18 



NATURE S TEACHINGS. 



" 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 



SCREW OF STEAMER. 



TAIL OF FISH. 



ACTION OF RUDDER. 



TAIL OF SEAL. 



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. 



