150 
Proceedings of Royal Society of Edinhurgh. [fee. 6, 
is fitted with a stop which prevents its rotating, relatively to the 
frame, beyond a certain position (which we may call its normal 
position) in one sense. One wheel, W, cannot rotate beyond its 
normal position in the positive sense, the other, W', cannot rotate 
beyond its normal position in the negative sense. W can rotate 
negatively, hut in doing so stretches a spring, and the spring is 
made strong enough to prevent any hut a very slight angular 
movement, with the greatest acceleration to which the instrument 
can he exposed. Similarly W' can rotate positively, hut in doing 
so stretches a spring ; in fact, W and W' are mirror images of each 
other, the two springs S and S' being as nearly as possible equal. 
If, now, the frame receives an acceleration of positive rotation, the 
two wheels tend to rotate negatively, relatively to the frame ; hut 
W' cannot do so, it is forced by its stop to rotate with the frame. 
But W does rotate relatively to the frame and stretches its spring. 
The extent to which the spring is stretched is approximately a 
measure of the acceleration. If we could keep the acceleration con- 
stant, the wheel would remain at the same angular distance from 
its normal position with its spring stretched. But if we make the 
acceleration zero, i.e., make the rotation uniform, the spring brings 
hack the wheel to its normal position. What is true of W and 
W' with positive acceleration, is of course true of W' and W with 
negative acceleration. While the frame is rotating, we cannot easily 
see whether the wheels are in their normal position or not, or how 
far they have rotated from them. It is necessary, therefore, to 
contrive some way of indicating this. In the model shown this is 
done by leading gas through the lower part of the axis of the frame, 
and by two pipes, one to each wheel. On the axle of each wheel, 
where the gas-pipe passes it, there is a stop-cock. In the normal 
position this stop-cock is nearly closed, so as to allow only a little 
gas to pass ; as the wheel rotates away from its normal position the 
stop- cock opens. From the stop-cocks the gas-pipes pass round to 
the upper part of the axis of the frame, and pass out through it 
through a joint to two fixed gas jets. Acceleration in the positive 
sense opens the stop-cock of W, and the corresponding gas jet 
flares up. Acceleration in the negative sense opens the stop-cock of 
W', and its jet flares up. When the rotation of the frame is 
uniform, whatever its rate may he, the wheels remain in their 
