Royal Astronomical Society. 227 



"If, however, after all, we should prefer to call in an aethereal me- 

 dium surrounding the sun, as the sole or partial cause of the remark- 

 able phenomenon in question, it will not be necessary to have re- 

 course to the idea of condensation arising from its mechanical pres- 

 sure, which, as we have seen, is repugnant to what we know of the 

 mode of propagation of pressure in fluids. A less repugnant explana- 

 tion offers itself in the presumable habitudes of the aethereal fluid with 

 respect to heat. Fourier has rendered it not improbable, that the 

 region in which the earth circulates has a temperature of its own, 

 greatly superior to what may be presumed to be the absolute zero, 

 and even to some artificial degrees of cold; and in my Essay on the 

 Study of Natural Philosophy*, (p. 157,) I have shown, I think 

 satisfactorily, that if this be the case, such temperature cannot be 

 due simply to the radiation of the stars, but must arise from some 

 other cause, such as the contact of an aether possessing itself a 

 determinate temperature, and tending, like all known fluids, to com- 

 municate this temperature to bodies immersed in it. Now, if we sup- 

 pose the temperature of the aether to increase as we approach the sun, 

 — which seems a natural, and, indeed, a necessary consequence of 

 regarding it as endued with the ordinary relations of fluids to heat, — 

 we are furnished with an obvious explanation of the phenomenon in 

 question. A body of such extreme tenuity as a comet may be pre- 

 sumed to take very readily the temperature of the aether in which it 

 is plunged j and the vicissitudes of warmth and cold thus experienced 

 may alternately convert into transparent vapour, and re-precipitate 

 the nebulous substance, just as we see an increase of atmospheric 

 temperature dissipate a fog, not by abstracting or annihilating its 

 aqueous particles, but by causing them to assume the elastic and 

 transparent state, which they lose, and again appear in fog when the 

 temperature sinks." 



In a subsequent letter to Mr. Baily, Sir J. Herschel gives an 

 account of a further observation obtained on the morning of the 4th 

 November. The following is an extract : — 



"After watching in vain for an opportunity of renewing my obser- 

 vations of the comet on the mornings of the 2nd and 3rd instant (Nov.), 

 the cloudy state of the sky on both mornings from 2 a.m. till day-break 

 precluding all possibility of observing it, I succeeded, on the morning 

 of the 4th, in getting a very satisfactory observation. Having set the 

 20-feet reflector on the spot indicated by Mr. Henderson's Epheme- 

 ris, I had hardly made two or three azimuthal sweeps of the tube to 

 and fro, when it made its appearance in the field of view as a large 

 and very bright nebula. Judging of its light as compared with that 

 of two nebulae of the 2nd class which occurred in the first azimuthal 

 sweep, I should say that its impression on the eye was at least 1 00 

 times that of one of the nebula?. I judged its diameter to be full 4'. 

 The condensation towards the middle was considerable, and the cen- 

 tre itself was occupied by a bright point about equal to a star of the 13th 



* See also my paper on the astronomical causes of geological phaeno- 

 mena. 



2 G2 



