Notices refpeB'tng New Booh. 357 



pole or the equator. He alfo mentions the difadvantages at- 

 tending the flopping of the balance-wheel in watches ; and 

 relates what Sully, Hook, Hautefeuille, and Du Tertre, in- 

 dividually did to obviate this (loppinfj; and alfo the experi- 

 ments of Le Roy, Tompion, Flameville, and Facio, in this 

 refpeft. BerthouJ taught the method of cutting the teeth 

 in the balance-wheel mathematically. John Prier of Lon- 

 don, and Samuel Anguilins, a Swede, made refearches alfo 

 on the fame fubjeft. The author mentions alfo Schott's pro- 

 pofal in regard to moderating pendulum clocks, as alio thofe 

 of Du Tertre and Le Roy. The recoil fcapement, invented 

 by Clement in 1680, was changed by Graham for a dead 

 fcapement, according to which the fcapement-wheel at each 

 fall of the pallet remains at reft. Graham alfo made the 

 fcapement in watches to reft by means of the cylinder and 

 pallet-wheel ; by which the fridion was lelTened, and the 

 machinery rendered fitter for greater and eafier vibrations. 

 This fcapement, however, requires oil, which is often hurt- 

 ful. In order to leflTen friftion ftill more with the dead 

 fcapement, Eerthoud improved an Invention of Mudtje, which 

 confifts in this, that the balance-wheel is not checked by the 

 balance itfelf, but by a particular catch which the balance 

 lets loofe, by which means the balance continues its ofcilla- 

 tions while the wheel is retained by the catch. The balance 

 makes two vibrations, one forwards and another backwards, 

 fo that the tooth brought to reft is fct free at the fecond vi- 

 bration. 1 his kind of fcapement, which is a ftriking proof 

 ot human ingenuity, is called the free. The author here 

 defcribcs, in an intercfting manner, the different kinds of 

 free Icapements invented by Magellans, Platier, Rendal, Ro- 

 bins, Grant, and Breguet. From the end of the 17th cen- 

 tury, celebrated mathematicians employed themfelves in en- 

 deavouring to difcovcr a general theory for all the parts of 

 watches, of which the author gives a particular account. 

 Another difiicuhy to be overcome in regard to clocks and 

 watches, was the influence which heat and cold has upon 

 the expanfion and contraftion of metals, and which alters 

 the motion of the whole. Graham is confidered as the firft 

 who made experiments with a view of freeing the pendulum 

 from fuch changes. He firft conceived the idea of mnking 

 pendulums of ebonv, fir-wi;()d, walnut-tree, &c.; but found 

 that the inoifture of the air produced other inconveniences. 

 Experiuicnts of the fame kind were afterwards made by Ma- 

 gellan, Foutana, Ludlam, Schroter, Croftwaite, and Knhler. 

 Grahain next conftriit-ted a pendulum of an iron tube, which 

 was filled to a certain height with quickfilver. Troughton, 

 Z 3 niftead 



