Turning the lever-ends of a middle-shaft consists in making them parallel and of a suitable diameter and length for the levers which are to be connected. These ends can be finished either before the holes in the lever-bosses are bored, or afterwards, with equal convenience, because no placing of the two objects together is requisite, the proper sizes being obtained with measurement. The shaft-end, when finished, is larger in diameter than the diameters of the hole to which it belongs, with the view of tightly fixing the lever to its shaft by shrinking. The shrinking of a lever-boss of great size on its shaft is the only effectual means of fastening it, and is also the only sure mode of obtaining a permanent connexion after they are first fastened together, and avoiding all future risk of getting loose.

The difference between the diameter of a shaft-end and the diameter of the hole in its lever-boss should never exceed a sixteenth of an inch, although the hole may be ten or twelve inches in diameter. The custom of many turners is to make the shaft-end nearly as large as they expect the hole to be when the lever is heated ; and because the diameter of the hole will be an eighth of an inch greater when the boss is heated, supposing it to be a foot in diameter, it is thought that the shaft-end should be an eighth of an inch larger than the hole while the boss is cold. But it may be easily seen that such a great difference between the two is, at least, liable to cause the lever-boss to stick when only about half way to its place, if it happens to be not quite hot enough, or if a piece of scale or clinker gets in the way and must be removed. It is also liable to render the boss, after being fastened, much less secure, than if only a small difference had been allowed. The differences between the ends of shafts and the holes in their respective lever-bosses, when both are cold, should average thus : for shafts whose diameters are between five and ten inches, only a thirty-second of an inch; for those between ten and fifteen inches in diameter, only a sixteenth, and for larger sizes a tenth, as an extreme amount. These differences are adopted for levers which are made of excellent tenacious iron, which is not bable

3 a2 to crack during the cooling, and which can be heated to a bright red heat. When steel levers are employed the differences for the respective diameters must be only half the amounts here given.

Whenever the hole of any lever-boss is enlarged by heat, whether to redness, or to nearly red, and is cooled, it will be found to have resumed its former diameter which it possessed previous to being heated, if nothing is in the hole to prevent it; and if the boss is steel or iron, and has been heated to redness, the hole is now smaller than before it was heated. Therefore if a cold shaft-end is large enough to just fill the hole of a boss while hot, the cooling must greatly stretch the metal of the boss, without tending to tighten it; and if it is brittle it will crack. Consequently, the result will be that the boss will not be so tight when cold as if it had been loose on the shaft, through a smaller difference between the two having existed.

A large lever-boss needs a little care while heating, to prevent some part getting burnt, and to prevent clinker adhering to some place in the hole, which would require to be removed with a broom and scraper having long handles. Pieces of clinker will sometimes stick tight in h hole, and therefore require a scraper with a sharp edge to detach them. To conveniently expand the hole of a small boss which is only a few inches in diameter, it need not be put into any fire, but can be heated with a few pieces of round iron, which are successively heated to near welding heat, and put into the hole. A small lever can be heated also on a thick iron plate which is placed over a forge fire, and kept red-hot a sufficient time to heat the boss, the boss being put upside down a few times, that both faces may be in contact.

For heating a large lever, having a hole of twelve or fourteen inches in diameter, a furnace is the best means. The lever is suspended with a chain while the shaft-boss is in the furnace and covered with fuel. The furnace-fire should have been properly made and heated, previous to placing the lever therein, which will cause it to get hot sooner, and without being liable to have some part of the surface slowly burnt off, or being made scaly. The fuel should consist principally of wood, with a little clean coals, thus avoiding the formation of clinker. While the boss is getting hot, the shaft is put into a proper position and situation, so that its length is horizontal, and the entire shaft at a suitable height for the lever to be swung out of the furnace to the shaft-end. A pendulum-hammer also is slung up at a proper height, to drive the lever quickly to its place before it has begun to cool, which quickly progresses immediately it enters upon the cold shaft. As soon as it is driven to the shoulder of the shaft, it is adjusted to place its key-way exactly over the key-way of the shaft, which must be quickly done either by hammering the top end of the lever, or by pulling it with a couple of chains attached to the top end.

In order to cause the key-way of the boss to be of the same width as the width of the key-way in the shaft, when the boss has been cooled and is tight, it is necessary to make the shaft's key-way a little wider than the boss key-way, so that after the boss has been cooled, both key-ways may be of very nearly the same width. But it should be observed, that if only a proper difference between the shaft-end and the hole in the boss is allowed, the width of the shaft's key-way need be but little or nothing wider than that of the boss.

Another mode of heating a large lever-boss may be mentioned, which consists in using a portable wrought-iron apparatus, similar to a bogie or devil. This is capable of being moved to any place which is convenient for the placing of the lever upon the shaft, and has a fire-place of ample size to admit the largest boss which may be put therein. When such an apparatus is used, the lever-boss is put into the firebox previous to making the fire, the wood and coals being afterwards packed around. A substitute for a bogie consists of a structure which is built up of old fire-bars, in any convenient place.

The cooling of a lever-boss must commence immediately it is adjusted to the desired place on the shaft. This is necessary because, by reason of the boss being of great size, it imparts a great amount of heat to the shaft-end within, which heat would greatly enlarge the shaft, and stretch the boss during cooling, if allowed to cool slowly, and the boss would probably be loose, because after the shaft-end has been enlarged with the heat and stretched the boss, the subsequent cooling of the shaft makes it smaller than the hole. To hasten the cooling, a stream of water as large as convenient, is therefore supplied to the boss as soon as it is adjusted.