420 
PROFESSOR F. BASHFORTH ON THE RESISTANCE OF THE 
The chronograph was completed in the summer of 1865, and in November and Decem- 
ber of the same year it was tried with ten screens, placed at intervals of 120 feet. 
Satisfactory records were obtained for eleven out of eighteen rounds of elongated shot 
tired from a 12-pounder B.L. gun, which very plainly indicated that the resistance 
of the air varied as the cube of the velocity. A full account of the chronograph and of 
the method of using it, accompanied by a detailed statement of the eleven successful 
rounds, was printed in the Proceedings of the Royal Artillery Institution, Woolwich, for 
August 1866, and was also published in a separate form. At present it will be sufficient 
to state that the axis of the cylinder is vertical, and in a line with the axis of the horizontal 
fly-wheel, to which it is attached. The fly-wheel is spun by hand. When the gun is 
ready to be fired, the markers are brought into contact with the paper ; and after the 
clock has recorded three or four seconds the gun is fired. The clock is allowed to 
record three or four seconds more, and then the markers are raised and the experiment 
is completed. The friction on the axis of the fly-wheel, the resistance of the air to the 
motion of the wheel, and the friction of the markers on the paper tend to reduce the 
angular velocity of the cylinder. But as the pendulum of a half-seconds’ clock raises 
a lever once each double swing, it interrupts the clock galvanic current once a second, 
and thus the clock-records show what spaces have passed under the markers each second. 
The changing angular velocity of the cylinder thus becomes accurately known. For if 
we commence measuring from some arbitrary point taken two or three seconds before 
the screen-records, and measure along the spiral traced by the clock marker, noting each 
record of the clock, and continuing our measurements two or three seconds beyond the 
screen-records, and if we difference these quantities, we shall find whether the angular 
velocity has been sufficiently regular in its change during the experiment. If so, we can 
by interpolation find what would have been the records if one had been made every 
tenth of a second. It has always been found to be sufficient to suppose the angular 
velocity constant during each tenth of a second, and to calculate smaller intervals of 
time by proportional parts. 
As the clock goes on all day breaking the current once a second, every record of the 
clock is made under precisely the same circumstances. If there be a loss of time between 
the breaking of the current and the making of the corresponding record, the loss of time 
may always be expected to remain the same for any single experiment, and therefore 
there can be no error ; for in experiments on gunnery the exact length of a second, 
only, is required, and not the exact time of the day. The galvanic current which works 
the screen-marker is kept constantly circulating through all the screens, excepting 
during the momentary interruption caused by the breaking or repairing of a screen, or 
some accidental rupture of the conducting wire. Although there is no necessity for 
this arrangement, it is found to be practically convenient. The ordinary screens used 
for other instruments are formed of fine copper wire stretched across a frame repeatedly, 
and through which the galvanic current circulates. When, then, a shot passes through 
the screen the current is permanently broken, unless some of the broken wires happen 
