MECHANICS AND USEFUL ARTS. 119 



heated, and a mandrel having a long- stem of somewlrit less diame- 

 ter is introduced into the bore. The tube is thru passed tlm>u'.:h 

 grooved rolls, the mandrel being held stationary by a collar on its 

 stem secured in a frame in front of the rolls, in such a position 

 that the head of the mandrel shall come directly in the centre be- 

 tween the two rolls so as to sustain the pressure. The motion of 

 the rolls draws the tube off the mandrel. Another slightly smaller 

 mandrel is introduced and the operation repeated till the tube is 

 brought clown to the diameter required. It will be seen that it is 

 quite possible also in this way to produce tubes with a solid end, 

 which are well adapted for guns. For gun-barrels there is a set 

 of rolls with recesses cut in the grooves to produce the enlarged 

 section required for the breech. The rolls are made to stop for two 

 seconds at each revolution, so as to allow the tube to be properly 

 inserted, though it has been found quite possible to do this without 

 this precaution. This process is very valuable for making hollow 

 railway axles, which, aside from being light, must necessarily be 

 sound. from the nature of the process. It is intended also to apply 

 it to the manufacture of boiler tubes. Scientific American. 



TRIAL OF A HEAVY UN. 



At the experimental firing of the 20-inch gun at Fort Hamilton, 

 March 15th, the charges used were, first shot, 125 pounds ; second 

 shot, 150 pounds; third shot, 175 pounds; fourth shot, 200 

 pounds ; mammoth powder. This gun was fired at a constant 

 angle of 25. Gen. Rodman's pressure plug was used in getting 

 the pressures. An examination of the record will show that from 

 some unknown cause the amount of pressure of the third discharge 

 exceeds that of the fourth, although 25 pounds less powder was 

 used. At the same time the range is 2,827 yards less. The same 

 irregularity may be observed between the second and third shots. 

 Future experiments may explain these variations. It would un- 

 doubtedly have been more rapidly loaded had the running gear 

 been free from dirt and properly oiled. The dust from the para- 

 pet (recently made) will insinuate itself, and unless removed 

 soon forms with the oil a mixture which, on the axle, seriously 

 retards the working of the gun. The mean recoil was 5' 10", or 

 less than half the length of the top carriage. Weight of shot, 

 1,080 pounds. 



The following is the record of the firing : 



Calculated Ranges. First shot, 6,110 yards, 3.47 miles ; second 

 shot, 6,802 yards, 3.86 miles ; third shot, 6,770 yards, 3.85 miles ; 

 fourth shot, 7,952 yards, 4.52 miles. These ranges are obtained 

 by computation. 



By Plotting, the Eanges are First shot, 6,144 yards difference, 

 34 yards ; second shot, 6,860 yards difference, 58 yards ; third 

 shot, 6,828 yards difference, 58 yards ; fourth shot, 8,001 yards 

 difference, 49 }~ards. 



Times of Flight. First shot, 26 seconds; second shot, 26 sec- 

 onds ; third shot, 27 seconds ; fourth shot, 27 seconds. Length 

 of base line, 2,310 yards. 



