THE ROYAL ARTILLERY INSTITUTION. 
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improve upon old methods, I decided to apply a known method of experi¬ 
menting which was new to this branch of science. 
After a due consideration of all circumstances of the case, it appeared that 
the following conditions must be satisfied by a chronograph worthy of perfect 
confidence:— 
(1) The time to be measured by a clock going uniformly. 
(2) The instrument to be capable of measuring the times occupied by a 
cannon ball in passing over at least nine successive equal spaces. 
(3) The instrument to be capable of measuring the longest known time of 
flight of a shot or shell. 
(4) Every beat of the clock to be recorded by the breaking of the same 
galvanic current, and under precisely the same conditions . 
(5) The time of passing each screen to be recorded by the momentary 
interruption of a second galvanic current, and under precisely the 
same conditions. 
(6) Provision to be made for keeping the strings or wires of the screens 
in a uniform state of tension , notwithstanding the force of the 
wind and the blast accompanying the ball. 
To gain assistance in carrying out the above conditions practically, I 
consulted a great variety of books on chronographs, electric telegraphs, 
clock making, &c. A description of one of the most recently constructed 
chronographs for an Observatory will be found in C. A. E. Peter’s Ueber 
die Bestimmung des Langen unterschiedes zwischen Altona und Schwerin. 
In Loomis’s Practical Astronomy (New York, 1855), it is stated that this 
method of recording transits had been employed exclusively at Washington 
Observatory since 1849, and allusion is made to the conical pendulum then 
(1855) in use at Greenwich to regulate the velocity of rotation of the cylinder 
of a similar instrument. 
The following, is a description of the chronograph as constructed, and of 
various useful appendages. Eig. 1 gives a general view of the chronograph. 
A is a fly-wheel capable of revolving about a vertical axis, and carrying with 
it the cylinder K } which is covered with prepared paper for the reception of 
the clock and screen records. The length of the cylinder is 12 or 14 inches, 
and the diameter 4 inches. I? is a toothed-wheel which gears with the 
wheelwork M so as to allow the string CD to be slowly unwrapped from its 
drum. The other end of CD being attached to the platform S allows it to 
descend slowly along the slide A, about £ inch for each revolution of the 
cylinder. B, B are electro-magnets; d, dJ are frames supporting the keepers; 
and f f are the ends of the springs which act against the attraction of the 
electro-magnets. When the current is interrupted in one circuit, as B, the 
magnetism of the electro-magnet is destroyed, the spring f carries back the 
keeper, which by means of the arm a gives a blow to the lever b. Thus the 
