The cost of the fuel must not only be given, but also its weight, because the price of fuel varies in different localities. The capability of using different sorts of fuel in the several stoves, and the amount of fuel so consumed for its equivalent of coal, should also be stated. These and other comparative inquiries having been made, the durability of that part of the stove which is subjected to the direct action of the burning fuel, must be examined. It will be made either of iron or of earthenware; and the relative merit of the various stoves will, as far as this point is concerned, consist in the facility and economy with which such parts can be removed, and the corresponding new parts be purchased and replaced in their proper position. It is always desirable for the consumer that the vendors of such articles should keep a stock of the parts liable to wear out, and that the latter should undertake to replace them at a fixed price. Those parts of the stove which project so as to be liable to accidental blows, and those which from their more constant use are much exposed to accident, as the hinges and the latches of doors, should then be examined. These, if of cast-iron or other brittle material, and constituting part of the substance of the door, should be sufficiently strong to resist fracture: if they are attached to it by rivets or otherwise, they will be lighter and stronger when made of wrought-iron. The last inquiry is into the expense of fixing the stove for use. It may be set in brickwork, within the chimney, in which case it will require a bricklayer and a large mass of materials in the shape of bricks and mortar, and possibly of stone. Or it may stand on its own base containing its own ash-pit, and by means of a small iron pipe the smoke may be conveyed into a flue. In this case almost any workman with hammer and chisel and a small quantity of mortar or cement, can fix it ready for use. Again, the stop-cock for the water-cistern may be either hard-soldered, riveted, or screwed in. If the latter, it can easily be unscrewed or reground when necessary. The same remark applies to the leaden supply-pipe; it may be connected by soldering, or by a union joint. In the former case these parts will require the aid not only of the tinman or coppersmith, but also of the plumber. § The expense of repairing a machine does not in all cases depend on the cost of the part replaced, or even on the actual cost of replacing that part alone. It often happened in the earlier days of locomotive engines, that the expense of some small reparation necessary to keep the machine in good working order, did not amount to ten shillings; whilst the expense of removing and replacing other parts, without which the workman could not get at the defective part, amounted to fifty or eighty shillings, or even to a still larger sum.