When calcium carbide is used as a source of illumination, the crystals are mechanically dropped into a tank containing water, and the gas generated is automatically collected in a small sliding tank, whence it passes through pipes to the various rooms.  The slaked lime, formed while the gas was generated, collects at the bottom of the tanks and is removed from time to time.

The cost of an acetylene generator is about $50 for a small house, and the cost of maintenance is not more than that of lamps.  The generator does not require filling oftener than once a week, and the labor is less than that required for oil lamps.  In a house in which there were twenty burners, the tanks were filled with water and carbide but once a fortnight.  Acetylene is seldom used in large cities, but it is very widely used in small communities and is particularly convenient in more or less remote summer residences.

Electric Lights.  The most recent and the most convenient lighting is that obtained by electricity.  A fine, hairlike filament within a glass bulb is raised to incandescence by the heat of an electric current.  This form of illumination will be considered in connection with electricity.

CHAPTER XVI

MAN’S WAY OF HELPING HIMSELF

149.  Labor-saving Devices.  To primitive man belonged more especially the arduous tasks of the out-of-door life:  the clearing of paths through the wilderness; the hauling of material; the breaking up of the hard soil of barren fields into soft loam ready to receive the seed; the harvesting of the ripe grain, etc.

[Illustration:  FIG. 91.—­Prying a stone out of the ground.]

The more intelligent races among men soon learned to help themselves in these tasks.  For example, our ancestors in the field soon learned to pry stones out of the ground (Fig. 91) rather than to undertake the almost impossible task of lifting them out of the earth in which they were embedded; to swing fallen trees away from a path by means of rope attached to one end rather than to attempt to remove them single-handed; to pitch hay rather than to lift it; to clear a field with a rake rather than with the hands; to carry heavy loads in wheelbarrows (Fig. 92) rather than on the shoulders; to roll barrels up a plank (Fig. 93) and to raise weights by ropes.  In every case, whether in the lifting of stones, or the felling of trees, or the transportation of heavy weights, or the digging of the ground, man used his brain in the invention of mechanical devices which would relieve muscular strain and lighten physical labor.

If all mankind had depended upon physical strength only, the world to-day would be in the condition prevalent in parts of Africa, Asia, and South America, where the natives loosen the soil with their hands or with crude implements (Fig. 94), and transport huge weights on their shoulders and heads.