56 
pressures in the receiving vessel, ranging from one half- 
pound to nearly 5 or 6 atmospheres. 
With smaller pressures in the discharging vessel the times 
occupied by the pressure in falling a proportional distance 
are nearly the same until the pressure in the receiving vessel 
reaches about the same relative height. 
What the exact relation between the two pressures is 
when the change in rate of flow occurs is not determined 
in these experiments. For as the change comes on slowly 
it is at first too small to be appreciable in such short intervals 
as 7*5 and 8 seconds. But an examination of Mr. Wilde’s 
table VI. shows that it lies between *5 and ’5 3. 
This very remarkable fact, to which Mr. Wilde has re- 
called attention, excited considerable interest 15 or 20 years 
ago. Graham does not appear to have noticed it, although, 
on reference to Graham’s experiments, it appears that these 
also show it in the most conclusive manner. See table IV., 
Phil. Trans, iv., 1846, pp. 573 — 632. Also Reprint, page 106. 
These experiments also show that the change comes on 
when the ratio of the pressures is between *483 and *531. 
R. D. Napier appears to have been the first to make the 
discovery. He found by his own experiments that the 
change came on when the ratio of pressures fell to *5 See 
Ency. Brit., Yol. xii., p. 481. Zeuner, Flieguer, and Him 
have also investigated the subject. 
At the time when Graham wrote, a theory of gaseous 
motion did not exist. But after the discovery of the mecha- 
nical equivalent of heat and thermo-dynamics a theory 
became possible, and was given with apparent mathematical 
completeness in 1856. This theory appeared to agree well 
with experiments until the particular fact under discussion 
was discovered. This fact, however, directly controverts the 
theory. For on applying the equations giving the rate of 
flow through an orifice to such experiments as Mr. Wilde’s, 
it appears that there is a marked disagreement between the 
