178 
Proceedings of the Royal Society of Edinburgh. [Sess. 
the knob G at the end of the axle H a half -turn, thus releasing R, and adding 
its weight to B. B and R together then overcome C and fall ; C rises, and 
the axle A with its arms revolves. After one revolution the motion is 
stopped because the pin projecting from the ball K (the ball Q has no pin) 
now comes in contact with the lower side of the lever D. When the 
experimenter gives another half -turn to the knob G, R is again raised, B is 
then overweighted by C, and the ball K revolves back to its original 
position, where it is arrested by the pin’s coming in contact with the upper 
side of D. Thus every time the experimenter gives the knob G a half- 
turn an exposure is made. 
It remains to be explained how the plate is moved after each exposure. 
The parts D and E may be compared to the “ escapement of a clock. 
Each time D is pushed from one side to the other by the pin of the ball K, 
a tooth on the wheel E is freed, because the teeth are spaced alternately 
nearer to and farther from the centre of the wheel, and E revolves under 
the action of the weight L* until it is stopped by the engagement of the 
next tooth with D. In this way the cord M wound round a small wheel 
on the axle of the wheel E is made to draw the carriage N one step 
along, at the conclusion of each exposure. This small wheel (S, fig. 3) 
is not fixed to the axle of the wheel E, and is only carried round with it 
because of its ratchet P, which is drawn forward by the arm Q attached to 
* The weight L requires to be removed each time a fresh plate is inserted. 
