Invention of the Crown Wheel Efcnpemetit-, is^c, 5! 



a contrate wheel called the crown wheel, having its teeth pointed and floped on one fide 

 only, fo that the points advance before any other part of the teeth during the motion. CD 

 are two pallets, or flaps, proceeding downwards from the verge E F., The pallets are 

 nearly at right angles to each other ; and when the balance F G fixed to the verge is at 

 reft, the pallets remain inclined to the plane of the wheel in an angle of about 45 de- 

 grees; but when it is made to vibrate, one of the pallets is brought nearer to the per- 

 pendicular pofition, while the other becomes more nearly parallel. The wheel muft be 

 fuppofed to have one of its teeth refting againft a pallet by virtue of the maintaining 

 power. This tooth will flip oft' or efcape as the pallet rifes towards the horizontal pofition, 

 at which inflant a tooth on the oppofite Gde of the wheel will flrike againft the other pallet 

 which is down. The returning vibration, by raifmg this laft pallet, will fufFcr that tooth 

 to efcape, and another tooth will apply itfelf to the firft-mentioned pallet. By this alterna- 

 tion the crown wheel will advance the quantity of half a tooth each vibration, and the 

 balance or pendulum will be prevented from coming to reft, becaufe the impulfe pf the 

 teeth againft the pallets will be equal to the refiftances from friftion and the re-adtion of the 



air. 



On this efcapement it maybe remarked, that the pendulum or balance is conftantly con- 

 nected with, and influenced by, the maintaining power, except during the exceedingly fmall 

 time of the drop of the whefl from one pallet to the other ; on which account the meafure 

 of time will greatly vary, when the force of vibration is merely equal, or not much greater 

 than the maintaining impulfe. This is fliewn in a ftriking manner by urging the movement 

 of a common watch by means of the key. If the key be prelTed in the ufual diredlion 

 of winding up, the beats of the vibration will become very flow, or even ftop; and if 

 the prefTure be made in the oppofite diredion, the vibrations will become very loud and 

 quick. 



dcfcent. Profeflor Vcnturi, in his Effa'i fvr Ifs Owjrages PhyJico-MulbimMiquei Je Leonard Je Find, page 18, 

 bas a curious note on this fubjeft, from which I here make feme extrafts. The common efcapement dc- 

 fcribed in the text was well known to de Vinci, who defcribes an inftrument a£ling by an efcapement of this 

 kind, fimilar, as he fays, to the verge of the balance in watches, which he does not feem to mention as a new 

 thing. He died al)out 1513. The ifochponifm of the pendulum was known to Galileo in i6oo, who before 

 his death, namely about 1633, propofcd to apply it to clocks. The aflual application by Huyghens was made 

 before 1658, when he publiflied his Horologi um ofciUatorium. He applied it by means of the common efcape- 

 ment already in ufe with the balance, and ftill retained in our table clocks. San£lorius had made the fame 

 application near forty years before that time, as appears by his Cammenlarii in A-vicennam, (qucji. 55) printed 

 in 1625, in which feveral inftruments are defcribcd as having been publicly exhibited and explained to his 

 auditors at his Itftures in Padua for thirteen years previous to that time. 



There is a manufcript (No. 7295) in the National Library of France, written about the middle of the 15th 

 century by H. Arnault, Phyfician to the Dukes of Burgundy, who died in 1465. This author defcribes a 

 planifphere which Jean de Fondeur (Fuforis) his mafter had conftrafted for the Duke, and which Arnault 

 himfelf had afterwards repaired. Whtnce it appears that the inftrument was conftrufted about the commence- 

 iiient of the ijth century. At folio c.<j a dtfign is feen of the watch which gives motion to the planifphere. It rs 

 nearly the moderm watch. The balance is called circulus affixui "virge faletorum, qui cum ea de -vi movelur. He 

 calls the crown wheel, which forms the common efcapement on the pallets of the verge of the balance, by the 

 name of folietus. At folio 60 the ftriking part is defcribed, where we have the terms •voietus, the hrochif ie- 

 •uanles malUum\ crux media inter woiiimtntum el foineriam ; cavak and ciii-volci for cbeiiiUe. M. Vcnturi thinki 

 that the watches of Wallingfort and Dondi, in the i4tli teiftviry, were conftruftiJ bn thfc principle of- the 

 balUncc. Robert Hooke added the fpring to the balance in the year 1658. 



H 2 This 



