SHORT NOTES ON PROFESSIONAL SUBJECTS. 
97 
67. The Chronograph described in the “ Proceedings” of the E.A. Institution 
for 1866* has been successfully employed in determining the resistance of the air to 
the motion of spherical and elongated projectiles, varying from 3 to 9 inches in 
diameter. A simple instrument on the same principle has been constructed for use 
in those cases where the determination of the velocity of a shot at a given point is 
all that is required. Any one acquainted with the use of the ordinary drawing 
scale would be able to take the management of this instrument, because the necessary 
calculations are of the most elementary kind, and the arrangements are such that 
no very nice adjustment of the strength of the electro-magnets is required. 
As in the original instrument a fly wheel is mounted upon a vertical axis, which 
carries a cylinder covered with paper. But in order to avoid the use of a clock, the 
time is given by a ball, which falls from rest through a known space. In the new 
instrument there is only one galvanic current, so that the time and screen records 
are made by the same marker, be it a mechanical dotter, or a spark from a 
Ruhmkorff’s coil. As the whole time occupied by an experiment is less than the 
time of revolution of the fly wheel, the stage which carries the marker is clamped in 
one position during each experiment. 
The principal difficulties which had to be overcome were:—(1) to avoid the dis¬ 
turbing influence of remaining magnetism in the recording electro-magnet; and (2) 
to provide an arrangement which would secure the breaking of the galvanic current 
with perfect certainty exactly at the instant when the ball was set free to fall. The 
manner in which the disturbing influence of remaining magnetism has been avoided 
may be thus explained without entering into details. The galvanic current is 
conducted to the gun, to the screens, to the chronograph C, to the apparatus 
Fig. 1. 
Gun and 
Screens. 
E C n 
$ 
ED for dropping the ball at the proper time. When the keeper D is pressed 
by hand against the electro-magnet E, a kind of switch is moved which allows the 
galvanic current to pass through the coils of the electro-magnet E, and at the same 
time the chronograph C is excluded from the circuit. The circuit is then A , q, r, JB, F. 
But if the current is interrupted for a moment the electro-magnet E becomes 
demagnetised, the keeper D is drawn back by a spring which moves the switch 
again so as to turn the current through C and exclude the electro-magnet E, The 
electro-magnet E can only recover its magnetism when the keeper D is again 
pressed up by hand. 
* "Vol. V. p. 161, &c. 
