360 Measurement of Powder Pressures in Cannon . [May, 
In 1855, Dr. W. E. Woodbridge conduced some experi- 
ments at Washington Arsenal with the view of measuring 
the powder pressures of fired gunpowder. The results of 
these experiments were not published by the U.S. Ordnance 
Department until November, 1878, when some additional 
experiments were recorded by the author. 
It was proposed in these experiments to ascertain the 
pressure of the gases evolved by the combustion of gun- 
powder, by including in the cavity within which the pressure 
should be restrained a piezometer, which, by registering the 
compression of a liquid contained within it, should afford an 
indication of the pressure to which it had been exposed. 
This instrument, as employed in these experiments, is a 
small cylindrical vessel of steel, inclosing a quantity of oil 
destined to receive the pressure of the fluid by which it may 
be surrounded, through the medium of a piston, which is 
carried inward a distance proportional to the amount of 
compression. To the piston is attached a stem of wire, 
extending inward, on one side of which a fine point is made 
to press, inscribing, when the piston is moved, a line on the 
stem equal in length to the extent of its motion. 
In preparing the piezometer for an experiment, two items 
are to be specially observed : it must contain no air, and 
the “ setting,” or adjustment of the quantity of oil contained, 
must be done at the precise temperature the instrument is 
to have at the moment of firing. 
The procedure is as follows : — All the parts are first oiled 
over their whole surfaces. The adjusting screw is inserted 
into the body of the instrument, which is then set upright 
in a socket attached to the middle of a small pan intended 
to catch any overflow, and is nearly or quite filled with oil, 
which should be made to flow down the side of the cavity 
rather than in a stream. The support of the marking-point, 
quite clean but covered with oil, is now screwed into its 
place, with the aid of a special implement, described. When 
this is withdrawn, it will be necessary to replace the oil 
caused to overflow by its insertion. The barrel is now 
slowly put in its place and screwed firmly down. The hole 
in the piston for receiving the stem is filled with oil, the 
stem screwed in, and the piston inserted in the barrel. The 
adjusting-screw is loosened a little, permitting the piston to 
be pressed just below the top of the barrel, and again 
tightened. The next step is to bring the instrument and 
its contents to the setting temperature. For this purpose a 
water-bath (a common wooden pail) was provided ; a narrow 
tin cup, deeper than the bath, and weighted at the bottom 
