ASTRONOMY AND METEOROLOGY. 321 



trace their effect upon the balance of our system, we entirely fail. It 

 appears from the experiments of Secchi upon the late comet with the 

 polariscope, that the light of the nucleus and the brighter part of the 

 " aigrettes " or jets, was never polarized, excepting very feebly on the 

 last day of possible observation, while that of the surrounding nebulos- 

 ity was strongly so, and the extremities of the jets exhibited an inter- 

 mediate condition. But even this very interesting result, confirmed 

 to a great extent by that of the great comet of 1861, in which the same 

 observer found the polarization of the light of the tail and the rays 

 near the nucleus very powerful, but no trace of it in the nucleus till 

 July 3 and the following days, when it was strongly indicated, gives 

 no distinct intimation as to the nature of the light. Secchi thinks that 

 the condition of the nucleus may have resembled that of terrestrial 

 clouds, which have no polarizing effect, while the coma and tail may 

 have been in a gaseous state ; but it is obvious on how slight a founda- 

 tion such a conjecture as to a totally unknown material is raised ; and 

 though this great and justly celebrated observer says that if we con- 

 tinue to make as much progress with future comets as we have done 

 with the last three conspicuous ones, our knowledge of their nature 

 w r ill be speedily complete, one cannot help thinking that "the wish was 

 father to the thought." In fact, there would be no greater difficulty 

 in maintaining the converse proposition, that we shall never obtain 

 any satisfactory idea as to their composition, unless we should get actu- 

 ally involved in their extent. There was. indeed, reason to suppose 

 that the edge of the tail swept over us in 1861, but from the imper- 

 ceptible passage of those very attenuated streams it would be hazar- 

 dous to infer the results of immediate contact with a glowing nucleus. 

 Up to the present time, the simple question cannot be considered to 

 be decided, whether comets shine by native or reflected light, or by a 

 combination of both ; even forms which have been considered to be of 

 constant occurrence, such as the curvature of the tail backwards from 

 the direction of its motion, and the superior density and distinctness of 

 its convex edge, have been shown by the recent comet to be less uni- 

 versal than had been supposed. Few, indeed, are the points which we 

 can consider adequately established. From the fixity, or limited 

 movement of the jets issuing from it, it may be considered certain that 

 the nucleus does not revolve ; a rotatory motion of the whole tail 

 round its axis has indeed been suspected, from the reciprocating form 

 of its branches in 1769, 1811, and 1825, and thought not impossible by 

 Secchi in 1860 ; but even should this be admitted in some instances, in 

 others it would be quite incompatible with observation. 



It is evident that the whole mass is vehemently acted upon by some 

 influence emanating from the sun, the continuation and accumulation 

 of which, after the perihelion passage, seems to point to a calorific 

 rather than a more instantaneous electric or magnetic action ; and it 

 would appear as though the nucleus even if translucent to light, and 

 reflecting it alike, as Sir J. Herschel suggests, from its interior part and 

 from its surface, and therefore having no shadow or dark side were 

 not equally permea'ble by that solar influence, whatever it may be ; 

 since the formation gf fans and envelopes takes place only on the side 

 exposed to the sun, while the opposite side is comparatively undisturbed, 

 and as it were in a sheltered state. An intermittent or discontinuous 



