ANNUAL OF SCIENTIFIC DISCOVERY. 



ure this pressure an instrument was devised, which is illustrated in 

 the accompanying engraving. 



A hole about a third of an inch in diameter is drilled through the 

 wall of the gun to the bore, and the outer portion of this hole is en- 

 larged to receive the end of a cylinder, , which has a piston work- 

 ing within it. In the cut, b represents the portion of the cylinder 



which is screwed into the hole in the cannon, and c is the piston, cor- 

 responding in size to the smaller portion of the hole. The gases, 

 pressing on the inner end of the cylinder, force it outward. Its 

 outer end is armed with a steel point, d, which is forced into a cop- 

 per bar, e, to a depth depending upon the amount of the pressure. 

 The copper bar and steel point are then placed under massive steel- 

 yards, and the force required to produce an indentation equal to that 

 produced by the gas is accurately weighed. Capt. Rodman says that 

 a difference of two pounds in 30,000 is plainly perceptible ; " so that 

 the indications of this instrument may be safely regarded as approx- 

 imating to within 1,000 pounds of the true pressure, even for the 

 greatest pressures exerted, and much nearer for the smaller pres- 



sures. 



We give some of the most interesting results obtained : 

 Pressure per square inch due to Proof Charges in a 42-Pounder Gun. 



Pounds. 

 21 Ibs. powder, 2 shot and 1 wad, gave a pressure at the bottom of 



the bore 64,510 



14 Ibs. powder, 2 shot and 1 wad, gave a pressure at the bottom of 



the bore 55,622 



21 Ibs. powder, 1 shot and 1 wad, gave pressure .... 47,785 



Table showing the velocity of shot, in feet per second, and pressure of gas per 

 square inch, in pounds, due to equal columns of powder behind equal columns 

 of metal, when fired in guns of different diameter of bore, each result being a 

 mean of ten fires . 



Pressure different at distances from bottom of bore. 



