SPACE 



27 



thus superimposed show the hyperboloid on the out- 

 side and its asymptotic cone inside. 



Conoids. — If the white thread, instead of revolv- 

 ing round the axis, 

 as in the experiment 

 which we have just 

 been reading, has im- 

 parted to it at one 

 extremity a rotatory 

 motion, while the other 

 extremity moves in a 

 straight line, one ob- 

 tains, according to the 

 relationship of the two 



movements, different 

 sorts of conoids, of 

 which Fig. 18 is. one 



Fig. 17. — Hyperboloid by revolution with its 

 asymptotic cone. 



example. 



Movements of a Curve in Space. Photography of the 

 Figures which it forms: Spheres, Ellipsoids, etc. — Among 



F.g. 18.— Conoid engendered by the move- 

 nient of a white thread. 



Fig. 19. — Sphere engendered by 

 the rotation of a semi-annular 

 white thread. 



the forms arising from the movements of a curve, the 

 most easy to produce is a sphere ; it is obtained when 



