AIR TO THE MOTION OF ELONGATED PROJECTILES. 
421 
to rest in contact. But for my chronograph it was necessary to make provision for the 
breaking and immediate restoration of the current, and that without changing the resist- 
ance of the circuit. The galvanic current passes along the top of each of the ten screens 
simultaneously. Equal weights are attached by long pieces of sewing-cotton to certain 
wire springs which project through holes in sheet copper. When the shot cuts one or 
more threads, the corresponding springs are released and fly from the bottom to the top 
of their holes. So long as any single spring is not in contact with the side of the hole 
through which it projects, the current is interrupted. This kind of screen secures a per- 
fectly constant resistance to the screen galvanic current. But it has been objected that 
if the galvanic current circulated about the screen electro-magnet for several minutes, 
which elapse between successive rounds, the loss of time between the breaking of the 
galvanic current and its corresponding registration would not be equal for all the ten 
screens. This error would be the more pernicious because it might be expected to follow 
some law , and therefore could not be eliminated. The following arrangements have 
been made partly to meet this difficulty, and partly for the sake of the convenience of 
keeping up a constant communication between the instrument and the range. 
A self-acting contact-breaker and ringer (figs. 1 & 2) is 
placed by the side of the gun. Ordinarily the lever a b is 
down, as in fig. 1, in which case the galvanic current takes 
the direct course, abed. When the lever a b is raised, the 
current is permanently interrupted, but the insertion of a 
metallic pin f (fig. 2) opens a passage through the contact- 
breaker, e, when all the bells in the circuit are continu- 
ously rung. After the screens have been mended the lever a b is raised to try whether 
the current is good. If so, it is lowered and the gun is loaded. When the range is 
clear and all is ready, the lever a b is raised and the pin f is inserted. The fly-wheel 
of the chronograph is now put in motion. The rapid interruptions of the current by 
the contact-breaker are recorded on the cylinder, till a pull at the lanyard, to fire the gun, 
simultaneously withdraws also the pin f, and so shuts out the contact-breaker. The 
screen-records often follow so closely that it is difficult, before measurement, to say which 
is the first screen-record. 
Another improvement, represented in fig. 3, has 
recently been introduced. The two ends of the 
circuit are at A and B, near the chronograph. 
When the markers are being raised from the paper 
by a lever, the spring l is simultaneously brought into contact with m, which turns the 
screen-current from the marker to the bell. Thus every interruption of the screen- 
current caused by the repair of the screens is signaled by the bell. When the bell rings 
continuously, it is known that the lever a b (fig. 2) is raised ready for firing. The fly- 
wheel is caused to spin, the markers are brought down upon the paper, and simulta- 
neously the spring l is brought into contact with n, when the bell is silenced and the 
J3ell 
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