clocks rfgiilated by the Fly : the BaUuta : the Pe:t{u.';,ii!. J7 



(.lefcemls very flowly and uniformly, bccaufe refilled by the friction of .\. fcrew in the la(t 

 arbor of the train, and a fly which terminates the whole. The late Mr. John Whitehurft, 

 F. R. S. conftrufted a clock on this principle for meafuring fliort portions of time, to 

 the hundredth part of a fecond, in which the regulating agent was a broad^ejfcd fly, 

 which aded upon the air. It was regulated in the hrft place by adjufling tiie weight, and 

 in the next place by altering the obliquity of the flriidng furface of the fly. It may readily 

 be imagined, as was in fa£l the cafe, that its rate would vary according to the denfity of the 

 air. Perhaps it might liave been of ufe in fome experiments to afcertain that varying rc- 

 fiftance. ' Huygcns, in his treatife De Horologio Olcillatorio, defcribes a clock regulated by 

 a fly, which derived its rcfiftance from the gravitating power of a ball at the end of a ful- 

 penfion by ftrings. When the maintaining power was weak, it fwung out to a fmall diftmcc 

 from a vertical axis, to the upper part of which the ftrings were faftened, and was therefore 

 'eafdy carried round; but when a grester force was appli.d, the ball fwung out farther, and 

 confequently afl-'orded a greater refiftance. The ftrings were made to apply againft a curve, 

 by virtue of which the diver-gencies of the ball produced ifochronal rotations. 



It is probable that the firft wjieel clocks were regulated by a fly ; an organ, of which 

 the inconveniencies are too great to require enumeration to fuch as are moderately fkillcd 

 in purfuits of this nature. Striking and repeating trains are ftilL regulated in this way. 

 M. Le Roy, in the Encyclopedic *, article Balaiic'ur, afliirms, that before the admirable 

 invention of the pendulum by Huygens, clocks were regulated by an horizontal ba- 

 lance, having a vertical axis, that paffcd through two holes, with liberty to play up and 

 down ; and that it was fufpended by means of a ftring pafled through a hole in the axis, and 

 faftened at both ends, fo as to form equal angles with the axis itfelf. Confequently, when 

 the balance revolved in one diredtipn, the ftring was wound upon the verge, and, being thus 

 fliortened, raifed it up until the weight of the balance had overcome the force of rotation: 

 after which it revolved the contrary way, and defcended to perform a fimllar afcent by 

 winding the Airing the oppofite way. He does not tell us the nature of the connexion 

 between this balance and the wheels which were regulated by it. 



The application of the pendulum to clock-work conftituted fo important an improvemeni 

 in engines for mcafuring time, that it will probably be very long before our cotemporaries, 

 or even their defcendants, will be able to bring the balance in competition with it. In this 

 organ, the regulating force is fo fteady, and the lofs from the rcfiftance of the air fo fmall, 

 that the imperfedions of the train through which the force of the firft mover nuift be 

 conveyed, arc capable of producing no more than a fmall part of that irregularity whicli 

 nccedarily follows when tlie aftuai regulator derives moft of its effefl: from the train itfelf. 



From the contemplation of this moft valuable meafure of time, and the fources of its- 

 regularity, the artilts of modern times have endeavoured to place it in a fituation which 

 (hall as nearly as ]ioflible refcmblc that of a pendulum detached from a train of wheels, 

 and vibrating in vacuo without fritlion or rcfiftance. This fimilarity is aimed at by ^ 

 fkilful difpofition of that part of the engine which connects the pendulum with the train, 

 and is called the cfcapcment, for which there are many contrivances well entitled to the 

 attention of artifts and men of fciencc. Another moft eflential particular is, that the organ 

 for nicafuring time, whether it be a balance or a pendulum, fliould invariably prcfcrve its 

 * Geneva edition, 17-8, vol. i» 



Vol.. I. — Mav 1797. I dimer.fioiis 



