96 Britain's Heritage of Science 



improvement. Owing to the expansion and contraction of 

 ordinary materials when the temperature rises or falls, the 

 time of oscillation of an ordinary pendulum alters with every 

 change of temperature ; but by properly combining different 

 materials, the difficulty may be overcome. Graham attached 

 a cylindrical vessel partly filled with mercury to the bob of 

 the pendulum; when the rod of the pendulum expands the 

 support of the mercury vessel descends, but the mercury 

 in the vessel also expands, which tends to raise the centre 

 of gravity of the whole arrangement. The expansion of 

 the mercury being considerably greater than that of the 

 pendulum rod, its volume may be adjusted so that the two 

 actions counterbalance each other, and the pendulum may 

 be made independent of moderate changes of temperature. 

 Another arrangement, the " gridiron " pendulum, was intro- 

 duced by John Harrison (1693-1776), the son of a York- 

 shire carpenter, who became a surveyor, and settled down 

 in London as a watchmaker. His pendulum compensation 

 has been very extensively used, but Harrison will chiefly 

 be remembered as the inventor of the chronometer. 



The demand for accurate timekeepers suitable for use on 

 board ship had become so urgent a question at the time, that 

 the Government had offered a reward of 20,000 to anyone 

 who would produce an instrument which satisfied certain 

 requirements. Harrison soon supplied a " time-measurer " 

 or " chronometer " which promised so well that the Govern- 

 ment helped him with grants of money and facilities for 

 testing his instrument on sea journeys. But it took him 

 twenty-six years of continued labour before he obtained the 

 full reward, producing a chronometer which, on a journey 

 to Jamaica and back, showed an accumulated error of less 

 than two minutes; this satisfied the required conditions, 

 and the prize was awarded to him. One of the features of 

 Harrison's chronometer, showing great ingenuity and manipu- 

 lative skill, consisted in the temperature compensation which 

 was applied to the balance wheel. 



Next to accurately going clocks, the astronomer requires 

 well-divided circles for the measurement of angles. Three 

 English instrument makers secured considerable reputation 

 in this work during the eighteenth century. The first of 



