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of this subject are, (1), the application of the laws of 
discharge of inelastic fluids, without any modification, to 
those which are elastic; (2), the confusion of the quantity 
of the discharge of elastic fluids after leaving the vessel, 
with the velocity of discharge through the aperture in the 
vessel ; and (3), the want of a sufif cient number of experi- 
ments, under varied conditions, and through sufficient range 
of pressure, to compare with the deductions derived from 
theory. 
It has hitherto been assumed, as a leading proposition in 
pneumatics, that air rushes into a vacuum with the velocity 
which a heavy body would acquire by fading 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 1,332 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 l6 
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 pres- 
sures 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, 
