The proper test for an electro-motor, therefore, is not merely its efficiency, or the quotient of the mechanical power given out, divided by the electrical energy put in, but also the number of feet it could raise its own weight in a given space of time, with a given current, or, in other words, the number of foot pounds of work each pound weight of the motor would give out. The Siemens D2 machine, as used in the launch shown in the diagram on the wall, is one of the lightest and best motors, it gives 7 horse power on the shaft, with an expenditure of 9 electrical horsepower, and it weighs 658 lb.; its efficiency, therefore, 7/5 or nearly 78 per cent.; but its "coefficient" as an engine of locomotion is 351--that is to say, each pound weight of the motor will yield 351 foot pounds on the shaft. We could get even more than 7 horse power out of this machine, by either running it at an excessive speed, or by using excessive currents; in both cases, however, we should shorten the life of the apparatus. An electro-motor consists, generally, of two or more electro-magnets so arranged that they continually attract each other, and thereby convey power. As already stated, there are numerous factors, all bearing a certain relationship to each other, and particular rules which hold good in one type of machine will not always answer in another, but the general laws of electricity and magnetism must be observed in all cases. With a given energy expressed in watts, we can arrange a quantity of wire and iron to produce a certain quantity of work; the smaller the quantity of material employed, and the larger the return for the energy put in, the greater is the total efficiency of the machine.