LIGHT OF COMETS. 



for in several ways. Yalz ascribed it to the pressure of the solar 

 atmosphere acting upon the comet ; that atmosphere being more 

 dense near the sun, compresses the comet and diminishes its 

 dimensions ; and, at a greater distance, being relieved from this 

 coercion, the body swells to its natural bulk. A very ingenious 

 train of reasoning was produced in support of this theory. The 

 density of the solar atmosphere and the elasticity of the comet, 

 being assumed to be such as they might naturally be supposed, 

 the variations of the comet's bulk are deduced by strict reason- 

 ing, and show a surprising coincidence with the observed 

 change in the dimensions. But this hypothesis is tainted by a 

 fatal error. It proceeds upon the supposition that the comet, 

 on the one hand, is formed of an elastic gas or vapour ; and, on 

 the other, that it is impervious to the solar atmosphere through 

 which it moves. To establish the theory, it would be neces- 

 sary to suppose that the elastic fluid composing the comet should 

 be surrounded by a nappe or envelope as elastic as the fluid 

 composing the comet, and yet wholly impenetrable by the solar 

 atmosphere. 



Several ingenious hypotheses * have been proposed and suc- 

 cessively rejected for explaining this phenomenon, but it seems 

 now agreed to ascribe it to the action of the varying temperature 

 to which the vapour which composes the nebulous envelope is 

 exposed. As the comet approaches the sun, this vapour is con- 

 verted by intense heat into a pure, transparent, and therefore 

 invisible elastic fluid. As it recedes from the sun, the tem- 

 perature decreasing, it is partially and gradually condensed, and 

 assumes the form of a semitransparent visible cloud, as steam 

 does escaping from the valve of a steam boiler. It becomes more 

 and more voluminous as the distance from the source of heat, and 

 therefore the extent of condensation, is augmented. 



86. Professor Struve made a series of observations on the comet 

 of Encke, at the period of its reappearance in 1828, and by the 

 aid of the great Dorpat telescope, made the drawings figs. 7 

 and 8. 



Fig. 7 represents the comet as it appeared on the 7th Novem- 

 ber, the diameter a b measuring 18'. The brightest part of 

 the comet extended from k to I, and was consequently eccen- 

 tric to it, the distance of the centre of brightness from the 

 centre of magnitude being k K. Between the 7th and the 30th 

 November, the magnitude of the comet decreased from that 

 represented in fig. 7, to that represented in fig. 8 ; but the 



* For several of these, see Sir J. HerscheFs memoir, " Proceedings of 

 Astronomical Society," vol. vi. p. 104. 



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