the Dead Escapement. 9 



The late Mr. dimming, F. R.S. E., in his Elements of Clock and 

 Watch Work, London, 1766, page 43, No. 176, states, that Graham 

 made his pallets take over twelve teeth. In his Plate II., in which he 

 represents the dead escapement drawn very large, and in detail, Mr. 

 Cumming places the centers at the distance of exactly one diameter 

 of the wheel apart, and makes the pallets take over eleven teeth : 



The same effect takes place in the pallet AB, but, from tlie relative situation 

 of the parts, in a much less degree ; the circular face of the pallet requiring 

 to be a portion of a much larger circle : and here it is worthy of notice, that 

 the faces of the two pallets being- portions of different circles, the one is, in 

 fact, a longer line than the other, and consequently with circular faces as 

 just described, there is more friction on one pallet than on the other ; and 

 more on botli than when the acting faces are straight lines. 



The proportional advance of the wheel and pallets was probably considered 

 by Mons. L. Berthoud, (chronometer-maker to the French Navy during the 

 period of the Republic,) of great importance in the case of the dead anchor 

 escapement when applied to watches ; he having given this shape to the pal- 

 lets of some of his box marine chronometers, that I have had an opportunity 

 of seeing. 



The friction is also unequal upon the rests of the pallets, without regard to the 

 shape of the inclined planes, whether straight or curved: for the arc of vibration 

 on each side of zero on the degree plate must necessarily subtend equal angles, 

 and the angles of lead on each side of zero being also equal, it necessarily fol- 

 lows that the angles of rest must be equal ; but the rests of the pallets being at 

 unequal distances from their center of motion of a quantity equal to the thick- 

 ness of the pallet, it also follows that, though the arcs which subtend the angles 

 of rest subtend equal angles, yet that one of them must necessarily be larger 

 than th other, being a portion of a larger circle ; and consequently the fric- 

 tion greater upon the one than upon the other; the difference, however, is 

 verv small, and this is an evil that from the construction cannot be avoided. 



It is scarcely necessary to add, that for the clock to be in beat, it is requisite 

 not only that the angles the pendulum is led should be equal, but that the 

 angles of rest on the circular faces of the pallets should also be equal ; other- 

 wise the total angles of vibration on each side of zero on the degree plate, 

 (which represents the perpendicular line when the pendulum is at rest,) will 

 not be equal, and, consequently, not be performed in equal times. The 

 above observations will, I believe, be found to be of universal application in 

 this construction of the dead escapement. 



For further illustration on the subject of the angle of lead, see " Astro- 

 nomical Observations," by the Rev. Win. Ludlam, 4to., Cambridge, 1769, 

 note, page 86. 



