104 



MECHANICS. 



olution, the other lias fallen one cog behind, and a hund- 

 red revolutions are required for the index, placed upon 

 one of them, to come around again so as to coincide with 

 its first position. 



The endless screw attaclied to the band- wheel already no- 

 ticed moves one cog at every yard advanced, and the in- 

 dex passing around in a hundred revolutions, it is obvious 

 that it will show 10,000 yards, or more than five miles. 



We sliall now attempt to describe thatpart of the ma- 

 chine which furnishes an accurate record of the force. In 

 doing this, we omit most of the details and vary some of 

 the parts, in order to make the explanation simpler and 

 clearer, the object being merely to explain the principle. 



The band- wheel a, fig. 114, (shown also in fig. 113,) re- 

 volves ouce for ev^ery yard of onward movement, as already 



stated, 



so, it causes 



In doing 



the 

 ai-m d <?, to vi- 

 brate backwards 

 and forwards, on 

 a pin at c?/ the 

 connecting rod 5 

 c being set near 

 the circumference 

 of the wheel ^, this vibrating movement is shown by the 

 dotted lines at / and i. The slide h moves on this vibra- 

 ting rod, by being connected with the spiral spring already 

 described, which indicates the force of the draught ; the 

 stronger the draught, the further this slide is moved toward 

 c. When there is no draught at all, the rod e remains at 

 the pivot c?, and has no motion; but as the slide h 

 is moved successively along the arm, this rod e is 

 thrust backwards and forwards, more or less, accord- 

 ing to the force of the draught. This thrusting move- 

 ment turns the ratchet wdieel g faster or slower as this 

 force varies. A self-recordino: index is connected with 



