2l8 



SULPHUR BACTERIA 



gives rise to liead resistance and frictional forces, p, the magni- 

 tude of which are dependent upon the speed of the body 

 through the hquid. These tend to resist the motion and act 

 in the opposite direction. 



Then QB = M/ + kv, 



where M -- mass of body, 



/= acceleration, 

 /c = a constant, 

 V =^ velocity. 



The above equation shows that any increase in the velocity 

 will produce a corresponding increase in the resistance. 

 Movement will be uniform after a balance has been attained 

 between the applied force and the opposing frictional force. 



OB- W 



where V is the ultimate velocity. 



Let us now consider the condi- 

 tions determining the rotational 

 equilibrium (Fig. 63). 



The couple F X MN tends to 

 turn the body of the organism in the 

 direction opposite to that in which 

 the helix is turning. The body will 

 set itself to counteract this force, 

 and in consequence a redistribution 

 of forces from the water will follow 

 until the couple is balanced. This 

 presumably cannot be accomplished in a straight cylindrical 

 body with very soft membranous outer layers without altering 

 its shape, and so if tiic spirillum is at rest it will at once assume 

 the spiral form on the resumption of movement. The forces 

 which here come into play may be illustrated by the following 

 example. If the elbow is placed on a table while a long stick is 

 held between the fingers and set into a whipping action, it will 

 be found necessary to counteract the resulting torque reaction 

 by offsetting the elbow to some extent in relation to the 

 shoulder. Any increase in the force of the whipping, or in the 

 addition of more weight to the stick will result in a corre- 



