MATHEMATICAL AND PHYSICAL SCIENCE. 



[Diss. VI. 



more nearly to the orbit of Mercury than the aphelion 

 does to that of Jupiter ; consequently at times the 

 perturbations due to the former planet may be very 

 great, and though the gravitating mass o the Comet 

 is utterly unknown, yet since the momentary direc- 

 tion of its motion depends solely on the ratio of the 

 attractive force of the Sun and Mercury, its observed 

 course gives the means of estimating that ratio. 1 

 (272.) The theory of a resisting medium was, on the whole, 

 Theory of well received, especially in England, where some of 

 a resisting our g rg j authorities gave it their adhesion. The then 

 altogether recent establishment of the Undulatory Theory of 

 favourably Light, was thought by many to receive a confirmation 

 received, f rom this evidence of something material filling the 

 planetary spaces. In Germany the hypothesis of re- 

 sistance received the complete opposition of Bessel's 

 high authority ; who declared that " a hundred other 

 reasons" might be found for the fact of the accelera- 

 tion, which he admits to be true. Encke, in reply, 

 reduces these 100 possible hypotheses to four, of which 

 we shall mention only one, as seemingly important, 

 namely, the forces exerted with so much intensity 

 within the body of the Comet itself, as indicated by 

 the projection of the tail. But he observes, with great 

 sagacity, that these forces, being apparently usually 

 excited in the line of the radius-vector joining the 

 Comet and the Sun, can hardly be supposed to affect 

 the periodic time. It having also been objected that 

 Halley's Comet shows no trace of acceleration, but, if 

 anything, of the reverse, M. Encke truly says, that 

 its perihelion distance does not lie within the assumed 

 limits of the denser ether. 



(273.) Nevertheless, the theory of a resisting medium in 

 and still space is not perhaps very popular, except in England, 

 questioned. Although M. de Huinboldt appears to favour it, I 

 understand that the German astronomers in general 

 scarcely regard it as in any degree proved. 

 (274.) Yet, if not true, the cardinal fact remains unex- 

 plained. The anomalous phenomena of the Tails of 

 Comets, considered by Herschel to be altogether inex- 

 plicable by the law of gravity, demand the closest 

 scrutiny ; and one can hardly help supposing that 

 the two difficulties may be in close connection. As 

 the Newtonian law is now considered (since the dis- 

 covery of Neptune, and the latest corrections of the 

 Lunar Tables) to be absolutely sufficient to account 

 for everything connected with planetary motion, the 

 Astronomy of Comets will be looked to with increas- 

 ing interest, as likely to reveal some laws of nature 

 not otherwise to be detected. In this respect, Pro- 

 fessor Encke's labours are likely to be more and more 

 important in their results. 



(275.) With reference to this very eminent astronomer, 

 M. Encke we have only to add, that he has for a great many 

 " y ears been at tne nea< ^ f ^ e Observatory at Berlin, 

 and in that capacity has published an Astronomical 

 Ephemeris of first-rate excellence. It is as a phy- 



sical astronomer, however, that he will be principally 

 remembered. Besides his admirable investigations 

 connected with the Comet, he improved the theory of 

 Vesta, and has very lately published a new Method 

 of Computing Perturbations, especially for orbits con- 

 siderably elliptical. Neptune was discovered at his 

 Observatory, by the assistant astronomer, M. Galle. 



Gambart's and Bielals Comet. JEAN GAMBART, (276.) 

 one of the most promising astronomers in France, died Gambart's 



,. , . , , i T v r and Brela'i 



ot consumption at a comparatively early age, 1 believe comet 

 in 1836. He was director of the Observatory at Mar- 

 seilles, which, notwithstanding its very unfavourable 

 position in the midst of the town, has acquired con- 

 siderable celebrity as regards the discovery and obser- 

 vation of Comets. Pons, by whom Encke's Comet was 

 found, both in 1805 and 1818, conducted the Ob- 

 servatory ; but its mounting was as bad as its situa- 

 tion, and Pons used despairingly to describe his tele- 

 scope as rather paralytic than parallactic. To this 

 crippled establishment M. Gambart succeeded, and by 

 his skill in managing his defective instruments, and 

 by his patience in sweeping for Comets, he discovered 

 and subsequently computed the orbits of a number of 

 these bodies between 1822 and the period of his death. 

 Gambart was highly esteemed, both by French and 

 foreign Astronomers. Pons also deserves great credit 

 for his extraordinary diligence in the discovery of 

 Comets, and M. Valz, who still directs the Obser- 

 vatory of Marseilles has cultivated this and other 

 branches of the science with success. 



Gambart's most remarkable discovery was the pe- (277.) 

 riodicity of the first Comet of 1826, having detected Periodicit; 

 that body independently at Marseilles, though it had in 6 2 J eari 

 been observed some days previously in Bohemia, by 

 Biela, an officer in the Austrian service. It is most 

 usually called Biela' s Comet, though it might with 

 equal right be termed Gambart's, who assigned its 

 path and predicted its return. Clausen, about the 

 same time with Gambart, assigned it a period of about 

 7 years ; and it was identified with former appear- 

 ances in 1772 and 1805-6. Its period thus appeared 

 to be 2460 days, or 6f years ; its aphelion is a little 

 exterior to Jupiter's orbit, and its perihelion is not 

 much within the Earth's. This Comet's orbit very 

 nearly intersects in one place the orbit of the Earth, 

 so that had the earth been one month forwarder in 

 its annual course in 1832, a collision would have 

 taken place, or at least the Earth would have been 

 enveloped in a cometary haze ; for it is difficult to 

 imagine a collision with a body whose tenuity is so 

 excessive, that Sir John Herschel perceived through 

 its entire thickness (estimated at 50,000 miles) stars 

 of the most excessive minuteness (16th or 17th mag- 

 nitude) as seen by his 20-feet reflector. It is an in- 

 teresting circumstance, that the first predicted peri- 

 helion passage, in 1832, took place within some hours 

 of the time fixed by MM. Santini and Damoiseau, 



1 On the Masses and Densities of the Planets, see Encke in Astron. Nachrichten, vol. xix., col. 187. 



