232 



SCIENCE 



[N. S. Vol. LI. No. 1314 



It is assumed here that the motion of the air- 

 plane has been kept constant. The motion 

 picture film which I shall show, which was 

 kindly loaned to us by the Aircraft Arma- 

 ment Section of the Ordnance Department, 

 will bring out clearly the tossing, pitching 

 motion of the bomb in its course to the earth. 



INTERIOR BALLISTICS 



In interior ballistics, there are a number 

 of unsolved problems. The fi.rst is concerned 

 with the pressure produced in a gun by the 

 exploding charge and its time rate of change. 



The ordinary method which has been in use 

 has been to measure the so-called maximum 

 pressure by the shortening produced in a cop- 

 per cylinder. But experiments have shown 

 that the amount which a cylinder of copper is 

 compressed by an applied pressure depends on 

 the amount of the pressure, the time of appli- 

 cation, the previous history as regards temper- 

 ing, annealing, compression, etc. It is known 

 for example that an application of a pressure 

 of say 36,000 pounds per square inch will give 

 an extra shortening to a cylinder previously 

 compressed to 40,000 pounds per square inch. 

 But the ordinary procedure has been to place 

 in the gun a copper cylinder which had been 

 precompressed to an amount nearly that to be 

 expected. Obviously such a cylinder may in- 

 dicate a pressure in the gun in excess of 

 40,000 pounds when in reality it was less. 

 Moreover, the copper cylinder need not indi- 

 cate the maximum. Rather it indicates a 

 simmaation of the total efFect of the gases 

 upon it. A smaller pressiu-e applied for some 

 time may produce a shortening equal to that 

 due to a larger pressure for a shorter time. 

 Notwithstanding this uncertainty in the be- 

 havior of a copper cylinder, that is the kind 

 of gage which has been used to standardize all 

 the powder used in guns. It is clear that we 

 may doubt whether these powders have been 

 standardized at all. What is wanted evidently 

 is a gage which will register the pressure 

 accurately at a certain instant and therefore 

 which will give the complete variation of the 

 pressure with time. 



Several gages have been devised which have 



points of excellence as well as defects. In the 

 Petavel gage the compression of a steel 

 spring was registered on a revolving drum by 

 a light pointer. But the mechanical processes 

 were not well worked out. Colonel Somers 

 improved on Petavel's design in the mechan- 

 ical details but neglected the optical. For 

 small arms, both mechanical and optical de- 

 tails have been worked out by Professor A. G. 

 Webster. In the gage the spring is a single 

 bar of steel about 5 mm. square and 20 mm. 

 long, which is bent by a plimger fitting into 

 a cylindrical opening through the wall of the 

 gun. Its moving parts have small mass and 

 high elasticity, and it seems capable of giving 

 an accTorate record of the changes in pressure 

 even when the whole time is of the order of 

 a few thousandths of a second. But its use 

 api)ear8 to be limited to the cases of guns 

 which can be rigidly clamped during the 

 explosion. 



In the Bureau of Standards, Drs. Curtis 

 and Duncan have been perfecting a gage 

 which has been used in the large naval guns. 

 Here a steel cylinder compresses a steel 

 spring. During the compression a metal 

 point makes electrical contact with conduc- 

 tors equally spaced. Consequently electrical 

 signals can be indicated by an oscillograph 

 for these equal steps. The time pressure 

 curve is then given if the spring can be 

 properly calibrated. There is however some 

 doubt on this point and there is also uncer- 

 tainty in electrical contacts and in the fric- 

 tion of the system. 



What is needed is a method of calibrating 

 accurately any gage by means of a known 

 rapidly changing high pressure. Such a 

 method has been worked out by the technical 

 stafi of the Ordnance Department, but the 

 mechanical and experimental work still has 

 to be done. 



I have given you here some applications of 

 the older physics to old and new problems of 

 war. The list even in this limited field might 

 be easily increased. By means of the photo- 

 graphy of sound waves from a projectile we 

 may determine many facts concerning its 

 motion, the frequency of its processional and 



