In the next illustration will be seen a model of her screw engines, whose position in the ship will be found on referring again to the longitudinal section. These were, it will be noticed, no longer of that early type which needed gearing, but worked directly, the cylinders being placed horizontally. The number of cylinders was four, each of which had two piston-rods, and steam was supplied by half a dozen double-ended tubular boilers of the rectangular or “box” type. For the benefit of the non-technical reader we may explain that the object seen in the foreground of the picture, extending from the centre to the right-hand side, is what is commonly called the “link motion gear,” which is employed for reversing the engines when it is required to send the ship astern. This controls the slide valves which allow the steam to enter the cylinders. The principle of the link motion is just this: two eccentrics are placed side by side on the shaft, but opposite to each other. Each of them is connected by a rod to one end of the “link,” which is curved in shape. In this illustration it will be easily recognised at the right-hand side in the front. Now, as the link is moved up or down, so it controls the eccentric. If it is lowered, for instance, then one eccentric only is working the valve, but if the link is raised the other eccentric will control the valve, and so the latter will work in the opposite direction to which it did before. Thus, by using one eccentric, steam enters the cylinder at one end first, while if the other eccentric is employed steam will enter first at the other. Thus it becomes possible to make the engine turn in whichever direction is desired by regulating the end of the cylinder by which the steam shall first enter. The _Great Eastern’s_ propeller had four blades, and an interesting arrangement was adopted so that when the ship was proceeding by means of her paddles, sails, or both, the screw propeller was kept revolving by means of two auxiliary engines in order that the speed of the ship through the water might not be diminished by the drag of the screw. Actual results showed that this ship could do her fifteen knots with screw and paddles, but her average speed was one knot less. Under screw alone she could do nine; under paddle power alone she did seven and a quarter. It will thus be noticed that when using both paddles and screw she ought to have done better, and this failing is explained by asserting that the paddle-wheels and the screw caused a resistance too great for their respective engines.