LOCOMOTION IN GENERAL. 103 



Let us suppose a man seated in a boat in the midst of a 

 tranquil lake. Under these conditions, his skiff will remain 

 perfectly motionless. If he wishes to advance, he must find 

 what is called a point of resistance. Suppose him to be fur- 

 nished with a pole, he will plunge it towards the bottom of 

 the water till it reaches the ground ; then, making an effort, 

 as if to drive from him this resisting body, he will cause his 

 boat to move in the opposite direction. This progression with 

 the point of resistance on the ground is similar to the ordinary 

 conditions of terrestrial locomotion. 



If the boatman be provided with a boat-hook, he will 

 get his point of resistance under different conditions. Laying 

 hold of the branches of trees, or the projections of the shore, 

 he will drag his pole towards himself, as if to bring near to 

 him the bodies to which it is fastened ; and if these bodies 

 resist his efforts, the boat alone will be displaced and drawn 

 towards them. 



Here are then two opposite modes of progression with 

 bearings on solid bodies ; in one the tendency is to repel, in the 

 other, to draw them nearer : the effect is the same in each case. 



But if the lake be too deep, or if the shores be too distant 

 to furnish the boatman with the solid fulcrum which he had 

 used before, the water itself will serve as a medium of 

 resistance. The boatman, armed with a flattened oar, endea- 

 vours to drive the water towards the stern of his boat ; the 

 water will yield to this impulse, but the boat, impelled in 

 an opposite direction, will go forward. The various kinds of 

 paddles for steam-boats, the screw, in fact, all nautical pro- 

 pellers, present this feature in common, of driving the water 

 backward, in order to produce in the boat an impulse in the 

 contrary direction, and to cause it to advance. 



Instead of an oar acting on the water, we may suppose the 

 boatman provided with a much larger paddle with which he 

 might drive back the air at the stern ; he will propel his 

 boat on the surface of the lake. He might make progress 

 also by turning a large screw like the sails of a wind- 

 mill, or by agitating at the stern some large fan which would 

 drive the air in the direction opposed to that in which he 

 desired to force his boat. 



