INVENTION or THE BALANCE SPRING. 363 



Having marie this calculation, in order to satisfy my curi- Spherule of 

 osity, 1 found that a solid spherule of pure 2;old, of the diame- ^vonld not work 

 ter of 7o-o,Vo (or exactly •^-ysVxg of an inch), ought to re- in water. 

 main suspended in water in consequence of the adhesion of 

 the particles of that liquid to each other. But I shall return - 

 to this subject on a future occasion. 



IT. 



An Account of the Invention of the Balance Spring, and the 

 Determination of the Conditions of its Isochronism in wide 

 and narrow Vibrations, by Robert Hooke, in the \6th Cen- 

 tury, and of the first free Escapement by Du Tertre ; toge- 

 ther tvith various other Historical Details relative to Time- 

 pieces. In a Letter from Mr. Thomas Re id of Edin- 

 burgh. 



To Mr. NICHOLSON. 

 5IR, 



JLn your Journals of late, I see you have given an account Reference to 



of the detached 'scapement, such as it is now ijenerally an- ^*'"^'^ ^"."^ 



plied to chronometers or timekeepers, and also of the differ- timepieces. 



ent forms, and the properties of the pendulum spring, both as 



explained by Messrs. Arnold and Earnshaw to the Honourable 



Board of Longitude. 



I beg you will allow me, through the channel of your in- Rbbert Hooke, 



teresting and useful Journal, to give you some account of the ^'/^ inventor of 

 = ' o J |.|,g pendulum 



invention of the pendulum spring and of its properties, by their spring unjustly 

 author, our countryman, the celebrated Dr. Hooke. I have >^°g^'5cted. 

 been often provoked to see his name so much kept in the back- 

 ground in legard to these matters, and particularly by foreign 

 artists, who, whenever they have occasion to speak or make 

 mention of the pendulum spring, enter much on the merits of 

 this important invention, and are full of enthusi.xsm in praise 

 «f M. Huyghens, for having made this wonderful discovery. 

 M. Huyghens was undoubtedly one of the most profound geo- Huyejhens. 



metricians that any age has produced, and Dr. Hooke .must ^*^^^''^® 



•^ = ^ ' ag-ainst Huy- 



ccrtainly be allowed to have been one of the greatest mechani- ghens' claim 

 clans; a bare recital of whose mechanical inveotions are of[" h'simen- 

 A a a 2 themselves 



