VELOCITY OF WHITE AND OF COLOURED LIGHT. 
245 
contact sends the electric current are moved during the drop of the scape wheel from one 
pallet to the other. In the figure (fig. 1) a tooth has just passed from the impulse face 
of the pallet P 3 . t Y now falls through the angle a 1 on to the dead face of the pallet P x . 
During this interval the tooth t of the smaller' 7 ' wheel mounted on the same axis as 
Fig. 1. 
the scape wheel moves the contact spring Cj S : against the contact spring C 3 S 3 , and 
the current passes. (The point of is agate, and the points of contact are platinum.) 
It will be seen that when the tooth t Y goes on to give impulse to the pallet P : through 
the angle b Y the tooth t has allowed the springs to separate and it passes through 
its angle b undisturbed. The motion of the tooth t 2 can be easily traced from the 
drawing. Careful workmanship was required to carry out this plan, as the angles 
<x, a 1; a 2 ought to be made smaller than in the drawing. The result has been very 
successful. 
Nevertheless, although this arrangement seems admirably adapted for not inter¬ 
fering with the regularity of the pendulum’s vibration, we considered that it was not 
perfectly satisfactory for our purpose. For the length of a second marked by it on 
the chronograph would depend upon the exact form and size of the teeth in the 
auxiliary wheel. Consequently, we extemporised a contact free from this defect. A 
hole of the same size as the arbor of the crutch was bored in a piece of cork, and 
* The wheel is here drawn smaller for clearness of description. It is really of the same size. 
