532 Mr. H. Wilde on the Velocity 



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 proposition. 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 dis- 

 charge 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 interro- 

 gated 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 pres- 

 sures, 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 135"'5 

 respectively ; but equal volumes of each passed through the 

 same tube at equally low pressures in 8 / 15" and 8 / 11" 

 v^ respectively. > Again, while the velocities of discharge of 

 inelastic fluids are as the ' square roots of the heads, some 

 mathematicians have justly considered that this law does not 

 apply to those which are elastic, and have assumed with good 

 reason (though what appears unlikely at first sight) that the 

 velocity of air discharged into a vacuum is the same for all • 

 pressures. But whatever differences of opinion there may be 

 amongst natural philosophers on this point, all are agreed in 

 estimating the quantity of air discharged from a higher into 

 air of a lower density, from the difference between the two 

 densities, as in the similar case of the discharge of inelastic 

 fluids, by the difference or effective head producing the pres- 

 sure. This mode of determining the amount of the discharge 

 from a higher to a lower density, like that of the velocity of 

 the atmosphere into a vacuum, has not, so far as I know, been 

 made the subject of experiment through any considerable 

 range of pressure. It therefore appeared to me that, as each 

 gas has its specific velocity of discharge, such a series of ex- 

 periments might be useful in confirming and extending our 

 knowledge of the dynamics of elastic fluids. In the course of 

 * Quarterly Journal of Science, 1818, vol. vii. p. 106. 



