The philosophy of a chimney is well explained by Dr. Arnott, in his _Elements of Physics_. He says: “Chimneys quicken the ascent of hot air, by keeping a long column of it together. A column of two feet high rises higher, or is pressed up with twice as much force as a column of one foot, and so, in proportion, for all other lengths; just as two or more corks strung together and immersed in water, tend upwards with proportionally more force than a single cork, or as a long spear of light wood, allowed to ascend perpendicularly from a great depth in water acquires a velocity which makes it dart above the surface, while a short piece under the same circumstances rises very slowly. In a chimney where one foot in height of the column of hot air is one ounce lighter than the same bulk of the external cold air, if the chimney be one hundred feet high, the air or smoke in it is propelled upwards with the force of one hundred ounces. In all cases, therefore, the _draught_, as it is called, of a chimney is proportioned to its length.” Defects of Open Fires. This being the general arrangement of a fire in a recess on one side of the room, and an open chimney above it, Dr. Arnott enumerates a long list of evils and inconveniences consequent on such an arrangement. 1. _Waste of fuel._--It has been found that in a common open English fire, seven-eighths of the heat produced from the fuel ascend the chimney, and are absolutely lost. This lost fuel is thus accounted for. One half of the heat is carried off in the smoke from the burning mass; one quarter is carried off by the current of the warmed air of the room, which is constantly entering the chimney between the fire and the mantel-piece, and mixing with the smoke; lastly, one eighth part of the combustible matter is supposed to form the black and visible part of smoke, in an unburned state. Some writers have even gone so far as to estimate the loss of heat in an open fire at fourteen-fifteenths of the whole. 2. _Unequal heating at different distances from the fire._--This forms a remarkable contrast with the uniform temperature in the air of a summer afternoon. In rooms with a strong fire, in very cold weather, it is not uncommon for persons to complain of being “scorched” on one side, and “pierced with cold” on the other; this is particularly the case in large apartments; for as the intensity of radiating heat (like light) is only one-fourth as great at a double distance, the walls of the room farthest from the fire are but little warmed, and, therefore, reflect but little heat to the backs of persons grouped round the fire. 3. _Cold draughts._--Air being constantly required to feed the fire, and to supply the chimney-draught, the fresh air which enters by the crevices and defects in the doors, windows, floors, &c., is often felt most injuriously as a cold current. “There is nothing more dangerous to health than to sit near such inlets, as is proved by the rheumatisms,