In stationary clocks, and in other cases in which the bulk of the machine is not an objection, a descending weight is used as the moving power. But in watches and portable chronometers, this would be attended with evident inconvenience. In such cases, a spiral spring, called the _mainspring_, is the moving power. The manner in which this spring communicates rotation to an axis, and the ingenious method of equalising the effect of its variable elasticity by giving to it a leverage, which increases as the elastic force diminishes, have been already explained. (255.) A similar objection lies against the use of a pendulum in portable chronometers. A spiral spring of a similar kind, but infinitely more delicate, called a _hair spring_, is substituted in its place. This spring is connected with a nicely-balanced wheel, called _the balance wheel_, which plays in pivots. When this wheel is turned to a certain extent in one direction, the hair spring is coiled up, and its elasticity causes the wheel to recoil, and return to a position in which the energy of the spring acts in the opposite direction. The balance wheel then returns, and continually vibrates in the same manner. The axis of this wheel is furnished with pallets similar to those of the pendulum, which are alternately engaged with the teeth of a crown wheel, which takes the place of the scapement wheel already described. A general view of the work of a common watch is represented in _fig. 111._ _bis._ A is the balance wheel bearing pallets _p_ _p_ upon its axis; C is the crown wheel, whose teeth are suffered to escape alternately by those pallets in the manner already described in the scapement of a clock. On the axis of the crown wheel is placed a pinion _d_, which drives another crown wheel K. On the axis of this is placed the pinion _c_, which plays in the teeth of the third wheel L. The pinion _b_ on the axis of L is engaged with the wheel M, called the centre wheel. The axle of this wheel is carried up through the centre of the dial. A pinion _a_ is placed upon it, which works in the great wheel N. On this wheel the mainspring immediately acts. O P is the mainspring stripped of its barrel. The axis of the wheel M passing through the centre of the dial is squared at the end to receive the minute hand. A second pinion Q is placed upon this axle which drives a wheel T. On the axle of this wheel a pinion _g_ is placed, which drives the hour wheel V. This wheel is placed upon a tubular axis, which incloses within it the axis of the wheel M. This tubular axis passing through the centre of the dial, carries the hour hand. The wheels A, B, C, D, E, _fig. 110._, correspond to the wheels C, K, L, M, N, _fig. 112._; and the pinions _a_, _b_, _c_, _d_, _e_, _fig. 109._, correspond to the pinions _d_, _c_, _b_, _a_, _fig. 111_. From what has already been explained of these wheels, it will be obvious that the wheel M, _fig. 111._, revolves once