3 66 Measurement of Powder Pressures in Cannon . [May, 
tear in pieces a brass or copper plate upon which they are 
gently heated. 
The variations of pressure sustained by the gun when 
fired with charges very nearly the same are greater, as 
might be expected, than the variations of initial velocity 
imparted to the ball under similar circumstances. 
When the combustion of the powder takes place with 
more than average rapidity, the pressure in the first instants 
of the explosion is augmented, but its adtion on the ball is 
not so well sustained as in the case in which the combus- 
tion is more slow and consequently longer continued. 
In an article “ On the Pressure of Fired Gunpowder in 
its Practical Applications,” in the “ American Journal of 
Science and Arts,” September, 1856, Dr. Woodbridge, after 
reference to the experiments of Count Rumford, who esti- 
mated the pressure of gunpowder fired in a space which it 
filled at not less than 54,750 atmospheres — from an erroneous 
estimate of the strength of his eprouvette, which was burst 
by the charge — gives the following experiment, which seems 
to show that the extreme force of gunpowder fired in small 
quantities does not exceed 6,200 atmospheres: — “ I inclosed 
in a hollow cylinder of cast-steel, ij ins. in exterior dia- 
meter and one-fourth of an inch in diameter interiorily, 20 
grs. of Hazzard’s Kentucky rifle powder, which filled loosely 
the cavity. This was fired by a flash of powder penetrating 
through the aperture of a valve (of steel) opening inward, 
but designed to prevent the escape of gas outward. The 
cylinder was not ruptured, and, being put under water, no 
gas was found to escape ; (the weight of the instrument 
was too great to test the loss of gas by my scales.) On 
pressing in the valve by means of a screw, an abundance of 
gas escaped, carrying with it the odour of sulphuretted 
hydrogen. The seat of the valve was found to remain per- 
fect; ; a facft which, when compared with a former trial in 
which the gases escaped in consequence of a slight defedt 
of the valve, is presumptive proof of its immediate adtion. 
The residuum was found to weigh 10*45 grs. The calculated 
strength of the cylinder would be equal to an internal pres- 
sure of about 93,000 lbs. per square inch, or 6,200 atmo- 
spheres of 15 lbs.” Dr. Woodbridge regards the method 
of ascertaining powder pressures by the registered compres- 
sion of a liquid as capable of greater accuracy than those 
which are based upon indentation or other change of form 
in pieces of metal adted on by a piston receiving the pres- 
sure to be measured. 
