The data of Karmarsch, Brix, and Gysser, already given, were obtained by the experimental method. They were, however, made mostly on a small scale, and, in some cases, without due regard to the peculiar requirements of the different kinds of fuel, as regards fire space, draught, etc. They can only be regarded as approximations to the truth, and have simply a comparative value, which is, however, sufficient for ordinary purposes. The general results of the investigations hitherto made on all the common kinds of fuel, are given in the subjoined statement. The comparison is made in units of heat, and refers to equal weights of the materials experimented with. HEATING POWER OF DIFFERENT KINDS OF FUEL. Air-dry Wood 2800 " Peat 2500 3000 Perfectly dry Wood 3600 " " Peat 3000 4000 Air-dry Lignite or Brown Coal 3300 4200 Perfectly dry Lignite or Brown Coal 4000 5000 Bituminous Coal 3800 7000 Anthracite 7500 Wood Charcoal 6300 7500 Coke 6500 7600 4.--_Modes of Burning Peat._ In the employment of peat fuel, regard must be had to its shape and bulk. Commonly, peat is cut or moulded into blocks or sods like bricks, which have a length of 8 to 18 inches; a breadth of 4 to 6 inches, and a thickness of 1-1/2 to 3 inches. Machine peat is sometimes formed into circular disks of 2 to 3 inches diameter, and 1 to 2 inches thickness and thereabouts. It is made also in the shape of balls of 2 to 3 inches diameter. Another form is that of thick-walled pipes, 2 to 3 inches in diameter, a foot or more long, and with a bore of one-half inch. Flat blocks are apt to lie closely together in the fire, and obstruct the draft. A fire-place, constructed properly for burning them, should be shallow, not admitting of more than two or three layers being superposed. According to the bulkiness of the peat, the fire-place should be roomy, as regards length and breadth. Fibrous and easily crumbling peat is usually burned upon a hearth, _i. e._ without a grate, either in stoves or open fire-places. Dense peat burns best upon a grate, the bars of which should be thin and near together, so that the air have access to every part of the fuel. The denser and tougher the peat, and the more its shape corresponds with that usual to coal, the better is it adapted for use in our ordinary coal stoves and furnaces. 5.--_Burning of broken peat._ [Illustration: Fig. 1--STAIR GRATE.]