Electric engines may be used with advantage in cases where the importance of utilizing power at a distance from an original motor is sufficient to compensate for the loss in converting the power into electric energy, and again in reconverting it into mechanical force at the place where it is to be applied, the total loss in these 2 processes being about 2/3 of the original force. The dynamo - electric machines employed are the same as those for electric lighting. A machine requiring 2 h. - p. to work it costs about 50/., one requiring 4 h. - p. about 80/., and one 10 h. - p. about 200/. The machine for giving out the force is the same as the one that receives it. There is also to be taken into account the wear and tear of the apparatus, and the interest on the capital expenditure.

The principal advantage which may be expected from electric transmission is in the utilization of natural forces, such as water - power or cheap fuel, at distant places. At present, however (1882), copper conductors for transmitting considerable power become costly and inconvenient for distances above 2 miles, and the system is practically limited, as it is seldom that the advantages so obtained compensate for the expense. The most favourable opportunity for using electric engines arises when the generator and conductor are already established for electric lighting. A small conductor J in. diameter will not only suffice for numerous lights, but would afford 1 or 2 h. - p. for a motor, and smaller motors suitable for a sewing - machine or other domestic purpose can be purchased for 3/. to 10/. Electric science is advancing so rapidly that the present difficulties are likely to be overcome, and the cheapness with which large motors can be worked as compared with numerous small ones will compensate for considerable loss in distribution and reconversion, if the difficulties and expenses of long conductors be removed. A motor of 1/2 h. - p. weighs less than 40 lb., and occupies very small space. Forms suitable for driving tricycles can be attached beneath the seat.

At the Agricultural Show at Bar - le - Duc, France, in May 1880, an electromotive machine was exhibited, suitable for agricultural use. A large field was successfully ploughed with one of Howard's double - furrow ploughs worked by it.

The experiment of Deprez at Munich gives much information for future use. With machines of the type described by him, 2000 volts is too high an electromotive force to employ. The reason is that it is impossible to prevent the contact of the brushes from being sometimes accidentally broken. In such a case it was found that the electromotive force developed by the extra current is sufficient to ruin the insulation. Another matter deserving of attention is the relative size of the generator and motor. Deprez employed 2 machines of equal size. This slightly simplifies the theory, but it is certainly not the most advantageous arrangement, and we are much in want of accurate measurements on this head. The Deprez experiment at Munich was, commercially, a failure. Since then Deprez has been occupied with experiments of a far more practical nature, with a line of 160 ohms resistance. In the latest experiments, where the effects of friction were deducted, a return of 47 1/2 Per cent. was obtained, and 4.4 h. - p. of work was actually given off by the motor. If a fall of water be used as the motive power, we can install a turbine and dynamos which shall transmit 6 h. - p. through a resistance of 12 ohms, at a cost of 200/., omitting the unknown cost of the conductor.

If this power were used in a place where coal costs 20s. per ton, the cost of fuel for 6 h. - p. would be about 60/. per annum. The interest and depreciation on the boiler and steam - engine would be about 30/. per annum, making in all 90/. per annum, exclusive of wages. Electrically transmitted, the interest on plant, at 15 per cent., would be 30/. per annum, exclusive of wages. This difference of 60/. per annum, after deducting from it the interest and depreciation of the conductor, is so enormous, that it is easy to see what a large saving would be effected in any installation where there is a large consumption of power. There are many factories where it is essential to use a high - priced coal, but if the power could be conveyed electrically from a distance of a few miles, an immense saving would be effected by employing a cheaper kind of coal. When water - power is used, it often happens that the cables can be conveyed along the bed of the river. This preserves the insulation, and keeps the conductor cool, so preventing the usual increase of resistance by heating. In some towns, notably Sheffield, the whole of the water supplying the town comes from reservoirs at a great height. The very large quantity of energy of this water is at present absolutely wasted.

At the site of the Severn Tunnel is a width of river of 2 1/2 miles, where the average rise of tide is 50 ft. If the average rate of flow across this section were 1 mile per hour, we could utilize 100,000 horsepower, and the market value of that power is something like 1,000,000/. per annum, which is now allowed to be wasted. It is worthy of the most serious consideration whether it would not be worth while to erect the enormous engineering works which would be required to utilize this wasted energy, or rather a portion of it, even assuming the interest and depreciation on the turbines and dynamos to be at the rate of 2/. per annum per horse - power (and it would be far less than this for a large installation). (Prof. Forbes.)