VOL. XLVII.] PHILOSOPHICAL TRANSACTIONS. 285 



contrivance to prevent the inequalities in the motion of the clock, arising from 

 the different lengths of the pendulum by the effects of heat and cold, yet he found 

 there were considerable errors still remaining, occasioned by the friction of the 

 pallets, as in the common way. He also suspended the pendulum on the wall 

 of the house, entirely independent of the clock, and clock-case : for he had ob- 

 served considerable alterations in the going of the clock, when the pendulum is 

 suspended as in the common manner. His pendulum vibrates in an arc of about 

 15 degrees, with a ball of about 3 lb. between cycloidal checks, which he him- 

 self found were necessary; though he had never heard of Huygens's book, till 

 after he had made them. He has also disposed the force of his pendulum-wheel 

 on the pendulum, by his sort of pallets, in such a manner, that the vibrations of 

 the pendulum will not be aflected by the different resistance of the air. On the 

 whole, this clock is made in such a manner, as to be almost entirely free from 

 friction ; in consequence of which he uses no oil, and therefore there is no ne- 

 cessity ever to clean the clock. When he settled in London, he sent for one of 

 these clocks from the country, and set it up in his house in Orange-street, in 

 the year 1739, where it has stood ever since, and in all that time has never va- 

 ried above one minute from the truth. He can depend on it to a second in a 

 month. 



About the year 1729, Mr, Harrison made his first machine for measuring 

 time at sea, in which he likewise applied this combination of wires of brass and 

 steel, to prevent any alterations by heat and cold. In 1746, he went on board 

 one of his majesty's ships of war with this machine to Lisbon, and returned, 

 where this machine was publicly shown. Since that time, he has made two more 

 of these machines or clocks for keeping time at sea, in both which he has like- 

 wise this provision, to prevent the effects of heat and cold. An account of these 

 and of the many contrivances which Mr. Harrison has made use of in them, for 

 answering their intended purpose, and an account of the success of his voyage 

 to and from Lisbon, is contained in a speech of the President Martin Folkes, 

 Esq. on his delivering to Mr. Harrison the gold medal of Sir Godfrey Copley ; 

 which speech is inserted in the minutes of the Society in 1749. 



Mr. John Shelton, who was the principal person employed by Mr. Graham in 

 making astronomical clocks, informs, that Mr. Graham, in 1737, made a pendu- 

 lum consisting of 3 bars, viz. one of steel, between two of brass, and that the 

 steel bar acted on a lever, so as to raise the pendulum, when lengthened by heat, 

 and to let it down, when shortened by cold. This lever, which is very strong, 

 rests on a roller, made moveable, so as to adjust the arms of the lever to their 

 true proportion. The whole was made to be as free from friction as possible in 

 such a construction. Mr. Graham made observations, by transits of the fixed 

 stars, of the motion of the clock with this sort of pendulum, and from the ex- 



