18 
MINUTES OF PROCEEDINGS OF 
eventually solved the difficulty—viz., that the condenser, which so greatly 
increases the effect of the coil, should perhaps be placed at the point 
of rupture of the primary wire. A condenser was therefore made, of the 
ordinary construction, and of such a size as to fit under the screens. Now, 
although at least 150 yds. of wire were attached between the screen and coil, 
sparks passed freely whenever the communication was broken, and with 
exactly the same battery power that had proved so unsuccessful before. 
Having thus solved the problem, it became a matter of curiosity to know how 
the inventors of other chronographs who likewise used the induced current 
had overcome this difficulty. On enquiry at Woolwich, I found they invariably 
employed powerful and expensive coils; whereas, by this method, a small and 
inexpensive one is all that is required. 
Theory of the Instrument. 
As the weight starts from rest, from a fixed point, and falls freely in air, 
the rate at any particular moment is given by the formula 
s= yt*-, 
where S = space fallen through, 
g = force of gravity at the place, 
t = the time in seconds. 
By the aid of logarithms, a scale may be easily constructed, starting from 0 
and going as far as the height of the instrument will allow. 
Eig. 2 shows a part of such scale, full size, the small divisions representing 
thousandths of a second. The greater the height from which the weight 
falls, the larger will be the divisions; so that any accuracy may thus be 
obtained. 
The action of the instrument is simply tins:—The weight being released a 
short time before the gun is fired, descends between the cylinders; the shot, on 
passing through the first screen, breaks the continuity of the primary wire of 
the coil, thus causing an induced spark to pass from one cylinder to the other 
through the brass wire of the weight—this being its shortest path. As the 
cylinders are covered with smoked paper, a minute spot registers the exact 
position of the weight at that moment. The weight continuing to fall, as the 
shot passes the second screen (the primary current being in the mean time 
re-established), the same result follows; and so on for any number of screens* 
The distance between the spots, as read off on the velocity scale, gives the time 
taken by the shot to pass the various screens. By means of the calculating 
scale shown in Eig. 3, the velocity may be at once ascertained for any 
distance between screens. It will thus be seen that there are no disturbing 
influences at work, such as the de-magnetization of electro-magnets, &c., to 
detract from the accuracy of the result. Eor a fresh experiment, all that is 
required is to turn the cylinders slightly round, so as to present a fresh sur¬ 
face of the black paper, and again suspend the weight. 
The Calculating Scale 
is in three divisions. The upper represents the distance in feet between the 
screens; the centre, time divided into thousandths of a second; and the lower, 
