xlii REPORT — 1853. 



found since the last meeting of the Association. Of these nine planets, our 

 countryman, Mr. Hind, has discovered four. The number now known, ex- 

 clusive of the large planets, but including the four old asteroids, amounts to 

 twenty-six, nor have we any reason to suppose that we have yet approximated 

 to the whole number of these minor planetary bodies. All those which have 

 been recently recognized appear like stars of magnitudes not lower than the 

 eighth or ninth, and are consequently invisible to the naked eye. The 

 search for them has now assumed, to a considerable extent, a more systematic 

 form, by a previous mapping of the stars up to a certain magnitude, and 

 contained within a belt of a few degrees in breadth on either side of the 

 ecliptic. Any small planet will in the first instance be inserted in the map 

 as a small star, but will on the re-examination of the same area some time 

 afterwards, be recognized in its true character from the fact of its having 

 moved from the place in which it was first observed. This mapping of the 

 ecliptic stars from the eighth to higher magnitudes is still comparatively 

 limited ; nor has the length of time during which any one portion, perhaps, 

 of the space been thus mapped, been sufficiently great to ensure the passage 

 through it, within that time, of any planet whose period is as long as the pos- 

 sible periods of those which may yet remain unknown to us. Analogy would 

 therefore lead us to conclude in favour of the probability of their number 

 being much greater than that at present recognised. All those which are 

 now known lie between the orbits of Mars and Jupiter, but many may exist 

 more distant and of much smaller apparent magnitudes ; and thus almost 

 the same careful telescopic research may be necessary to make us acquainted 

 with some of our planetary neighbours as with the remoter regions of space. 

 Nor is the telescopic mode the only one by which we may detect the existence 

 of remoter planets ; for as Uranus betrayed the existence of Neptune, so 

 may the latter hereafter reveal to us the retreats in which some more distant 

 member of the system has hitherto hidden himself from the observation of 

 man. 



There would seem to be a tendency in the human mind to repose on the 

 contemplation of any great truth after its first establishment. Thus, after 

 the undisputed reception of the theory of gravitation and the complete ex- 

 planation which it afforded of the planetary motions, men seemed to think 

 little of any further revelations which the solar system might still have to 

 make to us respecting its constitution or the physical causes which it calls 

 into operation. The recent discovery, however, of so many planets shows 

 how imperfectly we may yet be acquainted with the planetary part of the 

 system ; and the continual discovery of new comets seems to indicate that in 

 this department still more remains to be done. These curious bodies, too, 

 may possibly have to reveal to us facts more interesting than any which the 

 planets may still have in reserve for us. The experience of these latter 

 bodies, if I may so speak, is more limited, and their testimony, consequently, 

 more restricted. But they have already told us a noble tale. In moving, 

 as they do, in exact obedience to the law of gravitation, and thus establishing 

 that law, they have affirmed the highest generalization in physical science 

 which it has been accorded to the human mind to conceive. At the same time 

 the approximate circularity of their orbits prevents their passing through those 

 varied conditions to which comets are subjected. Thus, while the latter 

 obey, in common with the planets, the laws of gravitation, they frequently 

 present to us, in their apparent changes of volume, form, and general cha- 

 racter, phaenomena the explanation of which has hitherto baffled the inge- 

 nuity of astronomers. One of the most curious of these phaenomena has 



