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



might vary widely. This is to be expected if the explanation just given of the 

 irregularities is correct. For instance a cup-shaped cavity in the rod such as is 

 formed after a large number of shots will give a high value for the momentum, but if 

 not too pronounced it will not seriously affect the form of the relation between 

 pressure and time. 



The results have accordingly been reduced by taking in every case the percentages 

 of the total momentum found in the piece. The following table gives details of one 

 set of experiments. It was found that there was no systematic difference between the 

 service bullets and the soft bullets, and the results for both types are included in 

 the table : 



ROD, 1 inch diameter, 43 inches to 50 inches long. 2000 feet per second. 



The mean percentages given in the third column of the table are plotted against 

 length of piece in fig. 6. As the wave travels 2'04 inches in 10~ 5 seconds, 1 inch 

 length of piece represents 0'98 x 10~ 8 seconds.* The slope of this curve represents 

 pressure, and as already explained the maximum pressure is represented by the slope 

 at the origin. This is 22 per inch, and assuming an average total momentum of 

 61 '2 units the corresponding pressure is 



0'22x61'2xlQ 5 

 32'2xO-98 



= 42,600 Ibs. or 19'0 tons. 



It will also be noticed that the impact is practically complete in 6xlO~ 5 seconds, 

 97^ per cent, of the total being then accounted for in the piece. 



According to the simple theory, which regards each element of the bullet as coming 

 up to the end of the rod with its velocity v unimpaired and there suffering instant 



t The value of E for the mild steel of which the pieces were made was found to be 3 '00 x 10 7 Ibs. per 

 square inch. The density was 482 Ibs. per cubic foot. Both determinations are probably right within 



/TJT 



1 per cent. The velocity of propagation A/ -- is 17,000 feet per second. 



