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.