I/o 



CORRESPONDANCE. 1664. 



a 



purpofe; for though, when I lafl: writ, I was promifed, both by Mafter Faith- 

 orne 3) and Mafter Thompfon^), that I fhould hâve thofe 

 things which they had in their hands, finifhed within three or 

 four days; and though I hâve often called upon them, and 

 urged them ail I could, I hâve not been able to get them donc. 

 I hâve lately obferved many circumftances in the height of the 

 mercurial cylinder, which do very much crofs my former ob- 

 fervations; for a.t this very time the quickfilver is as high as 1 

 hâve a long time obferved it, and I don't remember, that it has 

 been higher: it has rifen a little for thefe four or fi ve days, 

 and has continued fo, notwithftanding the variety of winds, 

 and the multitude of rain, that has lately fallen; and, I think, 

 it rifes a little yet, but it is but little. 



I hâve taken notice alfo of two or three other very odd parti- 

 culars lately in it, which hâve croflTed feveral other obfervations. 

 The experiments we are now moft bufy about 5), are concerning 

 the adjuftening of the length of pendulums, thereby to fettle a 

 common ftandard for length; of which kind, MonfieurZulichem 

 has fent over'') fome very pretty théories; but upon very care- 

 ful trial with feveral accurate and large pendulums, madc with 

 balls of lignum vitae ''), fome of which balls are fix inches over, 

 others no bigger than the head of a pin, or a fmall f hot, and fuf- 











Faithorne était graveur, au service de la Société Royale. 

 Thompson était graveur à Londres. 



Dans les „Proceedings" de la Royal Society on lit, au sujet des premières expériences fai- 

 tes dans la séance du 7 décembre 1664 (V. st.), ce qui suit; 



The experiment for the verifying of Monfieur Iluygens's rule concerning 

 the univerfal meafure was made twice , there being taken a very fmall bullet 

 with a line of the lenght of 15 Vio inches: and two wooden balls, one of three, 

 the other of fix inches; which being adjufted with the former line, it was 

 found in the firft trial that the line of the bail of three inches diameter was 1 3^ 

 inches long: which added to the femi-diameterof the bail made the length of 

 the vibration from the point of fufpenfion to be fifteen inches, and the line of 

 the bail of fix inches diameter was twelve inches long, which added to the fe- 

 mi-diàmeter of the bail, made the whole length to be fifteen inches. In the fé- 

 cond trial it was as before, except that the pendulum with the bail of three 

 inches was adjufted to i3^§, to make it vibrate equally with the other two. 



This being calculated and compared with Monfieur Huygens's rule was 

 found to approach very near to it. 

 Consultez la Lettre N°. 1 274. 7) Lignum vitae était alors le nom du bois de gaïac. 



