AIR TO THE MOTION OF ELONGATED PROJECTILES. 
423 
second screen*. When the second screen is broken, a recording spark is given by the 
coil, and the current is passed on to the third screen, and so on to the end. All the time, 
the diapason is tracing its sinuous spiral. Unless a careful system of compensation be 
provided, this method of working the screens would cause great variation in the resistance 
of the circuit. In the arrangement of my screens, I was careful to maintain a constant 
resistance to the current, which end is secured by making the current pass through all 
the screens simultaneously, and by providing for it to be interrupted, but not broken. I 
am not aware that Captain Schultz’ instrument has been tried with more than two screens ; 
but Colonel Benet has given a Table showing the number of vibrations per second made 
by the diapason as determined at the Frankfort Arsenal. In this Table there are striking 
variations in the numbers of vibrations made in successive seconds, as in the second trial, 
extending to twenty-five seconds, we find 249T, 252-0, 249-5, 248-5, 246 0, 249-0, 
&c. The result of the trials is stated as if the constancy of the mean number of vibra- 
tions per second was all that was required. This is not the case. A succession of equal 
intervals of time must be marked out by the clock, or diapason, so that, when the gun 
is fired, the time of passing the screens may be noted by the side of a correct scale of 
time. Now it would make an important difference in the resulting velocity, if the gun 
were fired when the diapason was making 252-0, 248 - 5, or 246-0 vibrations per second. 
If the vibrations of the diapason be maintained by the vibrations of a second fork whicli 
alternately makes and breaks contact, as described by Helmholtz f, it is hardly to be 
expected that the number of vibrations per second can be maintained with sufficient 
constancy. There is another question. The point from which the spark is discharged 
cannot be allowed to touch the smoked surface of the cylinder ; and it may be asked 
whether the spark is not liable to deviate in its passage. 
After my chronograph had passed its first trial (in December 1865), it appeared to be 
desirable to institute experiments with a view to find the resistance of the air to various 
forms of heads of elongated shot, but such as were likely to be of practical utility. 
The proposed experiments received the sanction of the Right Honourable the Secre- 
tary of State for War, and ten of each kind of the following elongated shot were pre- 
pared for the 40-pounder M.L. gun : — 
(1) 
Solid. 
(2) 
Solid. 
( 3 ) 
Solid. 
( 4 ) 
Solid. 
( 5 ) 
Hollow. 
(6) 
Hollow. 
Hemispherical head. 
Hemispheroidal head (ratio of diameters 1 : 2). 
Ogival, struck with radius equal to a diameter of shot. 
Ogival, struck with radius equal to two diameters of shot. 
Ogival, having precisely the same external form and length as (3). 
Ogival, having precisely the same external form and length as (4). 
* “ In working the instrument it is essential that the current pass only through one target at a time, there 
being hut one coil and one battery no matter how many targets may be used. After the first target is ruptured, 
the current must be transferred to the succeeding one before the projectile reaches it, and so on throughout the 
series.” — Benet, p. 36. See also the Practical Mechanic’s Journal, Oct. 1, 1867, p. 195, to the same effect, 
t Tonempfindung, p. 584. 
