12 



Description of an Automatic 



wheel a on the axle y has ten teeth. Now when u has 

 made a complete revolution, it will have advanced a one 

 tooth or one-tenth of a revolution ; consequently the 

 wheel a will always express the hundredths. 



In order to transfer the motion of a to the axle x, we 

 fasten to a the wheel b, having ten teeth ; and by gearing 

 this in the wheel t, having ten teeth also, we transfer 

 the motion of a to t, hence we have the thousandths and 

 hundredths expressed on the wheels u and t. 



But let us go a step further, and see how we get our 

 tenths. The wheels a, b, we have shown, indicate the 

 hundredths : we therefore attach to them another wheel c, 

 having one tooth. Let the wheel h of ten teeth be placed 

 opposite. Now when the axle x, carrying the wheels a, 

 b, c, has made one complete revolution corresponding to 

 one tenth, the wheel h will have advanced one tooth ; con- 

 sequently the tenths will be represented on the wheel h. 



It is of course understood that the wheels u, t, h, are 

 separate, and free to move about the axis x. By repeating 

 this combination, we can employ any number of figures we 

 choose. 



Fig. 5. 



The wheels u, h, a and c, are made after the plan em- 

 ployed in the stop work in a chronometer. In fig. 5, a' and 

 c f indicate this form of gearing. It is seen that the teeth 

 of one wheel are cut in the arc of a circle, with the radius 

 equal to that of the wheel into which it gears. This ar- 

 rangement prevents any motion, except it be communicated 

 by the units wheel. The whole mechanism is therefore 



