4.76 NOTES. 



passing through the Observatory at Greenwich is assumed by the British as a 

 fixed origin, from whence terrestrial longitudes are measured. And as each 

 point on the surface of the earth passes through 360, or a. complete circle, in 

 twenty-four hours, at the rate of 15 degrees in an hour, time becomes a repre- 

 sentative of angular motion. Hence, if the eclipse of a satellite happens at any 

 place at eight o'clock in the evening, and the Nautical Almanack shows that 

 the same phenomenon will take place at Greenwich at nine, the place of ob- 

 servation will be in the 15 of west longitude. 



NOTE 96, p. 35. Conjunction. Let S be the sun, fig. 24, E the earth, and 

 J O J' C' the orbit of Jupiter. Then the eclipses which happen when Jupiter 

 is in O are seen 16 26 s sooner than those which take place when the planet is 

 in C. Jupiter is in conjunction when at C, and in opposition when in O. 



NOTE 97, p. 36. In the diagonal, 8fc. Were the line Fig. 26. 



A S, fig. 26, 100,000 times longer than A B, Jupiter's 

 true place would be in the direction A S', the diagonal 

 of the figure A B S' S, which is, of course, out of pro- 

 portion. 



NOTE 98, p. 36. Aberration of light. The celestial 

 bodies are so distant that the rays of light coming from 

 them may be reckoned parallel. Therefore, let S A, 

 S' B, fig. 26, be two rays of light coming from the sun, 

 or a planet, to the earth moving in its orbit in the di- 

 rection A B. If a telescope be held in the direction 

 A S, the ray S A, instead of going down the tube, will 

 impinge on its side, and be lost in consequence of the 



telescope being carried with the earth in the direction 



A B. But, if the tube be held in the position A E, so B A 



that A B is to A S as the velocity of the earth to the 



velocity of light, the ray will pass through S' E A. The star appears to be in 

 the direction A S', when it really is in the direction A S ; hence the angle 

 S A S' is the angle of aberration. 



NOTE 99, p. 36. Density proportional to elasticity. The more a fluid, such as 

 atmospheric air, is reduced in dimensions by pressure, the more it resists the 

 pressure. 



NOTE 100, p. 37. Oscillations of pendulum retarded. If a clock be carried 

 from the pole to the equator, its rate will be gradually diminished, that is, it 

 will go slower and slower: because the centrifugal force, which increases from 

 the pole to the equator, diminishes the force of gravity. 



NOTE 101, p. 40. Disturbing action. The disturbing force acts here in the 

 very same manner as in note 63; only that the disturbing body d, fig. 14, is the 

 sun, S the earth, and p the moon. 



NOTE 102, pp. 40, 42, 97- Perigee. A Greek word, signifying round the 

 earth. The perigee of the lunar orbit is the point P, fig. 6, where the moon 

 is nearest to the earth. It corresponds to the perihelion of a planet. Some- 

 times the word is used to denote the point where the sun is nearest to the 

 earth. 



NOTE 103, p. 40. Evectioii. The evection is produced by the action of the 



