(_Engineer._) Heat radiated from a fire passes through the air without increasing its temperature, in the same manner that the sun’s rays in warming the earth pass through and leave the atmosphere at the higher altitudes so bitterly cold that water and even mercury will freeze: it is for this reason that open fires should be lighted some time before the apartment is required for use, so that firstly a glowing fire be obtained (flames do not radiate any material quantity of heat, and practically heat by contact only), and secondly the surrounding objects, walls, &c., be heated by radiation, and these in their turn warm the air. In discussing the various methods of warming, it will be convenient to classify them under general heads. To put the reader upon a more familiar basis with this subject, a short explanation of the cause of heat will be here given. Combustion is the chemical union of oxygen (contained in the air) with some other substance for which it has an affinity; as applied to coal, it is the combining of oxygen and carbon producing carbonic acid gas, and it is known to every one that all chemical combinations evolve heat. Combustion may be said to be complete when coke, wood charcoal, or anthracite coal is burnt, as there is no smoke, the up current is colourless, and these fuels burn quite away, leaving nothing except a little ash, &c., which originally consisted of earthy impurities in the fuel. Ordinary coal contains bitumen (pitch) in its composition, which at a temperature of about 500° to 600° F., distils off as a smoky gas (carbon and hydrogen), but at a higher temperature this is ignited, forming flame by the union of oxygen with the smoke (carbon); the main principles of underfed, smoke-consuming grates are based upon this, with the object of causing all gaseous products from the fuel to pass through the incandescent portion of the fire and so render the consumption of the fuel complete, as will be explained later on. A good authority says that “the correct method of warming is to obtain everywhere, at will, the warmth most congenial to the constitution with air as pure as blows at the mountain top,” and it might have been added “without an unreasonable consumption of fuel.”