In large power-producing machinery an iron staircase leads up to a platform above the dynamos, giving access to the loftier parts of the apparatus, which then, in its general appearance and compactness, somewhat resembles a modern marine engine. On the walls are endless dials, recording the amount of current generated, localising the exact position of the trains on the line at any given moment, and checking the quantity of current picked up by each engine. There is absolutely no smell, no outward indication of resistless power, while almost Arcadian quiet reigns in the neighbourhood of the machines. That these small dynamos are capable of driving heavy cars filled with passengers at the rate of many miles an hour seems incredible; but faith, “the evidence of things not seen,” must come into play. The craving for mere size, however, will be amply gratified when the great power house at Chelsea, built to supply the Metropolitan, District, and other railways, is completed (_vide_ Chapter V.). But what on earth is a kilowatt, or a volt, an ohm, or an ampère?--expressions that are rapidly becoming as familiar as the word horse-power. Well, “horse-power” was a term invented long ago by engineers, who blandly asked one to imagine that an ordinary horse was capable of lifting a weight of 33,000 lbs. (or some 14½ tons) one foot high per minute. Now, electricity is a very exact science. There is no mere theory about it; and a unit is a definite quantity of power, known in that science as a “kilowatt hour.” Thus, a kilowatt, or 1,000 watts, is the equivalent in measured work of 1⅓ horse-power, equal to the lifting of 44,000 lbs. per minute, or the doing of so many units of work, either electric lighting, heating, machinery driving, or traction. VARIOUS FORMS OF ELECTRIC LOCOMOTION Electricity as a locomotive force is being presented to the public in various forms. There is the ordinary railway, like the Underground, that, cleansing itself, amending its ways, and becoming converted to the new order of traction, has been granted a new lease of life. Then there are new lines laid down, intended from the first to be electrical, with specially designed cars, diving beneath the Thames, and connecting the north and south of London. These are our metropolitan pioneer electric railways. There is also the system of railways specifically and popularly known as Tubes, most important factors in the travelling world of modern Babylon. Another division is the system known as Overhead Electric Railways; that is to say, rails laid upon iron girders supported by columns above the roadway, a notable example of which is the Liverpool Overhead Electric Railway.