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THOUGHTS ON THE INFLUENCE OP ETTTER 



Another phenomenon of comets, which has exercised the ingenuity of astronomers, and 

 one for which an explanation has been sought in ethereal influence, is the fact that, as 

 they approach the sun, their bulk becomes diminished, and increases after passing the 

 perihelion. 



If comets were elastic bodies, expansible by heat, and not exposed to any varying pres- 

 sure, these phenomena are the reverse of what ci priori reasoning should lead us to expect. 



The explanation referred to is the result of the labors of M. Valz, whose theory is thus 

 noticed by M. Arago. 



" To explain these changes of volume, M. Valz supposes that the ethereal substance 

 forms round the sun a true atmosphere, that, as in the terrestrial envelope, the lower strata 

 are condensed in proportion to the quantity of the superincumbent medium. He believes 

 that in passing through these regions, the comet must be exposed to a pressure propor- 

 tioned to the density of the strata. There would be no difficulty here, if we could admit 

 that the exterior envelope of the nebulosity was not permeable to the ether supposed to 

 pervade the universe." 



" Everybody knows that a bladder filled with air at the base of a mountain, swells if it 

 be transported towards the summit, and that it will burst if it be taken to a sufficient 

 height. But, where shall we find the pellicle to assimilate the nebulosity to a bladder, 

 which would prevent the ether from penetrating it in every direction, and invading its 

 smallest ramifications ]" 



"This difficulty for the present seems insurmountable, and we deeply regret it, for the 

 ingenious hypothesis of M. Valz has furnished him with the law of the variations in 

 volume of the nebulosity, as well for the comet of short period, as for that of 1618, with 

 an exactitude truly extraordinary."* 



Sir John Herschel makes the following remarks upon the proposed explanation: 



" M. Valz, who was the first to notice this fact (in Enckc's comet), accounts for it by 

 supposing it to undergo a real compression or condensation of volume, arising from the 

 pressure of an ethereal medium, which he conceives to grow more dense in the sun's 

 neighborhood. But such a hypothesis is evidently inadmissible, since it would require us 

 to assume the exterior of the comet to be in the nature of a skin or bag impervious to the 

 compressing medium." 



" The phenomenon is analogous to the increase of dimension above described, as observed 

 in the comet of Halley, when in the act of receding from the sun, and is doubtless referable 

 to a similar cause, viz., the alternate conversion of evaporable matter into the states of 

 visible cloud, and invisible gas, by the alternate action of cold and heat."f 



Is not this a melancholy confession for men of the standing of M. Arago and Sir John 



* Astronomie Populaire, vol. ii, p. 8*1. f Outlines of Astronomy, chapter xi. 



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