362 Measurement of Powder Pressures in Cannon. [May, 
The volume of oil subjected to pressure, the area of the 
piston, and the length of stroke being known, we derive 
from them the degree of compression. To complete the 
data for ascertaining the pressure to which the piezometer 
has been exposed in any given case, it becomes necessary to 
determine the relation of compression to the pressure 
required to effeCt it. 
The experiments made for this purpose were conducted 
with a good deal of care to secure accuracy, and were some- 
what elaborate. 
The barrel of the compressing pump is of cast-steel, 
placed horizontally, its exterior diameter being 2*2 ins., and 
that of its bore 07 ins. The piston, which is also of cast- 
steel, tempered, is forced in and retraced by means of a 
square-threaded steel screw 1*5 ins. in diameter. 
These parts are placed in a strong frame of iron. The 
screw is turned by means of a ratchet-wheel, which forms 
its head, and a lever and pawl connected with it. To 
facilitate the retraction of the piston a crank is attached. 
Screw-valves prevent, during the alternate movements of the 
piston, the escape of the liquid through the aperture by 
which it enters the barrel, and its return from the receiver 
into which it is forced. A safety-valve of tempered steel, 
ground to a seat, also in hardened steel, with its graduated 
lever and weight, is attached to the pump, and serves for 
measuring the pressures it is made to exert. 
A comparison of the pressure of a column of mercury of 
known height, with the weight on the valve of the gauge 
required to balance it, was adopted as the most accurate 
method of determining the relation between the weights to 
be placed on the scales and the desired pressures per square 
inch. 
The mercurial pressure-gauge consists of a cistern of glass 
containing mercury, and a series of perpendicular glass tubes 
attached to a graduated staff and joined continuously 
together, extending to the height of fifty-two feet. The 
tubes are so connedted with the cistern that by pressure 
upon the surface of the mercury it may be forced to ascend 
the tubes. 
In computing the pressure of the column, its height from 
the surface of the mercury in the cistern was taken, and the 
counteracting pressure of the weight of the oil by which the 
pressure was applied was allowed for in proportion to its 
height and specific gravity. 
To ascertain the compressibility of oil in steel piezometers, 
it was filled and set in the same manner as for use in the 
