ON METHODS FOR MEASURING PRESSURES IN THE BORES OV GUNS. 527 



The results of the Prussican experiments showed, with every charge 

 fired, two distinct maxima of tension. Other relative maxima no doubt 

 existed, but the mode of experimenting was not sufficiently delicate to 

 render them perceptible. 



Before passing to the more modern methods adopted for determining- 

 the tensions in guns, I must advert to one which has been repeatedlv 

 resorted to during the last 150 years. I mean the method of tiring the 

 same weight of charge and projectile from guns of the same calibre but of 

 different lengths, or, as has sometimes been done, by successively reducino- 

 the length of the same gun by cutting oft" a determinate number of 

 calibres from the muzzle. 



It is obvious that if, under the circumstances supposed, we know the 

 muzzle velocities of a projectile from a gun of, say, twenty-five calibres in 

 length and from a gun of thirty calibres in length, we are able from the 

 increased energy obtained to deduce the mean pressure acting upon the 

 projectile over the additional five calibres. 



The earliest experiments with different lengths of guns appear to have 

 been made in England as far back as 1736. These experiments, however, 

 have but little value, as the velocities were not directly determined, and 

 could only be deduced from the observed ranges. The same objection applies 

 to the long series of experiments carried on in Hanover in 1785, and those 

 cited by Piobert in 1801 ; but the interesting observation that the ranges 

 obtained from guns of twelve, fifteen, nineteen, and twenty- three calibres in 

 length were relative maxima cannot be relied on in any way as showing 

 abnormal variations in the muzzle pressure accompanying variations in 

 length. 



In Hutton's experiments, made with guns varying in length from 

 fifteen to forty calibres, the muzzle velocities were obtained by means of 

 the ballistic pendulum ; and, between these limits of length, the mean 

 powder pressure he realised can with sufficient certainty bo deduced. 



This remark applies also to the numerous similar experiments where 

 the muzzle velocities have been obtained by the more accurate chrono- 

 scopes that have been for many years in common use ; but this mode of 

 determining the pressure has many inconveniences, and ceases to be reliable 

 when the bore is of a very reduced length and the pressures approach 

 their maximum value. 



To the important and extensive series of experiments carried on by 

 Major Rodman for the United States Government in 1857 to 1859, the 

 main object of the experiments being to ascertain the effect which the 

 size of grain of the powder used has upon the pressure, we are indebted 

 for that officer's most ingenious pressure gauge ; and tlie crusher gauge, 

 which is now so extensively used, can only be considered a modification of 

 Major Rodman's instrument designed to remove certain difficulties at- 

 tending the use of the original instrument. 



Major Rodman's gauge is well known, but its construction is shown in 

 the accompanying drawing (fig. 1). Major Rodman applied his gauge 

 in the following manner : — • 



Desiring to ascertain the pressure at various points along the bore of a 

 gun, he bored at these points channels to the interior surface of the bore, 

 and in these channels cylinders with small holes drilled down the centre were 

 inserted ; to this cylinder is fitted the indicating apparatus, carried by 

 Major Rodman on the outside of the gun, and consisting of an indentino- 

 tool G with its knife (shown in elevation and section). Against the knife 



