Bi-focal lens for spectacles.
Economy of Heat.
From light we pass to its twin phase of energy, heat, for a glance at the forms of devices which enable us to use heat with economy. When we wish a furnace, crucible, or cooking vessel to maintain the highest possible temperature, we give it as little surface as possible. On the contrary when a warming apparatus is devised, its surface is freely extended. The traditional fireplace, for all its cheerfulness, yields but little heat. Benjamin Franklin copied its form in the stove which bears his name; as it stands out from a wall it warms the air all around itself, instead of on one side only. This model is familiar in gas stoves, whose heat thoroughly radiated and convected far exceeds that derived from fireplaces. In Canada forty years ago it was usual, especially in the country, to set up gallows-pipes and dumb-stoves, or drums, bulky, hollow structures of sheet iron, which obliged the heated products of combustion to take a roundabout course as they passed to the chimney. To be sure as thus cooled the gases were less effective as draft makers, but we must remember that one of the most wasteful uses of fire is in warming air or other gases for the sake of putting them in motion. In modern factories, central lighting stations, and the like huge installations, mechanical draft sends a quick current through a short chimney, saving much fuel. Excellent in design are the tile stoves of Germany and Holland. Their gentle heat does not parch the air; in moderately cold weather they render it unnecessary to light furnaces which develop, at such times, unduly high temperatures.
Canadian box stove with gallows-pipe.
In factories the heating coils filled with steam or hot water were at first fastened to the floor. Then came attaching them to the ceiling whence their heat is gently radiated; on the floor the coils may gather dust and dirt with risk of fire; with the other plan there is a saving of floor space, and accidental leaks are at once in evidence.
Canadian dumb-stove.
Tubes for warming are specially effective when dented or buckled in directions at right angles to each other and to the axis of the tube. This form gives the heating water or steam a swirling motion which causes it to part more rapidly with its heat than does a cylindrical tube of the same surface. Gold’s electric heater for street-cars, bath-rooms, and the like, is a spiral of resistant alloy, hung in a light metallic frame, the whole presenting a large surface to the air. Automobiles driven by heat engines require coils of the utmost possible surface whereat cooling can take place; in many cases this cooling is furthered by the action of a quick fan. In like manner the condensers of steam-engines, especially aboard ship, are made up of slender tubes presenting to the steam a chilling area of vast extent.