THE ROYAL ARTILLERY INSTITUTION. 
171 
7. The Bashforth Chronograph.* 
(Chap. III. to Art. 52.) 
The chronoscopes in existence were unable to register the passage of 
a projectile through more than two screens. They therefore determined 
the time of flight through one interval only; and manifest errors would 
have been involved had several such instruments been used for 
Mr. Bashforth's purpose—viz., to measure the times occupied by the 
same projectile in passing through successive intervals. He was there¬ 
fore led to the invention of the chronograph which bears his name, for 
the purpose of attaining this object with a single circuit. The screens 
were so constructed that the projectile passing through any one would 
not only interrupt, but instantly restore the circuit ; and the fact of its 
doing so was recorded in a spiral, by a slowly descending electro-magnet 
and pricker, on a drum which was made to revolve as uniformly as 
possible by being attached to a fly-wheel. The drum being annexed to 
a graduated disc, the distances of the records from a fixed zero could be 
... read off. 
The next difficulty was to convert these distances into time, since the 
rate of revolution of the drum could not be made perfectly uniform. 
To accomplish this, a clock beating half-seconds was constructed, so 
that at each second the pendulum broke and made contact in another 
circuit, also provided with an electro-magnet and pricker, which thus 
marked each second on the same drum. 
By comparing these two separate records, the exact times occupied 
by the projectile in passing each screen could be read off, as explained 
in the next section. 
The above is merely a brief outline of the method adopted, and omits 
all the interesting details-^such as the construction of the screens, and 
the very ingenious way in which the difficulties connected with remain¬ 
ing magnetism were got over. A complete description is given in 
Arts. 46-50. 
Before comparing the screen and clock readings, they required to be Art. 51. 
corrected by differencing. 
The results obtained (as explained below) favoured the adoption of 
the formula 
resistance = 25v 3 ; Art. 52. 
where 25 is not quite constant. 
Also, for different projectiles of the same class, the resistance was 
d? 
found to vary as —; where d = diameter and w = weight of pro¬ 
jectile. 
Also, the resistance of the air evidently varies as the pressure, which 
varies as the density. Hence, any result that is obtained for the resist¬ 
ance must be corrected for density by dividing by the number 
expressing it in terms of the unit of density: in fact, it is a simple 
proportion. 
* For full description see “ Proceedings, E.A. Institution,” Vol. Y. p. 161, August, 1866. 
[vol. ix.] 23 
