MAINSPRING. 



pieces where the space necessary for the play of the weight in its 

 descent and ascent can be conveniently obtained. This condition 

 is obviously incompatible with the circumstances attending pocket- 

 watohes, all portable and moveable timepieces, chimney, table, 

 and console clocks, and in general all timepieces constructed on 

 a small scale. 



The moving power applied to these universally is a mainspring, 

 which is a ribbon of highly tempered steel bent into a spiral form, 

 as represented in fig. 12. At one end, A, an eye is provided, by 

 which that extremity may be attached either to a fixed point or to 

 the side of the barrel to which the spring is intended to impart 

 motion. In the centre of the spiral an arbor, or axle, is intro- 

 duced, to which the inner extremity of the spring is attached. 

 Supposing the extremity A to be attached to a fixed point, let 

 the arbor B (fig. 13), be turned in the direction indicated by 

 the arrow. The spring will then be coiled closer and closer round 

 the arbor A B, while its exterior coils will be separated one from 

 another by wider and wider spaces. 



Fig. 13. 



After the spring has been thus coiled up by turning the arbor, 

 it will have a tendency to uncoil itself and recover its former 

 state, and if the arbor B be abandoned to its action and be free to 

 revolve, it will receive from the reaction of the spring a motion of 

 revolution contrary in direction to that which was given to the 

 arbor in coiling up the spring, and such motion would be imparted 

 to a wheel fixed upon the axle, and might from it be transmitted 

 to the hands in the same manner as if the arbor-wheel received 

 its motion from the power of a weight. 



31. But between such a moving force and that of a weight 

 there is an obvious difference. The tension of the cord by which 

 the weight is suspended, and consequently its effect in giving 

 revolution to the barrel upon which the cord is coiled, is always 



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