XIII 



ST VENANT'S EXPERIMENTS 



885 



e(|uilateral prism, such as is shewn in section in Fig. 437 A, if the part 

 of the rod represented in the section be twisted by a force acting 

 in the direction of the arrow, then the originally plane section will 

 be warped as indicated in the diagram — where the full contour-lines 

 represent elevation above, and the dotted lines represent depression 

 below, the original level. On the external surface of the prism, 

 then," contour-lines which were originally parallel and horizontal 

 will be found warped into sinuous curves, such that, on each of the 

 three faces, the curve will be convex upwards on one half, and 

 concave upwards on the other half of the face. The ram's horn, 

 and still better that of Ovis Ammon, is comparable to such a prism, 



A Fig. 437. 



save that in section it is not quite equilateral, and that its three 

 faces are not plane. The warping is therefore not precisely identical 

 on the three faces of the horn; but, in the general distribution of 

 the curves, it is in complete accordance with theory*. Similar 

 anticlastic curves ?ire well seen in many, antelopes; but they are 

 conspicuous by their absence in the cylindrical horns of oxen. 



The better to illustrate this phenomenon, the nature of which is 

 indeed obvious enough from a superficial examination of the horn, 

 I made a plaster cast of one of the horny rings in a horn of Ovis 

 Arnmon, so as to get an accurate pattern of its sinuous edge : and 

 then, filling the mould up -with wet clay, I modelled an anticlastic 



* The case of a thin conical shell under torsion is more complicated than either 

 that of the cylinder or of a prismatic rod ; and the more tapering, horns doubtless 

 deserve further study from this point of view. Cf. R. V. Southwell, On the torsion 

 of conical shells, Proc. R.S. (A), clxiii, pp. 337-355, 1937. 



