WHEEL-BEARERS. 229 



vigorously in the midst of them. They describe two great 

 circles, concentrical with the two wheels of the Brachionus, 

 and it is easy to see that their rotations are .the cause of 

 the movement. The motion of the cilia communicates 

 itself to the surrounding water, and produces circular cur- 

 rents, into which every floating atom within a certain dis- 

 tance is drawn, and in which it then continues to whirl 

 round with a rapidity which increases as it approaches the 

 centre of rotation. 



But the Brachionus suddenly lets go its foot-hold, and a 

 surprising change takes place. No more currents are made 

 in the water, but the animal itself glides swiftly away head 

 foremost with an even course, revolving on its axis as it 

 goes. What is the immediate cause of its movement ? 

 The ciliary action which before produced vertical currents. 



In order to explain this, let me suggest to you a homely 

 comparison. Suppose you see a boat on a still lake, and 

 in it a man pulling a pair of oars. He pulls vigorously, 

 but the boat does not move an inch, and you perceive that 

 she is fast moored ; a rope holds her to a post on the 

 bank. But does the man's rowing produce no effect ? 

 yes ; the successive strokes of the oars upon the water 

 have communicated motion to the fluid, and a strong cur- 

 rent is made on each side of the boat, in a direction oppo- 

 site to that in which he strives to row her forward, the 

 force of which is felt to a distance proportionate to the 

 vigour and continuance of his pulling. The reason of this 

 is that the boat is fixed, and all the force of the impact is 

 spent on the water. 



But now another man approaches the post, and unties 

 the rope. Instantly the boat glides ahead, and continues 

 to do so, urged by the repeated strokes of the oars, whose 

 eflect on the water in making currents is now slight and 

 imperceptible. The reason of this is that the water is now 

 a fixed, body (or nearly such), and the force of the impact 

 is mainly spent on the movable boat. 



