Where the ash-pit is clear, annoying but not dangerous obstructions are often due to the accumulation of cinders and clinkers above the grate. The old-fashioned furnaces were generally fitted with a "dumping-grate," consisting of a circular grating of cast iron, somewhat smaller than the lower opening of the fire-pot, and attached to a pivot in the centre, so that it could be turned over. The pivot projected through a slotted hole in the front of the furnace, so that the grate could either be moved laterally from side to side, by means of a suitable handle, or, by a different application of the handle, turned over. The lateral shaking served to sift out the fine ashes through the interstices of the grating, while the coarser cinders and clinkers, accompanied, usually, by some of the coal, fell over the edge of the grate into the ashpit; and, when it was desired to clear out the furnace, or build a new fire, the grate could, with the same handle, be turned into a vertical position, allowing everything above it to drop into the ash-pit. As only the clinkers and cinders at the edge of the grate were dislodged by the shaking, treatment with the hooked poker was generally necessary to get the lower part of the fire reasonably clear; and, even then, the middle portion of the grate could not be easily reached. Several devices have been introduced to meet this difficulty. In one furnace the grate is made in the shape of a basket, which can be turned, so as to bring any part of it to the front, and cleared of clinkers and cinders with a poker; and another form has a grate sloping downward from all sides to the centre, where there is a large hole, stopped by a plate, which can be dropped, so as to let the cinders and clinkers which have been shaken down to the hole fall out. The more recent furnaces are, however, generally equipped with some modification of the Smythe grate, similar to that applied to kitchen stoves, but on a larger scale, with the same revolving bars, with interlocking flanges, usually in two sets. These interlocking triangular flanges, when the bars are revolved, rise, clutch the cinders and clinkers over them, and, turning downward, drop them into the ash pit. In shaking a furnace provided with a grate of this kind, the first operation, after making sure that the ash-pit is cleared of the old ashes, should be to revolve each set of grate-bars once or twice, before any other shaking is done, following this by rocking the bars, to shake the fine ashes through. If the bars are rocked before turning them over, pieces of coal will be shaken down among the cinders, and will be dropped with them into the ash-pit, and lost; while, if the fire is left undisturbed until the bars are turned, the lower layer in the fire-pot will consist of nothing but cinders and ashes, or clinkers; and, after these are deposited in the ash-pit, by turning the bars over, the rocking, to shake out the fine ashes, can proceed without danger of shaking out coal. How far the removal of cinders by revolving the grate-bars should be carried depends on the coal used, and the previous condition of the fire; but, when a bright light is seen in the ash-pit, the fire may be considered, with this form of grate, to be quite clear.

Shaking a furnace.

With the older forms of grate, the ash-pit may be brightly illuminated from the edges of the grate, while the middle is still occupied by a huge mass of melted ashes and cinders, which can only be removed by laborious extraction through the door; and, as these inert masses diminish materially the efficiency of the fire, it is desirable to prevent them from forming, which can generally be done by keeping the fine ashes well shaken out, and avoiding, as far as possible, opening the lower door of the furnace. There is a great difference in coal as to disposition to form clinkers, but an infallible way to produce them, with any coal, is to send an ignorant servant to "start up the fire" in the morning.

Clinkers.

"Starting up the fire.'

Unless careful instructions are given, this ob ject is generally accomplished by closing the check-draught, if it has been open, and opening the lower door of the furnace, without disturbing the ashes which have been accumulating in the fire-pot during the night. The flood of air admitted under the grate effectually "starts up " the fire, which, with a reasonably good chimney draught, soon becomes hot enough to melt the ashes into solid masses; and, by the time the housekeeper arrives, and provides for mitigating the furious combustion, the mischief is done.

In place of these violent proceedings, which are very wasteful of coal, as well as destructive of the furnace, all regulation of furnaces should be carried out with caution. A good automatic regulator will soon save its cost in a large house; but, where this cannot be employed, much may be done by putting on a large quantity of fresh coal at night, and leaving it with a moderate draught, so that, by morning, the mass will be well ignited, without burning furiously, and without needing to be inordinately "started up."

In general, furnaces, when used with anthracite coal, work most economically and efficiently by keeping the fire-pot, in cold weather, filled as high as possible with coal, and allowing only a moderate combustion. As the air which passes through the air-chamber is warmed only by actual contact with the heated walls of the furnace and smoke-flues, it is found that a large extent of surface, at a moderate temperature, is much more effective, with the same expenditure of fuel, than a narrow zone, intensely heated by a small, but furious fire; and the latter is far more likely to crack the fire-pot, or cause other mischief, than a larger but more moderate fire. At the same time, if a large body of coal is maintained in the fire-pot, care must be taken not to open the lower door, and forget to close it again, as a large body of coal, burning furiously, is, naturally, more difficult to check than a small one.