264 EVENINGS AT THE MICEOSCOPE. 



atom witMn a certain distance 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 JBracMonus 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, revolv- 

 ing 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 ? Oh yes ; the successive strokes of 

 his oars upon the water have communicated motion to 

 the fluid, and a strong current is made on each side of the 

 boat, in a direction opposite to that in which he strives 

 to row her forward, the force of which is felt to a dis- 

 tance 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 un- 

 ties the rope. Instantly the boat glides ahead, and 

 continues to do so, urged by the repeated strokes of the 

 oars, whose effect on the water in making currents is 

 now slight and imperceptible. The reason of this is, 

 that the water is now the fixed body (or nearly such), 

 and the force of the impact is mainly spent on the 

 movable boat. 



The Brachionus is the boat, its cilia are the oars, 



