is found, 

 by multiplying the air expelled at the 

 preceding stroke by 3 and dividing it by 

 4 ; and the air remaining after each 

 stroke is also found by multiplying the 

 air remaining after the preceding stroke 

 by 3, and dividing it by 4. 



It appears by this computation, that 

 after the fifth stroke, the air remaining 

 in the receiver is less than one-fourth of 

 the original quantity. Less than one- 

 fourth of this will remain after the next 

 five strokes, that is, less than one six- 

 teenth part of the original quantity. If 

 we calculate that every five strokes ex- 

 tract three-fourths of the air contained 

 in the apparatus, we shall then under- 

 rate the rapidity of the exhaustion ; 

 and yet, even at this rate, after thirty 

 strokes of the pump, the air remaining 

 in the receiver would be only one 

 3096th part of the original quantity. The 

 pressure of this would amount to about 

 the sixteenth part of an ounce upon 

 the square inch. It is evident that by 

 continuing the process any degree of 

 rarefaction which may be desired can 

 be obtained. For all practical purposes, 

 therefore, a vacuum may be considered 

 to be procured ; but, in fact, we are as 

 far from having a real vacuum in the 

 receiver as ever, for such is the infinite 

 expanding power of air, that the smallest 

 particle will as completely fill the re- 

 ceiver and barrel as the most dense 

 substance could ; that is to say, no part 

 of the receiver or barrel, however small, 



will "be found absolutely free from air, 

 however long the process of exhaustion 

 may be continued. 



(34.) In the uses to which the air- 

 pump is applied, various degrees of ex- 

 haustion are necessary, and it becomes 

 very desirable to have some exact and 

 obvious indicator of the degree of rare- 

 faction which has been produced within 

 the receiver at any stage of the process. 

 We find in the barometer a simple and 

 most accurate means of measuring this 

 effect. 



Let a glass tube, open at both ends, 

 be inserted into the exhausting tube 

 AB (Jig. 16.), or into any other part of 



the apparatus, so as to communicate 

 freely with the receiver; and being 

 placed in a vertical position, let the other 

 end G be immersed in a cistern of nier- 



