SOLAR SYSTEM. 67 



the portion of that vapour, which from its proximity 

 to the sun hud become transparent, again by a 

 reduction of temperature made visible, and during 

 the comet's further recess from the sun is the vapour, 

 from a still greater reduction of temperature, con- 

 densed into a liquid, and thus the comet without a 

 tail and without a nebulous atmosphere presents 

 itself as a star of inferior magnitude a bright con- 

 centrated point ? 



Sir John Herschel, in his observations on Halley's 

 comet, which made its appearance in 1835, states, 

 that on the evening of the 28th of October, before 

 the termination of the twilight, he obtained an ex- 

 cellent view of Halley's comet, eighteen days pre- 

 vious to its perihelion passage ; its appearance was 

 about that of a star of the third magnitude, which, 

 as the darkness increased, appeared somewhat hazy. 

 In a night glass the tail 3 in length was con- 

 spicuous. After the perihelion passage had taken 

 place, Sir John Herschel observed that the cornet 

 was actually increasing in dimensions and with 

 such rapidity that it might almost be said to be 

 seen to grow. (The increase in the dimensions of 

 comets in their recess from the sun was pointed out 

 by M. Valz.) In the comet's further recess from the 

 sun the continued dilatation of the comet was ob- 



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