NOTES. 



461 



has also another meaning. A line, surface, or solid body, is said to be pro- 

 jected upon a plane, when parallel straight lines are drawn from every point of 

 it to the plane. The figure so traced upon a plane is a projection. The 

 projection of a terrestrial object is therefore its daylight shadow, since the sun's 

 rays are sensibly parallel. 



NOTE 21, p. 6. Space. The boundless region which contains all creation. 

 NOTE 22, pp. 6, 14. Conic sections. Lines formed by any plane cutting a 



Fig. 6. 



cone. A cone is a solid figure, like a sugar-loaf, fig. 5, of which A is the ape x 

 AD the axis, and the plane B E C F the base. The axis may or may not b e 

 perpendicular to the base, and the base may be a circle, or any other curved 

 line. When the axis is perpendicular to the base, the solid is a right cone. 

 If a right cone with a circular base be cut at right angles to the base by a plane 

 passing through the apex, the section will be a triangle. If the cone be cut 

 through both sides by a plane parallel to the base, the section will be a circle. 

 If the cone be cut slanting quite through both sides, the section will be an 

 ellipse, fig. 6. If the cone be cut parallel to one of the sloping sides as A B, 



Fig. 



Fig. 8. 



