WITH WHICH AIll RUSHES INTO A VACUUM. 147 



velocity which a heavy body would acquire by falling from 

 the top of a homogeneous atmosphere of the same density 

 as that on the earth^s surface ; and since air is about 840 

 times lighter than water, if the whole pressure of the 

 atmosphere be taken as equal to support 33 feet of water^ 

 we have the height of the homogeneous atmosphere equal 

 to 27^720 feet^ through which, by the free action of gravity, 

 is generated a velocity of 1332 feet per second. This, 

 therefore, is the velocity with which air is considered to 

 rush into a vacuum, and is taken as a standard number 

 in pneumatics, as 16 and 32 are standard numbers in 

 the general science of mechanics, expressing the action 

 of gravity on the surface of the earth. 



Now, so far as I am aware, no experiments have 

 hitherto been made directly proving this important pro- 

 position. It is true that attempts have been made to 

 determine the initial velocity by discharging air at 

 extremely low pressures into the atmosphere ; but, apart 

 from the conditions of the discharge into the air and into 

 a vacuum being different, the history of physical science 

 shows that it is unphilosophic to predicate absolute 

 uniformity of any law through the order of a whole range 

 of phenomena of the same kind ; as nature is full of 

 surprises when pushed to extremes, or when interrogated 

 under new experimental conditions. 



It was long ago shown by Faraday * that, in the passage 

 of different gases through capillary tubes, an inversion of 

 the velocities of different gases takes place under different 

 pressures, those which traverse quickest when the pressure 

 is high moving more slowly as it is diminished. Thus, 

 with equal high pressures, equal volumes of hydrogen gas 

 and olefiant gas passed through the same tube in 57" and 

 i35"'5 respectively; but equal volumes of each passed 



* Quarterly Journal of Science, 1818, vol. \n. p. 106. 



l2 



