We will now consider ourselves in the position of an electrical engineer, designing and supervising the lighting of a country house. This work will, of course, offer a great contrast to an installation in a mill or workshop, but these buildings are beyond the Scope of this book.

The first point to be considered is the number of lamps which will be required.

Unless there are unusual conditions, the capacity of the accumulators (see page 251) must be two-thirds the number of the lamps. As each lamp should for this purpose be considered as of 16 C.P., - although eventually some may be increased and others decreased in candle-power, and as, on a 100 volt circuit,1 each lamp requires 6 ampere, the total current for (say) 66 lamps will be 396 or (say) 40 amperes, which, for 10 hours, will equal 400 ampere-hours. Sufficient cells will be required to give a pressure of 100 when discharged to 1.9 volts each, at which point the makers advise their being recharged. The 100 volts, then, divided by 1.9, shows that 53 cells are necessary, and as it is usual to keep one extra in circuit, as a reserve in case of repairs being necessary, we will order 54 cells in glass boxes, each of 400 ampere-hour size, and at a maximum discharge of 4o amperes, complete with glass oil-insulators and teak trays. These oil-insulators are merely small glass cups, containing a little oil to prevent surface leakage (moisture being unable to creep over the oil), upon three or four of which is placed a shallow tray filled with sawdust or crumbled cork, so that the glass box of the cell (the bottom of which is often very irregular) will, when placed on it. be evenly supported.

Where gas is not obtainable, an oil-engine must be used, but a gas-engine is preferable, owing to its greater simplicity and reliability. To ascertain the power of the engine required, we must consider the current needed for the accumulators again. This is 40 amperes, and as each cell on completion of charging requires

1 I have fixed the EM.F at 100 volta, as I am imagining a case in which there are tome outbuildings, about eighty yard from the house, when we can install the plant without any noise from the engine or smell from the cells causing anncyance; snch a distance neceasitates rather a high E.M.F. to avoid having very large mains.

2½ volts, that will be 54 cells multiplied by 2½, or a total E.M.F. of 135. These volts multiplied by the 40 amperes will give a result of 5400 watts, and, as 600 watts to the horse-power is a safe figure to estimate upon, we divide the 5400 watts by the 600 and find that an engine of 9 horse-powers is required.

We shall then order a 9 brake-horse-power engine, being careful to use the. work "brake", since it is 9 actual horse-powers imparted to the dynamo that are necessary, and not 9 nominal horse-powers, which is obviously a vague expression. or 9 indicated horse-powers, since, although the engine may indicate a certain power, this is not definite enough, as a considerable proportion of that power is expended upon the compression of its gases, etc.What we want is a 9 brake horse-power engine, and of such a speed as will enable it to run without imparting the power to the dynamo in pulsations. This we will fix at -00 revolutions per minute. If the dynamo, and consequently the engine, were smaller, this latter would run faster, and vice versa. We must bear in mind to stipulate for a heavily-balanced fly-wheel on each side of the crank; the necessary tanks to connect with pipes to the jacket on the cylinder, in order that the water in them will circulate, and so keep the cylinder cool; and all other details and spare parts.

The selection of the dynamo is an easy matter, since all that it is necessary to state, is that it must be shunt-wound; that it must give 40 amperes at 135 volts, at (say) a speed of 1200 revolutions per minute with a double-flanged pulley of 12 inches in diameter, since the fly-wheel on the engine will be six feet in diameter. and will make 200 revolutions per minute. An endless double leather or link belt 5/8-inch thick will be required, and the dynamo must be driven with the belt tight on the lower side.

The accumulator shelves or racks, if there is plenty of room, can be about 30 inches from the floor, in a single tier ranged round the walls. They must be in a separate room from the dynamo, and between these rooms there must be no communication. For this reason, it is best to select one from which we have to come out into the open air before we can enter the dynamo-house, so that there will be little risk of the spray, which the cells give off on completion of the charge, being blown in upon the dynamo, and so damaging the insulation. The ventilation of an accumulator-house by means of a skylight or roof-ventilator is less likely to allow the fumes to attack young trees or shrubs, than an open window. The floor of this room must be such that it can be washed down into a drain.

The foundations may be made of Portland-cement concrete, that of the engine 4 inches, and that of the dynamo 10 inches above the level of the floor, which may be either boarded over, or covered with oil-cloth or tile-

The switchboard will be conveniently fixed about 18 inches from the wall, near the dynamo. The distance between the engine-shaft centre and that of the dynamo-shaft centre should, considering the high speed, be about 14 feet, and as the fly wheels of the engine will require pits, the floor must be channelled out, so that the belt may not touch as it runs.

The water-circulating tanks we will fix near the engine-cylinder, in the corner, on a stillage a few inches from the ground, and we will supply them with a ball-cock to ensure the water-level Wing maintained. It is a serious matter for the cooling of the cylinder to cease, as would be the case if the level of the water dropped below the uppermost pipe, because the cylinder might be damaged through the piston "siezing" in it.