Theoretically, then, with our weight of fully oxidized lead we should be able to travel for 82 hours; with the same weight of iron for 430 hours, or 18 days and nights continually, at the rate of 8 miles per hour, with one change. Of course, these feats are quite impossible. We might as well dream of getting 5 horse power out of a steam engine for one pound of coal per hour. While the chemist is busy with his researches for substances and combinations which will yield great power with small quantities of material, the engineer assiduously endeavors to reconvert the chemical or electrical energy into mechanical work suitable to the various needs. To get the maximum amount of work with a minimum amount of weight, and least dimensions combined with the necessary strength is the province of the mechanical engineer--it is a grand and interesting study; it involves many factors; it is not, as in the steam engine and hydraulic machine, a matter of pressures, tension and compression, centrifugal and static forces, but it comprises a still larger number of factors, all bearing a definite relation to each other. With dynamo machines the aim has been to obtain as nearly as possible as much electrical energy out of the machine as has been put in by the prime mover, irrespective of the quantity of material employed in its construction. Dr. J. Hopkinson has not only improved upon the Edison dynamo, and obtained 94 per cent. of the power applied in the form of electrical energy, but he got 50 horse power out of the same quantity of iron and copper where Edison could only get 20 horsepower--and, though the efficiency of this generator is perfect, it could not be called an efficient motor, suitable for locomotion by land or water, because it is still too heavy. An efficient motor for locomotion purposes must not only give out in mechanical work as nearly as possible as much as the electrical energy put in, but it must be of small weight, because it has to propel itself along with the vehicle, and every pound weight of the motor represents so many foot pounds of energy used in its own propulsion; thus, if a motor weighed 660 pounds, and were traveling at the rate of 50 feet per minute, against gravitation, it would expend 33,000 foot pounds per minute in moving itself, and although this machine may give 2 horse power, with an efficiency of 90 per cent. it would, in the case of a boat or a tram-car, be termed a wasteful machine. Here we have an all-important factor which can be neglected, to a certain extent, in the dynamo as a generator, although from an economical point of view excessive weight in the dynamo must also be carefully avoided.