Royal Astronomical Society. % c 2i> 



"The following night it proved cloudy, and my attention being 

 distracted toother objects, I have not since pursued the comet j 

 nor should I have thought it worth while to communicate observa- 

 tions confessedly so imperfect as the present, to the Society, but 

 for the high interest attaching to the object, as well as for an im- 

 pression left on my mind, by the excessive feebleness of its light as 

 seen in the equatorial, that it may have escaped the notice of most 

 observers at this early period of its appearance, for want of light, as I 

 am certain I could never have found it by any degree of attention 

 with that instrument, had not its precise place been given by the 20- 

 feet reflector. 



" I know not whether I may be pardoned the mention of a con- 

 jecture in this place, as to the origin of a very striking phaenomenon 

 in the history of comets, which seems to have been satisfactorily 

 established, at least in some instances, by positive observations ; 

 which is, their dilatation of volume as they recede from, and con- 

 centration within a smaller compass as they approach, the sun. 

 This phenomenon has been attempted to be accounted for by a 

 supposed pressure of the aether, whose density is assumed to 

 increase in the sun's vicinity. But, not to mention that the effect 

 would not follow from the cause without supposing the matter of 

 the comet impermeable to the aether (as a sponge is not compressed 

 by lowering it in water unless inclosed in a water-tight case), it 

 appears to me that the phaenomenon is explicable on a much less 

 gratuitous supposition -, viz. that of the extremely feeble attractive 

 force by which the matter of a comet must be held together, owing 

 to the probable extreme minuteness, of its mass. Cohesion can 

 hardly be supposed to exist in a gaseous or nebulous body of such 

 tenuity ; so that the only bond of union between its molecules 

 must be their feeble gravitation to each other, which is hardly more 

 than mere juxtaposition in space. Hence we must regard each 

 molecule as constituting almost a separate, independent projectile, 

 describing its own parabola about the sun. Now, the interval be- 

 tween two or more parabolas described about a common focus, and 

 having their axes coincident, is a minimum at the perihelion, and in- 

 creases as we recede from it, in a ratio easily calculable. The 

 volume, on this view of the subject, ought to increase in the sesqui- 

 plicate ratio of the radius vector. The observations of Encke's comet, 

 cited by M. Arago, indicate, no doubt, a much more rapid law of 

 increase j but, not to mention the difficulty of obtaining any positive 

 measures of a body so ill-defined as a comet, the circumstances under 

 which the observations must necessarily be made have a powerful 

 tendency to exaggerate the effect ; since, in proportion as a comet 

 recedes from the sun, it is continually seen projected upon a darker 

 and darker part of the heavens as it emerges from the twilight ; in 

 consequence of which, exterior strata of the nebulosity become per- 

 ceptible which were incapable of affecting the eye before. Those 

 who are at all conversant with the observation of nebulae, will not 

 foil to have remarked the rapid rate of the obliteration produced by 

 very trifling degrees of illumination, natural or artificial, of the field 



third Series, Vol. 2. No. 9. March 1833. 2 G 



