Several appliances have been from time to time introduced to correct the error in time-keeping caused by variations in the temperature, but none have come into such general use as that known by the term '_Compensation Balance_,' invented by Thomas Earnshaw, of London, and for which he received a government reward. This balance, when properly adjusted, causes the watch to keep the same time whether the temperature be 32 deg. or 90 deg.; while without it a watch will show a considerable difference in time, on being merely transferred from the pocket to the dressing-table, where, probably, the temperature would not be so high. Our woodcut represents a balance of this kind; the divided rim A A, is composed of steel and brass run together by fusion, the more expansible metal, brass, being placed outwards, the result of which is as follows:--Heat elongates the pendulum spring, and thereby causes a slower vibration of the balance. The same amount of heat will also expand the metals composing the balance; but as the inner rim of steel does not expand so freely as the outer one of brass, the conflicting action of the two tends to draw the free end of the circular rim inwards towards its centre, and thus decreases in all but one direction the diameter of the balance. This decrease tends to _quicken_ its vibration, and thus counteracts the effect of the elongation of the pendulum spring. In cold temperatures the pendulum spring is contracted, making the vibrations quicker, but the contraction of the brass rim draws the free end outwards, thus increasing its diameter, retarding its vibrations, and counteracting the effect of the contraction of the pendulum spring.