428 Mb DENISON, ON SOME RECENT 



all necessity and all excuse for spending money in polishing, and in other refinements, some 

 necessary and others superfluous, which are now introduced in all the best astronomical 

 clocks. And, as regards turret-clocks, it will remove what little excuse there ever was for the 

 prejudice against the use of cast-iron wheels among the clockmakers — at least those of them 

 who have had no experience of it, and refuse to listen to the experience of those who 

 have. 



It will moreover supersede the necessity for a train remontoire, except where it is desired 

 to have a visible jump of the minute-hand at every half minute, as in the clock at the Great 

 Northern Railway Station in London, and several others, and among them the great West- 

 minster clock, according to Mr Airy's original requisitions. But if the first blow of the hour 

 is made to take place the moment the going part lets off the striking, as it easily can be, this 

 will in almost all cases be a sufficiently exact and convenient mode of fixing the precise time, 

 and therefore a train remontoire will probably be very seldom used with this escapement. 

 Another saving in large clocks may be effected in the pendulum, as a long and heavy com- 

 pensated pendulum adds a good deal to the expense of a clock, and a short one will do as 

 well when the escapement gives a constant impulse without friction. The reason why a large 

 pendulum is advantageous to a clock with a common escapement is, that it mitigates the errors 

 due to the escapement; but it does not remove them. There are several turret-clocks now 

 making on this plan with l£ second pendulums (6l inches), instead of the longer ones which 

 they would have had if the escapement had been of the usual kind. 



There is another incidental advantage in an escapement of this kind, which I may as well 

 notice here ; viz. that it allows you to put the clock forward or backward any even number 

 of beats of the pendulum without touching the pendulum, and without any risk of injuring 

 the scape-wheel. For if you lift one of the arms, so as to make the wheel trip one tooth, you 

 put it forward two beats ; and if you stop the scape-wheel, of course you put the clock back : 

 whereas with a common escapement you cannot put the clock forward at all except by the 

 hand-adjusting work, which you cannot judge of to seconds; and it requires some care to stop 

 the scape- wheel without letting the pallets catch the point of one of the teeth. The still 

 smaller adjustments are to be made in the Westminster clock in what I believe to be a new way. 

 The place where any additional weight produces the greatest acceleration of a pendulum, is 

 half-way between the centers of suspension and oscillation ; at this point therefore a collar is 

 fixed, and on it lives a weight of the form called a 'shifter,' about -^-qo the weight of the 

 pendulum itself; and there is another weight like it, to be kept in the clock-room. Taking 

 off the permanent one for about 10 minutes will retard the clock 1 second, and putting on 

 the other will accelerate it. This method produces less disturbance of the pendulum than 

 any other I know of, though of course it requires some care in putting on the weight (it is 

 very easy to take it off) ; but every other method that I have seen requires much more care 

 in its application, as well as more trouble in construction. 



The permanent regulation of the pendulum is done in the same way, by small weights 

 laid upon the collar half-way down the pendulum. One ounce just accelerates this pendulum 

 of 6cwt. a second a day. This is far the best way of doing the pendulum regulation, as it 

 avoids all screwing up and down of the bob and other disturbance of the pendulum, and 



