•284 PHILOSOPHICAL TRANSACTIONS. [aNNO 1751. 



was any considerable difference of expansion between brass, steel, iron, silver, 

 &c. when exposed to the same degrees of heat ; conceiving that it would not be 

 very difficult, by making use of two sorts of metals differing considerably in their 

 degrees of expansion and contraction, to remedy in great measure the irregula- 

 rities, to which common pendulums are subject. He says also, that from the 

 experiments he then made, he found their differences so small, as gave him no 

 hopes of succeeding that way, which made him leave off prosecuting this affair 

 any more at that time : that, some time after, having observed an extraordinary 

 degree of expansion, by heat, in quicksilver, he thought of a proper manner of 

 applying a column of it to the pendulum of a clock, in order to prevent the in- 

 equalities arising from its different lengths by the effects of heat and cold; which 

 succeeded accordingly, and is what is now called Mr. Graham's quicksilver- 

 pendulum. Mr. Graham, in the same paper, takes notice, that, though the 

 pendulum of a clock was to remain invariable, yet there would still be some irre- 

 gularities in the motion of the clock, arising from the friction of the different 

 parts of the clockwork, and from the different degrees of foulness. 



In the year 1725, Mr. John Harrison,* of Barrow in Lincolnshire, made se-. 

 vera! experiments on wires of different metals, to find their different degrees of 

 expansion and contraction : for he thought, that by a proper combination of 

 wires of two different metals, differing considerably in their expansion and con- 

 traction, he might be enabled to keep the centre of oscillation of a pendulum al- 

 ways at the same distance from the point of suspension. In consequence of 

 these experiments, he made a pendulum consisting of one steel wire, at the end 

 of which is the ball or weight, and on each side of this wire 4 wires alternately 

 brass and steel, so disposed and contrived, as to raise the pendulum the same 

 quantity as it is lengthened by heat, and to let down the pendulum in the same 

 proportion as it is raised by cold. He made also a drawing of a clock, in which 

 the wheels are disposed in a different manner from those then in use ; which 

 drawing Mr. S. has seen, signed by himself in the year 1725. Two of these 

 clocks with pendulums, as described above, were finished in the year 1726. In 

 these clocks Mr. Harrison has made a particular sort of pallets, so as to be al- 

 most entirely free from friction ; for though he had thus happily succeeded in his 



* Mr. John Harrison, an ingenious clockmaker, was born in l693, at Foulby in Yorkshire, and 

 bred to his father's business, that of a carpenter. Having a good mechanical turn, particularly for 

 wheelwork, he constructed some wooden clocks, the accuracy ofwhich was so much admired, that 

 in 1728 he came to London with a drawing of a timekeeper, which he showed to Dr. Halley, who 

 recommended him to Mr. Graham, from whom he received great encouragement to prosecute his 

 design. In 1735 he visited London again with a complete machine, with which he was sent on a 

 voyage to Lisbon by the board of longitude, to make trial of its properties. From that time he went 

 on improving hislnvention, and at length received the reward of more than 20,0001. allowed by 

 parliament for the discovery of the longitude. He died in 1776", at 83 years of age. 



