174 
MINUTES OF PROCEEDINGS OF 
carefully placed opposite each record on the paper by means of a tangent 
screw (not shewn in the figure), and the vernier is read. 
The clock goes on breaking the galvanic circuit every swing of the 
pendulum, whether the marker m be in contact with the paper or not— 
consequently, whatever be the loss of time in the action of the marker, we 
may fairly suppose it to be constant.. But it has been objected that the 
current having been circulating through the screens for several seconds, or 
even minutes, without' interruption before the shot is fired, the records at 
the first and the following screens are not made under the same conditions. 
A careful inspection of the results actually obtained shewed that this objec¬ 
tion had no foundation to rest upon for that particular experiment; still 
it seemed desirable to get rid of the objectionable arrangement lest an 
irregularity might make its appearance in some important experiment. The 
first screen was placed 120 feet from the gun, the second at 240 feet, and 
so on. One preparatory break would be obtained by placing a screen at the 
mouth of the gun; but in addition to this it appears well to introduce the 
ordinary self-acting contact breaker, made to beat as near as may be the 
intervals of time to be measured. The raising of a spring lever interrupts 
the main current of galvanism through the screens. The insertion of a pin 
to keep up the lever, re-opens a passage for the screen galvanic current 
through the contact breaker; this may be made also to ring a bell in the 
instrument room, to give notice that all things are ready for the experiment. 
The fly-wheel is then put in motion, the signal to fire is given; the pulling 
of the lanyard withdraws at once the pin, restores the main current, and 
fires the gun. 
The construction of the-chronograph was commenced in August, 1864; 
it was ready for trial in June, 1865. It received its first partial trial before 
the Committee on Gun Cotton in July, 1865, in conjunction with Major 
Navez's Electro Ballistic Pendulum. The instruments gave a nearly con¬ 
stant difference of 20 f.s. in velocities of about 1500 f.s., and indicated a 
much greater consistency in the results than in those obtained by M. Melsens 
when engaged in comparing the two instruments of Major Navez and M* 
