98 
SHORT NOTES ON PROFESSIONAL SUBJECTS. 
The contrivance for dropping the hall W is represented in Fig. 2. The galvanic 
current circulates through a, b, c, d, &c. When the bolt ed is suddently drawn back 
in the direction of the arrow, the galvanic current is broken the instant contact 
ceases at c, and W begins to fall. The lever ab being unsupported, turns about 
its end a till the screw e comes in contact with the bolt dc. This restores the current 
ready for another record. 
In order to measure a velocity, the fly-wheel is put in rapid rotation. The keeper 
D is pressed against the electro-magnet E , which shuts the chronograph C out of 
the circuit. When the lanyard is pulled it breaks momentarily the galvanic current. 
This allows the keeper/) to be drawn back and so the galvanic current is turned through 
C. The marker is thus raised ready to record the exact time when, the ball W is 
dropped. Arrangements are made so that after D has turned the galvanic current 
through C, it also draws back the bolt cd, Fig. 2, so as to release the ball W and 
break the current. Afterwards the shot passing through two or three screens breaks 
the current, and lastly, when the ball W has fallen through its appointed distance 
freely under the action of gravity, it strikes a small disc and permanently stops the 
current. It is obvious that arrangements will have to be made so that the extreme 
screen records may fall, as near as possible, equally distant from the records made by 
the falling weight W. When these arrangements have been made the result may be 
represented as follows : 
A' d b‘ c' B' 
Where A' represents the record corresponding to the instant when the ball W 
begins to fall, and B' that when W has fallen through its appointed space, d\b\c' 
are the records of the time of passing three equidistant screens, the use of the 
central one being to test the value of the records, because b'c ought to exceed 
db' by a certain known quantity. But dc would be used in calculating the 
velocity. Suppose the height of fall of W to be adjusted so as to occupy half a 
second, then A'B' may be taken to represent half a second on the same scale as 
ac' represents the time occupied by the shot in passing from the first to the third 
screen, say 200 feet, for the angular velocity of the fly-wheel would not sensibly 
vary during the short time of half a second. But even if there was a slight 
sensible uniform loss of velocity in this short space of time, provided b' fell near 
the middle point between A' and B', no sensible effect would be produced in 
the resulting velocity. The time therefore occupied by the shot in passing over 
200 feet would be 
ac 
equal to 4 >^7 X half a second; and the velocity of the shot 
- 4 ± H 
A'B 1 A 1 B' 
would be equal 200 X -— 7 - x 2 = 400 x - 7 - 7 - feet per second. The resulting 
a c a c 
velocity might be read off at once without calculation 011 a good slide rule. 
