MECHANICS AND MECHANICAL MODELS 



sides, so that the same amount of energy would be expended as before 

 in imparting to the moving fluid the same average velocity. In the case 

 of living cells, however, a physiological response to gravity might be made, 

 and more or less energy be liberated on the side where the stream ascends 

 than on that where it descends (Sect. 9). 



The mechanical model suffices to show that, given materials of definite 

 structure, motion may be set up in a fluid by interaction with a non- 

 moving layer which it touches and which by friction reduces the velocity 

 of the layers immediately touching it to nil. When the average internal 

 velocity is constant, the total work done by the expanding steam must 

 just counterbalance the total work done against friction by the moving 

 layers. These exert a shearing strain upon 

 the outer rigid wall tending to drag it round 

 with them, and under theoretical conditions 

 this forwardly directed force will be just 

 counterbalanced by the backward reaction of 

 the expanding steam. Hence when freely 

 suspended, the model as a whole should re- 

 main at rest with regard to surrounding 

 objects, and the same should be the case 

 when the velocity is decreasing, for the pro- 

 pelling force, and hence the equal and opposite 

 backward reaction, can never be less than the 

 force due to friction. When, however, the 

 velocity is increasing, the propelling force and 

 its backward reaction are greater than the 

 force due to internal friction, hence the model 

 when freely suspended will spin round as a 

 whole in the opposite direction to that of the 



internal current. As a matter of fact this is always the case, partly 

 because the average internal velocity is never constant, and partly because 

 there is a certain amount of lateral friction and displacement. 



If short, living cells of Char a and Nitella, in which the direction of 

 streaming is nearly parallel to the long axis of the cell and on opposite 

 sides of it, are suspended in still, moist air by single fibres of unspun silk, 

 they soon come to rest, but the fibre, when viewed through a horizontal 

 microscope, exhibits no distinct torsion to one side or the other. A 

 mechanical stimulus sufficient to cause a temporary cessation of streaming, 

 lasting until the swinging vibrations have ceased, can readily be applied 

 to cells suspended in this manner. A slight tremor Can often be observed 

 when streaming recommences. Occasionally a slight but distinct swing 

 occurred, followed by one in the opposite direction caused by the torsion 

 in the fibre. On examination the first swing was seen to be in the 



FIG. i. Mechanical model of rotating 

 cell. The arrows show the direction of 



