26 



erted in the direction of tliat line. For example, if 

 we suppose a straight tube of smaller diameter than 

 the depth of the concavity of the C, to be held oyer 

 the upper end of the rod, while the lower end is im- 

 movably fixed, the spring will discharge itself in 

 the line of the tube. 



§ 20. Let us next apply the counter-pressures at 

 diagonally opposite points. The pressure from 

 either point forms, as before, a concave beneath 

 the point, but this concave no longer forms one 

 with that under the other point. On the contrary, 

 the concaves being now on opposite sides are each 

 met by the pressure of an intermediate point, and 

 form an S, or two C C with their hollows in oppo- 

 site directions. 



The straight line joining the two points of diag- 

 onal pressure, must cross the rod in a part of its 

 length determined by the nature of the rod, and 

 the ratio of the pressures to each other. In this 

 crossing all counter -tivisting forces from the points 

 of pressure will meet, and here more especially op- 

 pose each other. 



We shall designate by the expression " Cross- 

 linej" this portion of the line joining the two diag- 

 onal points. This "cross-line" plays a very im- 

 portant part in the following theory of locomotion. 



