NOTES. 



449 



NOTE 113, p. 40. Synodic revolution of the moon. The time between 

 two consecutive new or full moons. 



NOTE 114, p. 40. Horizontal refraction. The light, in coming from a 

 celestial object, is bent into a curve as soon as it enters our atmosphere ; 

 and that bending is greatest when the object is in the horizon. 



NOTE 115, p. 40. Solar eclipse. Let S, fig. 28, be the sun, m the 

 moon, and E the earth. Then a E 6 is the moon's shadow, which some- 



Fig. 28. 



times eclipses a small portion of the earth's surface at e, and sometimes 

 falls short of it. To a person at e, in the centre of the shadow, the eclipse 

 may be total or annular ; to a person not in the centre of the shadow a 

 part of the sun will be eclipsed ; and to one at the edge of the shadow 

 there will be no eclipse at all. The spaces P6E, P' a E, are the 

 penumbra. 



NOTE 116, p. 43. From the extremities, $c. If the length of the line 

 a b, fig. 29, be measured, in feet or fathoms, the angles p- ng 

 S 6 a, S a b, can be measured, and then the angle a S b is 

 known, whence the length of the line S C may be com- 

 puted, a S b is the parallax of the object S ; and it is 

 clear that, the greater the distance of S, the less the base 

 a 6 will appear, because the angle a S' 6 is less than 

 a$b. 



NOTE 117, p. 44. Every particle will describe a circle, 

 #c. If N S, fig. 3, be the axis about which the body 

 revolves, then particles at B, Q, &c., will whirl in the 

 circles B G A a, Q E q d, whose centres are in the axis N S, 

 and their planes parallel to one another. They are, in 

 fact, parallels of latitude, Q E q d being the equator.. 



NOTE 118, p. 44. The force of gravity, fyc. Gravity 

 at the equator acts in the direction Q C, fig. 30. Whereas 

 the direction of the centrifugal force is exactly contrary, 

 being in the direction C Q ; 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 a c h 

 to the surface, but the centrifugal force acts perpendi- 

 cularly to N S, the axis of rotation. Now the effect of the centrifugal force 



