VOL. LII.] PHILOSOPHICAL TRANSACTIONS. 605 



he had brought with him, go considerably slower than it had gone in France. 

 But Sir Isaac Newton first of all showed, how the variation of gravity in different 

 latitudes depended, not only on the centrifugal force, but also on the figure of 

 the earth, which he likewise determined, as well as the proportion of the force 

 of gravity in different latitudes, as far as theory alone could limit them. 



Mr. Colin Campbell made a very curious experiment, of the diminution of 

 gravity from London to Jamaica, by means of an excellent clock, made by Mr. 

 Graham, an account of which is given by Dr. Bradley in N'* 432 of the Philos. 

 Trans., to which he has added a table of his own, expressing the proportion of 

 the force of gravity in different latitudes, and has subjoined the proportion of 

 the equatorial to the polar diameter of the earth, which should follow from the 

 experiment, according to Sir Isaac Newton's principles. A like experiment was 

 made by the learned French astronomers, who went to the polar circle, to mea- 

 sure the length of a degree of the meridian, by a clock made by the same excel- 

 lent artist, expressly for the same purpose. We are likewise obliged to the gen- 

 tlemen of the R.A.s. at Paris, for several experiments made by them, in order 

 to determine the force of gravity in different places, by measuring the length of 

 the second pendulum. 



Mr. M. could not fail of being desirous of improving the opportunity, .which 

 his voyage to St. Helena afforded, of examining this curious point, among other 

 experiments: and the r.s. were pleased to furnish him with an excellent clock, 

 with a gridiron pendulum adapted to it, executed by that diligent and ingenious 

 artist Mr. John Shelton, for that purpose. Dr. Bradley set the clock up at the 

 Royal Observatory at Greenwich, and there examined its going, where he found 

 that it lost 1 1 seconds per day on sidereal time, the thermometer of Fahrenheit's 

 construction, which was placed withinside of the clock-case, standing about 50 

 degrees, at a medium, during the time in which the experiment was made. 



Soon after his arrival at St. Helena he set up the clock in the valley near 

 James's Fort, in a place elevated 85 feet above the level of the sea ; and by a 

 proper meridian mark, and a transit instrument, he observed the transits in the 

 annexed table ; where the first column expresses the day of the month of the ob- 

 servation ; the '2d the time shown by the clock, at the instant of the transit of 

 the sun's centre, across the middle vertical wire of the instrument on that day ; 

 the 3d column is made from the 2d, by applying: 



rr^, \ "^ , "^ ,.^ Time by the Time by the clock, 



the equation of time to it. iherefore the dif- i7oi. clock at the with the equation 



ferences between the numbers in the 2d column 



show the rate at which the clock gains on the -^P*"'' ^^ 



sun; and the differences between the numbers 



in the 3d column show the rate at which the 



clock gains on the mean time, the thermometer 



being at 7 1^ being on a medium, at the rate of 



1"^ 59'.2 per day. 



sun'stransit. of lime applied. 



