effect, but the substitution of one lamp for another is carried out so rapidly that the eye gets the impression of perfect steadiness. The Dussaud system is of little use for ordinary lighting purposes, but for lighthouse illumination, photographic studio work, and the projection of lantern slides and cinematograph films, it appears to be of considerable value. Electric light has many advantages over all other illuminants. It gives off very little heat, and does not use up the oxygen in the air of a room as gas does; while by means of flexible wires the lamps can be put practically anywhere, so that the light may be had just where it is wanted. Another great advantage is that the light may be switched on without any trouble about matches, and there is none of the danger from fire which always exists with a flame. The current for electric lamps is generally taken from the public mains, but in isolated country houses a dynamo has to be installed on the premises. This is usually driven by a small engine running on petrol or paraffin. In order to avoid having to run the engine and dynamo continually, the current is not taken directly from the dynamo, but from a battery of accumulators. During the day the dynamo is used to charge the accumulators, and these supply the current at night without requiring any attention. Electric lighting from primary cells is out of the question if a good light is wanted continuously for long periods, for the process is far too costly and troublesome. If a light of small candle-power is required for periods of from a few minutes to about an hour, with fairly long intervals of rest, primary cells may be made a success. Large dry cells are useful for this purpose, but probably the most satisfactory cell is the sack Leclanché. This is similar in working to the ordinary Leclanché cell used for bells, but the carbon mixture is placed in a canvas bag or sack, instead of in a porous pot, and the zinc rod is replaced by a sheet of zinc surrounding the sack. These cells give about 1½ volt each, so that four, connected in series, are required to light a 6-volt lamp. The lamps must take only a very small current, or the cells will fail quickly. Small metal filament lamps taking from a third to half an ampere are made specially for this purpose, and these always should be used. A battery of sack Leclanché cells with a miniature lamp of this kind forms a convenient outfit for use as a night-light, or for lighting a dark cupboard, passage or staircase. Lamps with ruby glass, or with a ruby cap to slip over the bulb, may be obtained for photographic purposes. If the outfit is wanted for use as a reading-lamp it is better to have two separate batteries, and to use them alternately for short periods. With this arrangement each battery has a short spell of work followed by a rest, and the light may be kept on for longer periods without overworking the cells. CHAPTER XIII ELECTRIC HEATING