PHYSICAL SCIENCE. 



xxxin 



In 1773, Laplace having found that the varia- 

 tion of eccentricity of Jupiter's orbit must cause 

 a corresponding alteration in the motion of the 

 satellites, transferred the same idea to the per- 

 turbations of our moon, and thus discovered the 

 true theory of the secular equation, or rather of 

 that vast cycle in which the lunar revolutions 

 are alternately accelerated and retarded. Dur- 

 ing this discussion he demonstrated that the at- 

 tractive force or gravity must be transmitted 50 

 million times faster than light, which travels at 

 the rate of 200,000 miles in a second. We may 

 therefore consider it as quite instantaneous. This 

 Conclusion is important, because it sets aside all 

 mechanical attempts to explain gravitation by 

 the interposition of an ether, &c., and demon- 

 strates it to be a principle ordained by the wis- 

 dom of the supreme architect. Laplace continu- 

 ing his researches, at last discovered that the 

 secular equation of the moon affecting her mean 

 motion, and that of her perigee and her nodes, in 

 the ratio of 4, 12, and 3, is produced by the slow 

 variation of the solar attraction, occasioned by 

 the changing eccentricity of the earth's orbit, 

 resulting from the influence of the larger planets, 

 though they cannot alter the great axis which 

 determines the mean periodic revolution. 



In 1785 he proved that Jupiter and Saturn can 

 have no secular equations. But remarking that 

 their mean periods are commensurable, and very 

 nearly as 2 to 5, he found their reciprocal accelera- 

 tion and retardation to follow the same ratio. 

 The cycle began in 1560, and comprehends 929 

 years. So that, in 1750, Saturn had his period 

 shortened 48' 44"; while that of Jupiter was 

 lengthened by 19' 28". In 1788 he discovered 

 two curious laws that connect the periods of 

 Jupiter's satellites, and gave a complete theory 

 of their motions, which served as the basis of 

 Delambre's excellent tables. In 1808, Lagrange 

 gave a general solution of the problem of dis- 

 turbing forces, and reduced his equations into 

 a form of the utmost simplicity and elegance. 



4. Lunar and solar parallaxes. The nearest 

 celestial bodies are seen from the surface of the 

 earth in a position somewhat different than if 

 viewed from the centre. This difference, called 

 varallax, is obviously greatest at the horizon, and 

 diminishes constantly as we approach the zenith. 

 To ascertain parallax, therefore, with tolerable 

 precision, observations must be made at distant 

 stations. Lacaille selected the Cape of Good 

 Hope, where he determined the mean parallax of 

 the moon to be 57' 39". The parallax of the sun 

 being very small, is more difficult to determine* 

 Kepler had made it a minute, Halley estimated 

 it at 25" ; but succeeding astronomers had reduced 

 it to 10". Halley proposed a very ingenious 

 method of determining it with accuracy from the 



next transit of Venus, by measuring the accelera- 

 tion of the time of her passage over the disc of 

 the sun, as viewed from remote points on the sur- 

 face of the globe. Aware that his own life would 

 not be prolonged till that event took place, he 

 warmly exhorted his successors to prepare them- 

 selves for observing it on the 5th of June, 1761. 

 Astronomers were accordingly despatched by the 

 maritime powers of Europe to all the stations 

 that were considered as most accessible and 

 eligible : but the result did not answer their ex- 

 pectations ; some of the stations were not well 

 chosen, some of the most expert astronomers had 

 not reached the station assigned them, while 

 others were prevented from observing by the 

 state of the weather. Pingre deduced a parallax 

 of 10^", while Short made it only 8./. The un- 

 certainty was finally removed by the numerous 

 and skilful observations of the transit of the 3d 

 of June, 1769. The several results differed 

 scarcely a quarter of a second, and their concur- 

 rence fixed the parallax at 8"'6. This agrees 

 with the theoretic calculations of Laplace, from 

 the lunar anomalies. Bessel, with immense 

 labour, combined and recomputed the original 

 observations, and detected a small inaccuracy, 

 the correcting of which reduces the parallax to 

 8"'575. Consequently the mean distance of the 

 sun is 95,158,440 English miles. 



5. Discovery of Uranus. Dr Herschell, who 

 had devoted himself to the construction of 

 telescopes, and to an indefatigable observation of 

 the heavens, announced to Dr Maskelyne, that, 

 on the night of the lc:h of March, 1781, he ob- 

 served a shifting star, which from its smallness he 

 judged to be a comet, though it was distinguish- 

 ed neither by a nebulosity nor by a tail. The 

 motion of the star was so slow as to require dis- 

 tant observations to ascertain its path. It was 

 for several months presumed to be a comet ; but 

 the hypothesis of a parabolic orbit led to very 

 discordant results. The president Saron was 

 the first who conceived it to be a planet, having 

 inferred from the observations communicated to 

 him that it described a circle with a radius about 

 twelve times the mean distance of the earth from 

 the sun. Lexell, before the end of the year, had 

 computed the elements of the new planet with 

 considerable accuracy, making the great axis of 

 its orbit nineteen times greater than that of the 

 earth, and the period of its revolution eighty- 

 four years. Bradley, mistaking it for a fixed 

 star, had observed it on the 3d of December, 

 1753, and it was again seen by Mayer on the 

 23d of September, 1756. 



Herschell gave this new planet the name of 

 the Georgium sidus ; but the term Uranus, ap- 

 plied to it by Bode, is almost universally adopt- 

 ed. Herschell discovered the satellites of this 



