OF ARTS AND SCIENCES. 371 



of it are given likewise, in some modified form, in several other 

 places. 



This instrument and its use will be understood by any engineer or 

 intelligent mechanic from the following description. Suppose a row 

 of six small holes to be drilled through the side of a cannon, into its 

 caliber. These holes are placed 14 inches apart, and, commencing 

 near the breech, extend to near the muzzle, or a distance of 84 inches. 

 The instrument consists of a small but strong iron frame, having a 

 shank or plug forged upon one of its sides. This plug is 1^ inches 

 in diameter and 1^ inches long, and is formed into a screw, the thread 

 of which corresponds with a similar screw-thread cut into the outer 

 portion of each of the holes in the side of the gun, by which means the 

 frame may be secured to either of the holes, at pleasure. A small hole 

 is bored through the axis of this plug, making a free passage to the 

 caliber of the gun. A piston is nicely fitted to this hole in the plug, 

 and thus the end of the piston receiving the whole force of the fired 

 gunpowder will be driven outwards at each discharge. A large steel 

 head or block is fitted upon the outer end of the piston, and from 

 this head rises a pyramidal or lozenge-shaped point, or blunt edge. 

 Against this edge, and firmly fixed in the frame, is a piece of thick 

 copper. Now when the gun is fired, the piston, being driven outwards, 

 forces this dull point or edge into the copper, and leaves a mark or 

 impression upon it corresponding in depth to the force with which the 

 piston was driven against it. By comparing an impression, made in 

 this way, with another impression made upon another piece of copper 

 by the actual pressure of weights placed upon a similar tool, Captain 

 Rodman supposes that the force of the piston may be estimated very 

 nearly. It will be seen, on proceeding a little further, that the force 

 of the point, by which the impression is made by the instrument, 

 is not the equivalent of the pressure upon the caliber of the gun, 

 and that it was a gross oversight in Captain Rodman to consider 

 them as equals. It will, of course, be understood, that, when one of 

 the holes is used by the instrument, the others are closed by screw- 

 plugs made to fit them. 



Let us, then, see whether the instrument will perform what is 

 claimed for it. First, then, it is said that the indentation or impression 

 made by the blunt or diamond-shaped point indicates the pressure of 

 the fired gunpowder ; that it acts by pressure purely, as a heavy weight 

 acts when placed slowly upon a support, and not by a blow, as the same 



