NOTES. 



479 



NOTE 116, p. 50. Every particle will describe a circle, $c. If NS, fig. 3, be 

 the axis about which the body revolves, then particles at B, Q, &c., will whirl 

 in the circles BGAa, QEqd, whose centres are in the axis NS, and their 

 planes parallel to one another. They are, in fact, parallels of latitude, QEqd 

 being the equator. 



NOTE 117, p. 50. The force of gravity, <f-c. Gravity at the equator acts in 

 the direction Q C, fig. 30. Whereas the direction of the centrifugal force is 



Fig. 30. 



exactly contrary, being in the direction CQ; hence the difference of the two 

 is the force called gravitation, which makes bodies fall to the surface of the 

 earth. At any point, m, not at the equator, the direction of gravity is m b, 

 perpendicular to the surface, but the centrifugal force acts perpendicularly to 

 NS, the axis of rotation. Now the effect of the centrifugal force is the same 

 as if it were two forces, one of which, acting in the direction bm, diminishes 

 the force of gravity, and another which, acting in the direction mt, tangent to 

 the surface at m, urges the particles towards Q, and tends to swell out the earth 

 at the equator. 



NOTE 118, p. 51. 

 the same density. 



Homogeneous mass. A quantity of matter, everywhere of 



NOTE 119, p. 51. Ellipsoid of revolution. A solid formed by the revolution 

 of an ellipse about its axis. If the ellipse revolve about its minor axis Q D, 

 fig. 6, the ellipsoid will be oblate, or flattened at the poles like an orange. If 

 the revolution be about the greater axis AP, the ellipsoid will be prolate, like 

 an egg. 



NOTE 120, p. 51. Concentric elliptical strata. Strata, or layers, having an 

 elliptical form and the same centre. 



NOTK 121, p. 52. On the whole, $c. The line NQSg, fig. 1, represents the 

 ellipse in question, its major axis being Qq, its minor axis NS. 



NOTB 122, p. 52. Increase in the length of the radii, $c. The radii gra- 



