Let us now pause for a moment to witness some of the changes which were going on in regard to the machinery for steamships. In the engines which were installed in the Russian ship shown opposite page 84 we saw how the beam had become the side-lever, and why it had been placed in this position in the steamboat. This had become the customary type for steamships which were still propelled by paddle-wheels, and the perfected development had been due to Boulton and Watt, dating from about 1820. Until about 1860 this type was used most generally, until ocean-going steamers discarded the paddle-wheel for the screw. It is, therefore, essential that before proceeding farther we should get well-acquainted with it, and we shall find that following the lead which had been given them, especially by the famous Robert Napier, marine engineers began to build these types, as well for deep-sea ships as for river-going craft. The illustration here facing, which has been taken from a model in the South Kensington Museum, represents the regular side-lever type, the full-sized engines having been made by a Poplar firm in 1836 for the _Ruby_, which plied between London and Gravesend, a vessel of 170 tons, and the fastest Thames steamer of that time. On referring to our illustration, the side-lever will be immediately recognised in the fore-ground at the bottom. To the left of this are the two cylinders, side by side. The side-lever is seen to be pivoted at its centre, whilst at the reader’s left hand the end of this is joined by a connecting rod. Thus, as the piston-rod is moved upwards or downwards, so the left-hand half of the side-lever will move. At the opposite, right-hand, side of the latter the connecting rod will be observed to be attached to the side-lever, whilst the other end of the connecting rod drives the crank; the latter, in turn, driving the shaft on either end of which will be placed a paddle-wheel. In this engine before us there are two cranks, of which one is seen prominently at the very top of the picture. Each connecting rod is attached to two side-levers, one on either side of the cylinder, by means of a cross-head. Similarly at the piston-rod there is also a cross-head, with a connecting rod on either side, of which one only is visible. Later on a modified form of this type of engine was introduced in order to economise space, for one of the great drawbacks of the side-lever engine was that it took up an enormous amount of room, which could ill be spared from that to be devoted to the carrying of cargo or the accommodation of the passengers. In this modification the cylinders, instead of being placed side by side, or athwartships, were fore and aft, the one behind the other.