Just below it is a Cydippe of its ordinary size, showing the paddle-bands, one of which is drawn at the side much magnified, so as to show the arrangement of the little paddles. As to the tentacles which trail from the body, we shall treat of them when we come to our next division of the subject of the work.
Lastly, there is a representation of the self-feathering feet of the Duck, the left foot expanded in striking the water, and the right closed so as to offer no resistance when drawn forward for another stroke. The swan’s foot shows this action even more beautifully than does that of the duck.
We now come to another mode of propulsion, namely, that which is not due to direct pressure of a more or less flat body against the water, but to the indirect principle of the screw, wedge, or inclined plane.
Space being valuable, I will only take two instances, namely, the well-known mode of propelling a boat by a single oar working in a groove or rowlock in the middle of the stern, and the ordinary screw of modern steamers.
Most of my readers must have seen a sailor in the act of “sculling” a boat. A tolerably deep notch is sunk in the centre of the stern, and the oar is laid in it, as shown in the central illustration, on the right-hand side. The sailor then takes the handle of the oar, and works it regularly backwards and forwards, without taking the blade out of the water. The boat at once begins to move forward, and, when the oar is urged by a strong and experienced man, can be propelled with wonderful speed. The well-known “Tanka” boat-girls of China never think of using two oars, a single oar in the stern being all-sufficient for the rapid and intricate evolutions required in their business.
The mechanical process which is here employed is nothing more than that of the inclined plane, or rather, the wedge, the oar-blade forming the wedge, and the force being directed against the stern of the boat, and so driving it through the water.
The Rudder affords another example of a similar force, although it is used more for directing than propelling a vessel. Still, just as the scull is used not only for propelling, but for steering the boat, the rudder, when moved steadily backwards and forwards, can be used for propulsion as well as steerage. In the absence of oars, this property is most useful, as I can practically testify.
So different in appearance are the screw and the inclined plane, that very few people would realise the fact that the screw is nothing but an inclined plane wound round a cylinder, or rather, is a circular inclined plane. The ordinary corkscrew is a good example of this principle, the cylinder being but an imaginary one.
Now, if the screw be turned round, it is evident that force is applied just on the principle of the wedge, and this principle is well shown in the various screw-presses, of which the common linen-press is a familiar example, as was the original printing-press, which still survives as a toy for children.