364 Proceedings of the British Association for 1850. 



ber of comets, lie had endeavoured to arrange such facts 

 respecting them as observation made known. These were : 

 1. A comet consisting of a nucleus, and one or more gaseous 

 envelopes. 2. The nucleus is solid and material, but ex- 

 ceedingly small. 3. The nucleus is eccentrically situated in 

 the gaseous body. 4. Comets of the longest period have 

 the largest bodies, 5. The more eccentric the orbit, the 

 more eccentric the body of the comet. 6. The comet rotates 

 on its shorter nucleoid axis in the same time that it takes to 

 move round the sun. 7. This axis is not always at right 

 angles to the plane of the orbit. 8. There is also a quicker 

 rotation round the longer axis. 9. A comet shines by re- 

 flected light, and shows a sensible phase. 10. In proportion 

 to the eccentricity of the orbit a comet increases in density, 

 and decreases in size in approaching the perihelion, and 

 vice versa. 11. The longer axis of a comet is straight at 

 the perihelion and aphelion, but between these points is con- 

 cave towards the latter, the curvature being inversely as the 

 eccentricity of the orbit. 12. (Sir John Herschel's.) The 

 component molecules of a comet are only held together by 

 their mutual gravitation, each constituting an almost separate 

 projectile, and describing its own parabola round the sun. 

 Professor Smyth adverted particularly to the 9th and 10th 

 axioms. Arago had proved that comets shine by reflected 

 light, but as to their showing a sensible phase, a contrary 

 opinion had hitherto been held. He shewed, however, by 

 several diagrams, that they had phases, and how this was 

 caused. The atmosphere being eccentrically disposed of by 

 the force of gravitation to the nucleus, the greater part of 

 the light will fall on the denser part of the atmosphere ex- 

 posed directly towards the sun. Behind the atmosphere and 

 the nucleus there will be an axial darkness, while the ex- 

 terior parts of the atmosphere above and below will present 

 the form of a double tail, with the light growing dimmer to- 

 wards its extremities. With regard to the 10th axiom stat- 

 ing a decrease rather than increase in bulk of comets near 

 "the sun, this had been explained by Sir John Herschel, and 

 was verified in a remarkable manner by the great comet of 

 1843, which had a far less perihelion distance than had ever 

 been known before, — only about 60,000 miles. 



