342 PHILOSOFHICAL TRANSACTIONS. [aNNO 1794, 



head of the screw till the clock came to lose -^ of » second in 24 hours, I did 

 not think it worth while to bring it nearer ; I then began to observe, and carried 

 on the observations, when the weather permitted, for about Q months, during 

 which the thermometer had fallen so low as 15° of Fahrenheit, in the clock- 

 case, and risen as high as 84; and with considerable variations. Unfortunately 

 I have mislaid or lost the particulars of each observation; but I have preserved 

 the greatest difference from the rate of its going. Counting on, according to 

 the rate of its going, during the whole time it never exceeded the sum half a 

 second, nor was ever less than half a second, whether it was taken from day to 

 day, month to month, or from any one to any other period during the observation. 



Undoubtedly therefore, notwithstanding the errors that might have arisen 

 from the expansion of the wood by moisture, and from the unsteadiness of the 

 building in which it was placed, it certainly performed better than any other 

 time- piece that has been made; and perhaps affords a principle which may be 

 used in fixed observatories for keeping time with certainty, by easy and not very 

 expensive means; and of determining, with the rest of Mr. Whitehurst's appa- 

 ratus, the difference between the lengths of 2 pendulums swinging equal arches 

 of circles of different diameters, in any 2 given different times. 



The astronomer royal has also suggested an improvement: viz. instead of 

 grinding the 2 crystalline pieces in a cylindric form, the lower part should be 

 ground in a cycloidal form; then it would have the advantage of cycloidal cheeks, 

 which no contrivance hitherto has been able to attain. There are some further 

 observations necessary to be made, to enable workmen to construct clocks ac- 

 cording to this principle, and some reflections on its operation, Tlie manner of 

 hanging a leaden weight to the pendulum, its proportion to the maintaining 

 power, the manner of applying the pendulum to the clock, and the structure of 

 the clock, are to be found in Mr, Whitehurst's pamphlet; with only this dif- 

 ference, that the steel wire should go through a tube placed in the axis of the 

 spherical lead weight, and be fixed at the bottom instead of the top of it. This 

 however is of no great consequence if there be a power of altering the height of 

 the fixed point i; because Mr. Whitehurst's pendulum consisting partly of steel, 

 partly of lead, therefore the point i must be adjusted to the joint expansions of 

 lead and steel, if the wire be fixed at the top of the ball. 



The first reflection that I shall make is, that the steel wire, the brass tube, 

 and the materials which connect the points i, e, being of different sizes, and 

 different in their disposition to be heated or cooled, some one of them might be 

 heated or cooled faster than another. But where good clocks are kept, the 

 changes of the heat of the atmosphere are so slow, tliat no great difference can 

 take place in the time that each of the parts rises to the heat of the atmosphere 

 in the room where the clock is kept ; none that could make any sensible error. 



