4 EXPANSION AND CONTRACTION OF SOLIDS BY HEAT. 



repulsion is increased, and the substance acted upon is larger in bulk. 

 If heat be abstracted from a solid, repulsion is weakened, the colresive 

 force is augmented, and the solid occupies diminished space. If a solid 

 be heated equally in all its parts, it will maintain its figure ; expansion 

 will cause all its parts to increase in the same proportion, Avhich is 

 easily proved by taking a brass plate pierced with a hole, into which a 

 bar exactly fits, the bar and plate being of the same temperature. Let 

 the bar be heated to a considerable temperature, and it will be found 

 that the hole in the brass plate will be too small to admit the bar. 

 Reduce the bar to the temperature of the plate, and it will resume its 

 original dimensions. 



Of solids, metals are the most expansive ; and of metals, lead. The 

 effect of the expansion of metals has been regulated with respect to the 

 construction of clocks ; and the labours of experimental men have 

 brought these instruments " to such perfection," says Sir John Her- 

 chel, " that an irregularity in the rate of going, to the extent of a 

 a single second in twenty-four hours in two consecutive days, is not 

 tolerated in one of good character ; so that any interval of time less 

 than twenty-four hours may be certainly ascertained within a few 

 tenths of a second, by their use." The effect of raising the tempera- 

 ture of a pendulum clock would be to cause the pendulum to expand, 

 which would remove the centre of oscillation further from the point of 

 suspension, and cause its rate of vibration to be slower ; diminution of 

 temperature would be attended with a contrary effect. To remedy 

 these defects, Graham, in the year 1715, made several experiments, 

 which led to the construction of the mercurial pendulum. The point 

 of suspension in this pendulum is always kept at the same distance 

 from the bob, or centre of oscillation, by means of mercury, which 

 ascends when exposed to increased temperature, and descends when 

 heat is abstracted, thereby regulating the rate of vibration. In 1726, 

 Harrison, originally a carpenter, at Barton in Lincolnshire, invented a 

 pendulum formed of brass and steel rods. The object in this invention 

 is attained by the greater expansion of the brass rods, which force the 

 bob, or mass, upwards, as much as the steel rods are elongated, or 

 forced downwards. In making patterns for cast-iron moulds, the work- 

 man allows one-eighth of an inch per foot for the contraction of the 

 metal. If a goblet glass, at about the temperature of 50, be filled 

 half full of hot water, probably the glass will crack, because its surface 

 in contact with the water will rapidly expand, and be forced away 

 from that part not heated, and beyond the reach of the water ; but if 



