St Paul's Cathedral Clock.
Old Clock at the Royal Free Hospital.
such time-pieces to be very faulty, and to necessitate the substitution of wheels of the superior metal.
The best clocks are usually made with wheels of the best gun metal. The teeth are cut by steam power, with an improved cutting engine; and at the same moment that the teeth are cut, they are finished by the engine without the aid of the file, sand-paper, or other polishing materials, so that the most minute difference cannot possibly occur, their accuracy being secured even to the thousandth part of an inch. In the old times this work was done by a man turning a fly-wheel, but that method necessarily occasioned an unevenness of cut which had afterwards to be removed by filing and hand polishing. Wheels thus made could not of course have that precision of movement which is essential in a public clock, and which can only be obtained by a perfect mechanical fit of the teeth of the wheels, such true mechanical fitting being only secured by truly accurate cutting machines. Hand cutting varies with each artisan, and therefore cannot be equally trustworthy. In cheap clocks, constructed to suit public companies who give their contract to the lowest tender, iron is frequently used instead of steel, both in the pinions and arbors, and cast-iron takes the place of gun metal or hard brass in the wheels and bosses,—the result usually being that the Public Clock gets into disrepute through its requiring to be repaired so frequently, and more money is expended upon such repairs than would have sufficed for the purchase of a thoroughly perfect time-keeper. It is urged by the advocates of iron wheels that a clock can be manufactured at a considerably less cost by their employment, but in estimating expense there seems to have been overlooked the important question, as to what will be the probable durability of the machine.
I should be sorry to condemn wholesale all clocks, the main wheels of which are made of iron, but very certain it is that a large proportion of clocks constructed of this material and by London houses of great reputation (despite of their possessing an escapement invented by amateurs who consider themselves the depositories of all horological knowledge), have been found most faulty time-keepers, and after a few years have become entirely worn out and useless.
It is argued (and rightly so) by the advocates of iron wheels that case-hardened pinions should not be used, in consequence of their wearing with great unevenness, but such persons should be reminded that this objection is much greater in the instance of cast-iron wheels. A case came under my notice some time since of a clock made by a London house, with iron wheels, which after comparatively little time became entirely worn out and had to be removed, a result not at all surprising to those who are aware of the porous nature of iron. The Teeth of wheels have to be made with the greatest skill and care in order that the entire mechanism shall work without friction, and shall not only temporarily keep time with regularity, but shall last for many years without renewal. Teeth should fit into one another without a squeezing pressure (which is equivalent to friction), but with exact uniformity of contact, the action being almost entirely between the teeth separating from each other and not between those which are approaching, i.e. in technical language, the action should be after the line of centres of the wheels and not before it.
Church clocks were accustomed formerly to be made to go for thirty-four hours, and to be wound up every day; by the frequency of which winding the clock could be made to keep time with great accuracy, for regulating could be attended to as frequently, and no great variation could well occur in twenty-four hours. But the regulating, as a matter of course, requires a regulator, or standard, of time, which is not always to be found in country places, nor even is the man in charge of clock-winding always in possession of a watch sufficiently accurate to convey the time from the regulator if there were one to the Church clock. Of late, Church clocks are made to go eight days, and so the labour of frequent winding has been saved, while at the same time by extra care in the manufacture and fixing of a clock, there need be no necessity for frequently regulating it.