422 
PROFESSOR F. BASHFORTH ON THE RESISTANCE OF THE 
marker registers the breaks of the contact-breaker till the pin f is withdrawn and the 
gun is fired. The clock is allowed to make a few beats ; and then the markers are raised 
from the paper, and contact is reestablished between l and m. Thus the galvanic cur- 
rent only circulates for eight or ten seconds about the screen electro-magnet for each ex- 
periment, and the current is always being rapidly interrupted quite up to the firing of 
the gun ; so that there is no opportunity for the development of a varying strength of 
remaining magnetism. The whole arrangement is found to work so satisfactorily that 
on one occasion nine rounds (23 to 31) were fired in forty-five minutes. 
After all possible precautions have been taken, it is found that there are small correc- 
tions required in order to make the successive records of both clock and screen consist- 
ently regular. The unit of the scale used in measuring is about half an inch, and the 
scale is read off to two places of decimals, or to the -%ho °f an inch. The corrections are 
carried to three places of decimals of the scale. The final calculations are carried to 
four places of decimals of a second. This is done to secure accuracy to the nearest 
of a second of time, giving an opening for an error of rbW o o °f a second of 
time, or 6 or 8 inches of space, in finding the time occupied by the shot in passing 
from the first to any succeeding screen. These corrections of the readings of the scale 
are rendered necessary because the screens cannot be practically maintained at perfectly 
equal distances. The point of the shot may strike fairly upon a thread at one screen, 
and between two threads at the next screen. One spring may act more promptly than 
another. One string may bend more than another before breaking. These corrections 
are often merely nominal, but there are some sufficiently large to warn us to beware of 
trusting implicitly to any measurement of a velocity by two screens only. 
Shortly after the publication of the description of my chronograph, my attention was 
directed to a chronograph with a cylinder, the invention of Captain Schultz*, of the 
French Artillery, which had been tried in France and America. The instrument is 
adapted for making any number of records ; and, like my own, its success does not de- 
pend upon the uniformity of rotation of the cylinder. My instrument makes the clock - 
and screen-records side by side, on glazed paper which covers the cylinder, so that the 
original records of the experiments can be preserved for future reference. Captain 
Schultz makes his records on the slightly smoked metal surface of his cylinder, which 
are effaced when they have been read off. Captain Schultz uses a large tuning-fork, 
usually called a diapason, the vibrations of which are sustained by electro-magnetism, to 
effect the mechanical division of the second into 250 or more equal intervals. The dia- 
pason, vibrating as the cylinder turns, traces a sinuous spiral line. The pendulum in swing- 
ing interrupts a galvanic circuit once a second, and causes a spark from a Ruhmkorff’s 
coil to strike the cylinder and make a record. Thus it is found how many vibrations the 
diapason makes per second. The clock is then taken out of the circuit, and the current 
is made to pass through the Ruhmkorff coil and the first screen. When the first screen 
is broken the coil gives a spark, and the galvanic current is made to pass through the 
* Colonel Benet’s ‘Electro-ballistic Machines,’ 1866, p. 32. 
