A locomotive was constructed upon this plan in 1815, and was found to answer extremely well. But at that period the mechanical skill of the country was not equal to forging cranked axles of the soundness and strength necessary to stand the jars incident to locomotive work. Stephenson was accordingly compelled to fall back upon a substitute, which, although less simple and efficient, was within the mechanical capabilities of the workmen of that day, in respect of construction as well as repair. He adopted a chain which rolled over indented wheels placed on the centre of each axle, and was so arranged that the two pairs of wheels were effectually coupled and made to keep pace with each other. The chain, however, after a few years’ use, became stretched; and then the engines were liable to irregularity in their working, especially in changing from working back to working forward again. Eventually the chain was laid aside, and the front and hind wheels were united by rods on the outside, instead of by rods and crank axles inside, as specified in the original patent. This expedient completely answered the purpose required, without involving any expensive or difficult workmanship. Thus, in 1815, by dint of patient and persevering labour,—by careful observation of the works of others, and never neglecting to avail himself of their suggestions,—Stephenson succeeded in manufacturing an engine which included the following important improvements on all previous attempts in the same direction:—viz., simple and direct communication between the cylinders and the wheels rolling upon the rails; joint adhesion of all the wheels, attained by the use of horizontal connecting-rods; and finally, a beautiful method of exciting the combustion of the fuel by employing the waste steam, which had formerly been allowed to escape uselessly into the air. Although many improvements in detail were afterwards introduced in the locomotive by George Stephenson himself, as well as by his equally distinguished son, it is perhaps not too much to say that this engine, as a mechanical contrivance, contained the germ of all that has since been effected. It may in fact be regarded as the type of the present locomotive engine. CHAPTER VI. INVENTION OF THE “GEORDY” SAFETY-LAMP.