600 BIBLIOGRAPHY OF HEESCHEl's WRITINGS. 



Herschel, W.: Synopsis of the Writings of— Continued. 



A. D. Vol. p. 



1812 102 140 Of the result of a Comet's Perihelion Passage. 



The quality of giving out light is immensely increased by an ap- 

 proach to the sun. 

 141 The act of shiniug denotes a decomposition in which at least light 

 (and perhaps many other elastic volatile substances) is given out. 

 I look upon a perihelion passage in some degree as an act of con- 

 solidation. 



141 If this idea be admitted, may we not conclude that the consolidation 



of the comet of 1807, when at perihelion, had already been carried 

 to a much higher degree than that of the present one, by some for- 

 mer apxjroach to our sun, or to other similar celestial bodies, such 

 as we have reason to believe the fixed stars to be? 



142 It is probable that comets may pass around other suns than ours. 

 142 Have we not reason to suppose that the comet of 1807 was compara- 

 tively a much older comet ? 



142 Should the idea of age be rejected, wo may suppose that the comet of 

 1811 since the time of some former perihelion passage may have 

 acquired a quantity of " iinperiheUoned matter" by passing through 

 extensive strata of nebulosity. 



142 I think it not unlikely that the matter of comets is originally nebu- 



lous. It may jiossibly happen that highly condensed nebulre may 

 be drawn towards the nearest celestial body of the nature of our 

 sun, and, after their fu-st perihelion passage round it, proceed 

 towards some other similar body, and finally may come into the 

 regions of our sun, where at last we perceive them as comets. 



143 The brilliant appearance of our comet may be ascribed either to its 



having but lately emerged from a nebulous condition, or to its 

 having carried off' some [foreign] nebulous matter. The first case 

 will lead us to conceive how planetary bodies may begin to have 

 an existence ; the second how they may increase and grow up to 

 maturity. 



Wm. Hekschel. 

 [Dated] Slough, near Windsor, December IG, 1811. 



1812 102 229 Ohservationsof a second Comet, ivith Ee^narks on its Construction. [Comet 

 1811, II.] By William Herschel, LL. D., F. E. S. Eead March 

 12, 1812. 

 229 I call this the second comet; the other of this year the first. 

 The Body of the Comet. 



231 The nucleus was 5". 2744 in diameter. 



232 The real diameter of its nucleus cannot be less than 2,(537 miles. 



232 The chevelure of the Comet. 



233 Tlie Tail of the Comet. Its length on January 20 must have been about 



659,000 miles. 



233 Remarks on the Construction of the Comet. 



234 When the two comets are compared they are found to be extremely 



different. 



234 The light of the second comet is probably reflected from the sun. 



The nucleus of the comet is surrounded by an elastic atmosphere, 

 which is transparent. 



235 The little extent and extreme faintness of the tail might be ascribed 



to the great perihelion distance of the comet if the comjiarative 

 view of the comets of 1807 and 1811 in my last paper {lid not prove 



