2 7 2 



SECTION MECHANICS. 



I may briefly point out the nature of one of these (Fig i.) The one 



element is an equilateral curve-triangle, 

 which possesses the property that any pair 

 of parallel tangents to it are at the same 

 distance apart. The second element is 

 a square, the side-length of which is equal 

 to this distance. The two elements al- 

 ways touch each other in four points, the 

 normals to these points intersect in one 

 point (as O ), and at any instant motion 

 can take place about this point only. The 



C 



K 



Figure i. 



two bodies form a pair of elements in exactly the same sense as before, 

 but with one important difference. In the lower pairs the paths of all 

 points in the moving element are similar. Every point in the moving 

 element of the twisting pair, for instance, moves in a helix of the same 

 pitch, all points in that of the turning pair in concentric circles, &c. 

 Here, however, the paths of all the points are different. Some of 



Figure 



