448 MR. B. HOPKINSON ON MEASURING THE PRESSURE PRODUCED IN THE 



comparative tests were made with rods off inch, 1 inch, and l|- inch diameter. The 

 lengths of the rods were roughly 48 inches, 43 inches, and 30 inches, respectively. 

 The results are exhibited in the table on p. 447, in which the figures for the 1-inch 

 rod are the same as those already given. 



It will be seen that the diameter of the rod has no appreciable effect up to a 

 length of 4 inches, but that for greater lengths the large rod gives appreciably lower 

 values. In other words the apparent maximum pressure is not much affected by the 

 diameter, and is presumably correctly given by all three rods, while the duration of 

 the blow is largely overestimated by the 1^-inch rod, and presumably somewhat 

 overestimated by the other two, though as they are in substantial agreement on 

 this point the error cannot be very large. It may be surmised that some at any 

 rate of the difference between the observed and calculated times of impact is due to 

 this cause, though, as already pointed out, the rigidity of the bullet is competent 

 to account for part of it. 



Experiments at Lower Velocities. 



Measurements were also made with cartridges giving velocities of about 1240 feet 

 per second and 700 feet per second respectively, the same types of bullet being used. 

 The results for the 1240 feet per second cartridges are exhibited in the following 

 table, which corresponds to that already given on p. 442 for the 2000 feet per second 

 cartridges : 



ROD, 1 inch diameter, about 40 inches long. Velocity of bullets 1240 feet per second. 

 (Mean of 5 shots : maximum 1257, minimum 1229.) 



The mean total momentum registered (37 shots) is 37'7 units ; the calculated total 

 is 1240 x 0'0306 = 38 units. 



The percentage figures are plotted in fig. G (curve marked " 1240 feet per second"). 





