Engler — Foci of Sections, Cone of Revolution. 55 
In Fig. 5, with the vertex, S, as a center and any radius, 
describe an arc cutting the elements of the cone (SD, and 
SD,) ino, and 0, respectively. Draw 0, k, 0, k, perpendicular 
to the axis, and 0, g,, and 0, g, parallel to the axis, meeting 
P 
FIGURE 5. 
the tangent at p in g, and g, respectively. Join Sq, and Sq,. 
If the radius taken is considered unity, 
o, k=p 9,=sina=tan=— pq, 
= tan 6, 
0, k =p q,=sin a=tan < p Sq, 
tan 0, 
Therefore, Sq, and Sq, are elements of the focal cones for or the ae 
section Z,Z,, and F, and F, are the foci of that section 
Issued April 1s, 1892. 
