expands more with heat and contracts more with cold than steel; so that when the spokes become elongated by a rise of temperature, the pieces bend inwards at their free ends (Fig. 207); if the temperature falls, the spokes are shortened, and the rim pieces bend outwards (Fig. 208).39 This ingenious contrivance keeps the leverage of the rim constant within very fine limits. The screws S S are inserted in the rim to balance it correctly, and very fine adjustment is made by means of the four tiny weights W W. In ships' chronometers,40 the rim pieces are sub-compensated towards their free ends to counteract slight errors in the primary compensation. So delicate is the compensation that a daily loss or gain of only half a second is often the limit of error.
