A Treatise On The Principles And Practice Of Dock Engineering

512 DOCK ENGINEERING. time; and in countries where the thermometer is often below zero, it would be difficult to secure perfect immunity from interruption of working. To this drawback must be added the great cost of laying mains and forming culverts for their reception. Water pressure is also very materially affected by bends and changes of direction, so that where these are inevitable there will be a corresponding loss of power. Electricity.—As a distributive agent, electricity has very largely corne into favour during the last ten to fifteen years. Its principal merits are the extreme cleanliness and compactness of its working parts, and the tenuity and flexibility of its supply mains, both of which features stand out in prominent juxtaposition to the soot and smoke of the steam engine and the bulky and awkward canalisation of hydraulic power. Moreover, the first cost of wire mains is much less than that of any corresponding pipe system. As a motive force, electricity is able to discharge all the functions of steam for actuating mechanism identical in character. The main shaft of a machine may be driven indifferently by a steam engine or an electric motor. But whereas steam power is rarely capable of centralisation, electricity is most admirably adapted to systematic distribution from a common centre. Since steam is most commonly employed for the genera- tion of the electric current itself, it is not contended that the latter system is as economical as the former; but it may be pointed out that one large electrical generating station, worked by steam power, will probably involve less expenditure in fuel, repairs, maintenance, and attendance than a number of separate steam engines, each with its own special outfit and upkeep. Moreover, the generating plant may find an additional use at night-time for lighting purposes, and this at a period when, lifting appliances being more or less idle, there would be little or no interference with the discharge of its primary functions. There must inevitably be considérable saving arising from the adaptation of a single installation to the supply of both power and light. The advantages arising from the combination are, however, largely discounted during the winter, when the shortness of the days necessitates early lighting. The amount of electrical energy consumed is sensibly proportional to the work done, and in this respect electric power differs advantageously from hydraulic power. Electric distributors, however, are more complicated than the working parts of the other two systems; they are, therefore, less easily kept in repair, and they necessitate the attendance of skilled workmen. Moreover, they do not act with the smoothness and precision of hydraulic machines, nor with the independence and directness of the steam engine. Comparative Expenditure of Energy—In order to institute a comparison between the several systems in regard to their expenditure of energy and the cost of its production, it is necessary to establish the relationship existing between their respective units of power. The primary unit of work is the