98 On a Jlandard A4eafure from a Pendulum. 



pe!\clulurn could be connedled to a piece of mechanifm to number the vibrations without affect- 

 ing them, I dropped the idea for that time. I learnt, however, fome time afterwards, that Mr. 

 John Whitehurft, a very ingenious perfon, had been in purfuit of the fame objeft, with better 

 fuccefs, and had contrived a machine fully correfponding to his expeftations and my wiflies. 

 This he afterwards explained to the world in a pamphlet entitled " An Attempt to obtain 

 " Meafures of Length, Sec. from the Menfuration of Time, or the true Length of Pendulums," 

 publiflied in 1787. Mr. Whitehurft having therein done all that related to the ftandard 

 meafure of length, and fuggefted that of weight, it appeared to me that it remained only to 

 verify and complete his experiments. 



§. 3. For this purpofe, by the kind afliftance of my friend Dr. G. Fordyce, who at Mr.. 

 Whitehurft's death had purchafed his apparatus, I was furniflied with the very machine with 

 which Mr. Whitehurft had made his obfervations. I alfo procured to be made by Mr. 

 Troughton a very excellent beam-compafs or divided fcale, furniflied with microfcopes and 

 micrometer, for the moft exait obfervations of longitudinal meafure ; as alfo a very nice beam 

 or hydroftatic balance, fenfible with the -roa of a grain when loaded with 61b. troy at each end. 

 Mr. Arnold made me one of his admirable time-keepers, in order to carry time from my fidereal 

 regulator in my obfervatory, with which it was adjufted, to the room wherein I had fixed Mr. 

 Whitehurft's pendulum j and who having taken a journey into Warwickfljire, was fo good 

 as to afEft in the beginning of thefe experiments. Thus equipped, I went to work in the 

 latter end of Auguft, 1796, when the temperature was about 60", firft to examine the length 

 of the pendulum ; when, to my great mortification, I found that the thin wire of which the 

 rod confifted was too weak to fupport the ball in a ftate of vibration ; and that after 15 or 20 

 hours' adion it repeatedly broke. The lame misfortune attended my trials with three other 

 different forts of wires that I had obtained from London. Whether this accident happened 

 from any ruft in the old wire, or from want of due temper in the new, or from its being too 

 much pinched between the cheeks*, I cannot tell. I can only obferve, that all the wires 

 that I ufed were confiderably heavier, and, therefore, probably ftronger than what Mr. White- 

 hurft mentions, viz. 3 grains in weight for 80 inches in length ; nay, mine proceeded as 

 far as from 5 to 6 grains for that length, and yet I could never get it to fupport the ball 

 during the whole period of my experiment. This being the cafe, and being in the country, 

 far removed from the manufactory of this fine wire, I was reluctantly compelled to relinquifh. 

 this part of the operation to fome more favourable opportunity. In the mean while, however, 

 I thought it defirable to meafure the difference of the lengths of Mr. Whitehurft's pendulum 

 from his own obfervations ; for very fortunately the marks diat he had made on the brafs 

 vertical ruler of his machine were ftill vifible, and this interval, which he calls " 59, 892 

 inches," I determined on my divided fcale made by Troughton from Mr. Bird's ftandard to 

 be = 59,89358 inches, from a mean of four different trials in the temperature of 64'' i that 

 mean differing from the extremes only =,0003 inch f . 



* c c Fig. I. of Plate II. in Mr. Whitehurft's pamphlet. 

 f See the remark in Pbilof. Journal, III. 32. note. — N. 



§. 4. By 



