19 
It was long ago shown by Faraday^' that, in the passage 
of different gases throngh 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 
1S5‘5" respectively ; but equal volumes of each passed 
through the same tube at equally low pressures in 8' -15" 
and 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 philoso» 
phers on this point, all are agreed in estimating the quantity 
of air discharged from a higher into air of a lower density, 
form 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 pressure. This mode 
of determining the amount of the discharge from a higher 
to a lower density, like that of the velocity of the atmos- 
phere 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 its specific velocity of discharge, such a series of 
experiments might be useful in confirming and extending 
our knowledge of the dynamics of elastic fluids. In the 
course of these experiments, I have met with some results 
which I thought of sufficient importance to bring before 
the Society. 
The apparatus employed in this investigation consisted 
of two strong cylinders of cast iron, shown in the engraving. 
* Quarterly Journal of Science, 1818, vol. vii., p. 106, 
