u 



Lord Kelvin. 

 Fio. 0. 



[Jan. 18, 



two of the four corners of each being projected on one point in the 

 diagram. Fig. 9 shows, on the same scale of magnitude with corre- 

 sponding distinction between heavy and light lines, the orthogonal 

 projection on a plane parallel to a pair of square faces. 



16. If the rule of 15 with reference to the division of each arm 

 of a skeleton tetrahedron into four equal parts by points in which it 

 is cut by other lines of skeletons is fulfilled with all details of 14 

 and 15 applied to any oblique parallelepiped, we find a tetrakaideka- 

 hedron which we may call orthoid, because it is an orthic tetrakai- 

 dekahedron, altered by homogeneous strain. Professor Crurn Brown 

 has kindly made for me the beautiful model of an orthoid al tetra- 

 kaidekahedron thus defined which is placed before the Royal Society 

 as an illustration of the present communication. 



Fig. 10 is a stereoscopic picture of an orthic tetrakaidekahedron, 

 made by soldering together thirty-six pieces of wire, each 4 in. long, 

 with three ends of wire at each of twenty-four corners. 



17. I cannot in the present communication enter upon the most 

 general possible plane-faced partitional tetrakaidekahedron or show 

 its relation to orthic and orthoidal tetrakaidekahedrons. I may 



