of the Cambridge Ohsei^atory . 161 



in determining the longitude of the Cambridge Obsei*vatory. Our 

 plan was soon arranged, and immediately put in execution. 



The chronometers were carried on the person of a servant, in 

 a belt of particular construction made under the direction of 

 Mr. Sheepshanks, and which had been found remarkably useful in 

 carrying the chronometers to the bottom of a Cornish mine. One of 

 the government chronometers was disabled by the breaking of its 

 mainspring; but its jilace was supplied by one lent by the maker, 

 Mr. Molyneux, of which we well knew the value. On Oct. 12, 

 about 1 p. M. Mr. Sheepshanks compared each of the chronometers 

 with the transit clock at Greenwich, and brought them to London, 

 where they were received by my servant, who came to Cambridge 

 by the Boston mail. I compared them about 3 a. m. with the 

 Transit Clock at Cambridge, and immediately returned them by 

 the Observatory servant, who went to London by the Times coach. 

 Mr. Sheepshanks received them and compared them a second time 

 with the Greenwich transit clock : they were again sent to Cam- 

 bridge by the mail, and I again compared them with the transit 

 clock. My servant then carried them again to London, and 

 Mr. Sheepshanks compared them a thinl time with the Green- 

 wich clock : after which they were deposited at the Royal Ob- 

 servatory. 



The chronometers were compared with the clocks by the 

 method of coincidence of beats. As some members of the Society 

 may not have had occasion to use this method, I will endeavour 

 to explain its principles. The transit clock necessarily goes 

 sidereal time very nearly: the chronometers were regulated to 

 mean solai- time. And as 36-3 solar days are equal to 366 sidereal 

 days, the sidereal clock goes faster than a solar chronometer in 

 the ratio of 366 : 365. Consequently, in one .second of time the 

 sidereal clock gains on the solar chronometer ^^ of a second. 

 If then the clock is behind the chronometer ttt of a second, in 



yol. III. Part I. X 



