VOL. LII.] PHILOSOPHICAL TRANSACTIONS. 631 



LXXXVIL An Account of Mr, Masons Paper concerning the Going of Mr. 

 Ellicott's Clock, at St. Helena. By James Short, M.A., F. R. S. p. 540. 



In this paper Mr. Mason tells us that, in order to determine the regularity of 

 the motion of Mr. Ellicott's clock, ne resolved to make observations of the occul- 

 tations of stars, by a ridge of rocks, the altitude of which was about 30° above the 

 place of observation, and at about a quarter of a mile distance; but that this 

 method was soon improved by the Rev. Mr. Maskelyne, who proposed to make 

 use of the equal altitude instrument for that purpose, by observing the vanishing 

 of the stars out of the field of the telescope. By observations of this sort, from 

 the 31st of October to the IQth of November 1761, he found that the clock went 

 very regularly, not varying so much as a second in that time; but from the 19th 

 of November to the 3d of December, he found that the clock had gone slow, or 

 lost 2 seconds of time; and this alteration he imputes to the wedges, behind the 

 clock, having got loose, shrunk as he supposes by the dryness of the place; he 

 therefore secured the wedges, and found that from the 3d of December to the 

 'i2d of December, the clock did not vary in its motion above one second of time. 

 On the 5th of January the clock was stopped; and it appears that the clock did 

 not vary so much as one second of time from the 9th of January to the 22d of 

 January. The thermometer was hung by the side of the clock, and he never 

 saw it higher than 744- divisions, nor lower than 67, from the 12th of Jan. 

 1761 to the 18th of January 1762. He has given no description of the clock. 



Remark. The method proposed by Mr. Mason, of making these observations 

 by means of the occultations of stars behind the ridge of rocks, was certainly 

 better than the other, by means of the equal altitude instrument ; for it has been 

 found by experience, that any instrument, however securely fixed, is liable to 

 alterations in its direction, owing perhaps to the effects of heat and cold, moisture 

 and dryness, in the parts to which the instrument is fastened ; and an equal alti- 

 tude instrument was the most improper for this purpose, because it could not be 

 rectified by looking at a distant mark, to correct any alterations it might have 

 suffered in its position or direction. Mr. Mason further says, that by comparing 

 the observations of the going of the clock, made at St. Helena, with those 

 made at the Cape of Good Hope, the difference of the effect of gravity at the two 

 places may be found. 



Remark. No observations of the difference in the going of a clock, made at 

 different places, can with certainty determine the difference of the effect of gra- 

 vity at these places; because it has been found by experience that the same clock, 

 placed at different times on different walls, in the same room, will make a dif- 

 ference in the going of the clock, even though every part of the clock remains 

 the same. .. 



