394 



NOTES. 



or mp, is the space the iwoon would fall through in the first second of 

 her descent to the earth, were she not retained in her orbit by her cen- 

 trifugal force. 



NOTE 19, p. 5. Action and reaction. When motion is communicated 

 by collision or pressure, the action of the body which strikes is returned 

 with equal force by the body which receives the blow. The pressure of 

 a hand on a table is resisted with an equal and contrary force. This 

 necessarily follows from the impenetrability of matter, a property by which 

 no two particles of matter can occupy the same identical portion of space 

 at the same time. When motion is communicated without apparent 

 contact, as in gravitation, attraction, and repulsion, the quantity of motion 

 gained by the one body is exactly equal to that lost by the other, but in a, 

 contrary direction ; a circumstance known by experience only. 



NOTE 20, p. 5. Projected. A body is projected when it is thrown ; u 

 ball fired from a gun is projected ; it is therefore called a projectile. But 

 the word has also another meaning. A line, surface, or solid body, is 

 said to be projected upon a plane, when parallel straight lines are drawn 

 from every point of it to the plane. The figure so traced upon the plane 

 is a projection. The projection of a terrestrial object is therefore its day- 

 light shadow, since the sun's rays are sensibly parallel. 



NOTE 21 , p. 5. Space. The boundless region which contains all creation. 



NOTE 22, pp. 5, 12. Conic Sections. Lines formed by any plane cut- 



Fig. 6. 



Fig.l. 



Fig. 8. 





