the Velocities of Machincnj . 43 



improperly he raUed a tachometer. You will at the same 

 time receive a drawini^, with an account of Ihc instrument, 

 and the mode ofits application. 



I aai, sir, most respectfully. 



Your obedient humble ■^erv^uf. 

 Fort Pl^ce, Permondsey, BrYAN DoNKIN. 



Aprii !1, 181 . 



To C. Taijlor, M.D. Sec. 



Riference to, ovd Description of, Mr. Donkln's TachometeTy 

 or Insirimient for indicating the Velocity of Machinery ^ 



In the employment of machinery It is evidently of great 

 importance to be provided with an easy and readv niethod 

 ior discovering at all times whelner the motion of the ma- 

 chine is quicker or slower than what is known to be best 

 adapted for the object in view. This advantage, it is hoped, 

 may be derived from the tachometer; for it is an instrument 

 which requites only to be adjusted once for all, to anv par- 

 ticular machine, and then it will always be ready without 

 the help of calculation or of a time-piece, to indicate in- 

 stantly upon inspection the slightest excess or defect in the 

 actual velocity. 



A Iront view of the tachometer Is repre«enled in fig. I, 

 and a side view in fig .2, of plate I. XYZ, fig. 1, is the 

 veitical section of a wooden cup, made of box, which is 

 drawn in elevation at X, fio. tj. The whiter parts of iha 

 section, in fig. 1, represent u hat is solid, and the dark parts 

 whit is hollow. This cup is filled with n)ercurv up to the 

 level LL, fig. 1. Into the mercury is immersi^l the lower 

 part of the upright glass tube AB, which is filled with co- 

 loured spirits of wine, and open at botli ends, so that some 

 of the mercury in the cup enters at the lower orifice, and when 

 everv thing is at rest, supports a long column of spirit*, ai 

 represented in the figure. Tlie bottom of the cup is fas- 

 tened by a screw to a short vertical spindle D, so that when 

 the spindle is whirled round, the cup, (whose figure is a 

 solid of revolution,) revolves at the same time round its axis, 

 V, bich coincides with that of tlie spmdle. 



In consequence of this rotation, the mercurv in the cup 

 acquires a centrifu'ial force, by which its particles are tlirowii 

 outi^aids, and that with ilie greater iniensilv, acc(;iding as 

 tlicy are more distant from the axis, and accordiiur as the, 

 angular vclc^city is greater. Hence, on account of lis fluidilv, 

 the mercury rises hifflier and higher as it recedes frouT live 



axis. 



