450 



Mr. A. T. Hare 



[Feb. 21, 



is out of contact with it, and so the action of the train cannot disturb 

 the pendulum except by the friction of the dead faces. 



There is, however, a source of error to which Mudge's escape- 

 ment is hable which was sufficient to condemn it. The driving- 

 power had to be ample, and there was danger either that the gravity 

 pieces might be thrown clean off the wheel, allowing the latter to 

 race and destroying all timekeeping, or that, if this complete 

 " tripping " did not occur, they might, at all events, be thrown a 

 little too high, so that the teeth of the scape-wheel, instead of resting 

 in the exact coruer, as tooth 1 is seen to be doing, would rest on the 

 dead face nearer its extremity, and probably hold up the gravity 

 piece, by friction, higher than it should. This fault was called by 

 Lord Grimthorpe "approximate tripping," and if it occurred the 

 constancy of the maintenance would be lost. This might probably 



Fig. 1. 



Fig. 2. 



Fig. 3. 



have been cured by the use of a dashpot, with which Mudge's escape- 

 ment would have been very considerably improved. 



Mudge's escapement was followed by Bloxam's, the action of 

 which will be obvious from Fig. 2. It is still to be seen in action in 

 Bloxam's own clock, which is now, by his nephew's permission, at 

 the Science Museum. The noteworthy feature in it is that the 

 locking arms are much longer than the lifting teeth, so that the 

 friction of unlocking is much reduced. 



It was on Bloxam's design that Lord (rrimthorpe improved in the 

 construction of his well-known " double three-legged gravity escape- 

 ment," used for the first time in the great clock in the Houses of 

 Parliament. The principal feature in this escapement is the long 

 wind-fly, which moderates the shock of impact of the teeth on the 

 pallets, and wliich the large angular movement of the scape-wheel 

 (00^ at each tick as against 20^ in Bloxam's) rendered effective. 



