334. 



REPORT — 1801. 



thousandths, the instrument is capable of recording the millionth part of 

 a second. 



The precise rate of the discs' rotation is ascertained from one of the 

 intermediate shafts, which, by means of a relay, registers the revolution 

 on a subsidiary clironoscope, on which, also by a relay, a chronometer 

 registers seconds. The subsidiary chronoscope can be read to about the 

 ■ ^\,,i th part of a second. 



The registration of the passage of the shot across any of the fixed 

 points in the bore is effected by the severance of the primary of an in- 

 duction coil causing a spark from the secondary, which writes its record 

 on prepared paper gummed to the periphery of the disc. The time is thus 

 registered every round at sixteen points of the boi'e. 



In the earlier experiments with this instrument the primary was cut 

 by means of the arrangement shown in fig. 5, and this was entirely 

 satisfactory when velocities of from 1,400 to 1,600 f.s. were in ques- 

 tion. But with the Aery high velocities now employed, with velocities, 

 for example, between 2,.500 and 3,500 f.s., the knife, instead of being 

 knocked down, frequently cuts a long groove in the cast-iron projectile. 



Fig. 5. — Original Apparatus for Cutting Wire by Moving Shot. 





|ifSS#^ 



on some occasions reaching the driving band of the shot before being 

 forced into its place. 



On account of this defect I haAe in all recent experiments adopted 

 the arrangement shown in fig. 6, which giAes extremely satisfactory results, 

 if care be taken that the plug is sufficiently secured to prevent its being 

 forced out of its place by the rush of conii^ressed air displaced by the 

 passage of a projectile. 



I have ascertained by experiments which I need not here describe that 

 the mean instrumental error of this chronoscope, due chiefly to the 

 deflection of the spark, amounts only to about three one-millionths of a 

 second. 



I must not conceal the fact that the determination of the pressure by 

 this method is attended with very great labour. As an illustration I 

 have prepared a diagram (fig. 7) of a I'ecent set of experiments. Usually the 

 pressures are deduced from the mean of three consecutive rounds fired 

 under the same circumstances. 



In this case, owing to the bore being clean, a much higher velocity 

 Avas obtained from the first round, and the velocities and pressures were 

 therefore calculated both for the mean and independently for each of the 

 three rounds. 



