84 Proceedings of the Royal Society of Edinburgh. [Sess. 
suspension spring. The second rod is the one which presses on the 
pendulum to convey the impulse. The third carries an adjustable counter- 
poise at one end, by means of which the magnitude of the impulse can be 
varied, and the other end rests, normally, upon a stop, which may occupy 
an upper or a lower position. As the pendulum swings from left to right 
it picks up the crutch, in its higher position, and carries it along with it. 
Its contact with the crutch closes an electric circuit, which operates a 
magnet that moves the stop to its lower position. The pendulum and 
crutch move as one piece about their common axis, without rubbing, up to 
the extreme excursion and back again until the stop, catching the crutch, 
separates them. As the stop which arrests the crutch is in a lower position 
than that from which it was picked up, work has been done upon the 
pendulum, and serves to maintain its motion against air friction or any 
other, as well as loss of energy (by collision) in picking up the crutch. 
Immediately the crutch meets the stop contact is broken sharply, and the 
stop and crutch return to their upper position, while the pendulum continues 
its swing freely until the round of operations is repeated. 
Mr Cottingham’s clock is therefore extraordinarily simple. It contains 
only two moving parts besides the pendulum. I do not propose to discuss 
here the electric connections, but would merely mention that with proper 
precautions no difficulty is found in the pendulum itself making and break- 
