Illustration: SCREW STEAM-VESSEL. Illustration: FIG. 1. SCREW STEAM-VESSEL, SHOWING THE FAN. Illustration: FIG. 2. (‡ PROPELLER.) These are instruments placed at the back part of steam-vessels for the purpose of propelling them through the water. Fig. 1 will show the position they occupy, and fig. 2 the shape of the propeller. When first used, they had one or two entire turns round the axis, but are now made with two blades, each forming about one-sixth part only of one turn, and this is found to give more power with less friction. The propeller is turned rapidly round in the water, from which it meets with resistance in a direction perpendicular to the surface of its blades, but as this is oblique to the direction of rotation the force is exerted in two directions, one directly opposes this rotation, and is overcome by the power of the steam-engine, the other is in a direction towards the ship, overcoming the inertia of the vessel and the friction and resistance of the water, so that the ship is moved along, and the propeller winds its way through the water in a spiral direction as an ordinary screw does through the hollow screw made to fit it, the vessel travelling at a speed proportionate to the screw’s revolutions. ANCHORS. Illustration: FIG. 1. (‡ ANCHOR WITH WOOD STOCK.) Illustration: FIG. 2. (‡ ANCHOR WITH IRON STOCK.) Illustration: FIG. 3. (‡ SPACE-SAVING ANCHOR.) Illustration: FIG. 4. (‡ ANCHOR AT REST.) Illustration: FIG. 5. (‡ “WEIGHING” ANCHOR.)