ASTRONOMY. 



[MOTION or THE KTARS. 



inferior and more simple systems ; and Lambert went so 

 far as to imagiiM that the separate clusters performed 

 revolutions round a great central body, in the same 

 manner as our planet and satellite move round their 

 primary. 



The attention of Sir W. Henchel was particularly 

 directed to the subject of the construction of the heavens ; 

 and bis labours in this field must be regarded aa among 

 the most important of his work*. In order to bring the 

 theories started by his predecessors to a teat, and to 

 obtain an idea of the form ami dimensions of the stellar 

 universe, Henchel had recourse to a long and laborious 

 method, which he properly termed "gauging the hea- 

 vens." This was done by directing a powerful telescope 

 (the twenty-foot reflector) to different parts of the sky, 

 and counting the number of stars in each field of view. 

 In order to insure greater accuracy, he counted the num- 

 ber of stars in ten contiguous fields, and took the average 

 to express the comparative riches or poverty of the dis- 

 trict In some portions of the sky, only three or four 

 stars of all magnitudes were seen per field for a consider- 

 able distance round ; whilst at other times the field was 

 crowded with many hundreds ; and those latter portions 

 were always fouud in or near the Milky Way. By 

 combining the numerous "gauges" which he made, he 

 endeavoured to determine the various depths (from the 

 different degrees of obliquity in which the stars were 

 viewed) of the Milky Way. Supposing the individuals 

 composing it to be placed at pretty equal distances, he 

 concluded the whole visible heavens to be of a lenticular 

 form, and not a stratum of stars inclosed by plane sur- 

 faces. The proportion the thickness bean to the dia- 

 meter of this le>a, he considered was as one to five and a 

 half : and he further concluded that the sun was removed 

 but little from the centre of the group. Subsequently, 

 when the motion of various double stars, consequent on 

 their physical connection, was discovered by Herschel, his 

 ideas were considerably modified ; he now imagined that be 

 perceived evidences of this physical connection in the great 

 'groups of the Milky Way, and that this "clustering 

 power," as he termed it, tended to break it up into frag- 

 ments. 



Nor was this the only evidence of the motion of the 

 stars, for many of them are endowed with an indubitable 

 motion, as was apparent from their positions, compared 

 with neighbouring stars ; and, whether due to their own 

 proper motion, or to that of the sun, it must be con- 

 sidered as an absolute proof of the instability of these 

 bodies in space. Among the stars which are thus known 

 to have considerable proper motion, we may mention 

 f Cassiopeia?, which has a proper motion of o" -82 an- 

 nually, in right ascension, and 1"'55 in north polar dis- 

 tance. The star 61 Cygnus, whose distance lias been 

 determined with some exactness, is one of those which 

 has a large proper motion ; and it was in consequence of 

 this circumstance that it was chosen by Bessel for the 

 determination of the annual parallax. The proper mo- 

 tion, in right ascension, amounts to 6"'39, and in north 

 polar distance, to 3* '30. In 40 Eridani, the proper motion, 

 in right ascension, amounts to 2* -10, and in north polar 

 distance, to 3' 45. The star 1830 of Groombridge's, 

 which was considered to show the greatest amount of 

 parallax, is one of those whose proper motion is most 

 considerable, amounting to 6" '10 in right ascension, and 

 6'70 in north polar distance. Other stars, in almost 

 every constellation, have large proper motions ; but the 

 quantities are, as yet, somewhat doubtful, as it is only 

 by the comparison of accurate modem observations with 

 ancient authorities nearly as accurate, that these small 

 quantities can be deduced ; and it is but since the tnn> - 

 .ulley that the places of the objects can be depended 

 upon. 



:h, however, has been effected to show the reality 

 of their movement, and iU direction. If this were due to 

 the motion of the start themselves, it might be supposed 

 that they would move in all directions, north or south, 

 east or west ; and though this is found to bo the case 

 with some, yet, with far the greater number, the direction 

 taken is much more regular. It has hence been con- 



jectured that it is the sun itself which is in motion the 

 consequence of which would be, that those stars which 

 are situated in that part of the sky which we approach, 

 would appear to be separated more and more, as the 

 I angular distance would increase the nearer we approach 

 to the objects. In that part of the celestial regions 

 which we are leaving behind us, the stars would appear 

 to be falling closer together, for the contrary reason. 

 Although this supposition, explaining the proper motions 

 of stars as being due to the simple effect of parallax, was 

 made by both Mayer and Lambert, it was Sir W. Herschel 

 who first attempted to show the direction of the solar 

 motion, which he concluded was towards a point in the 

 constellation of Hercules, whose position, in 1783, was 

 at 257 of right ascension, and Co of north polar dis- 

 tance. An attempt has since been made, by Argelamler, 

 to solve this problem ; and that celebrated astronomer 

 arrives nearly at the same result as Herschel, finding, 

 from an examination of the proper motions of tluv 

 hundred and ninety stars, that the part of the heavens 

 towards which the sun is progressing, is probably situated 

 at 200 61' of right ascension, and 68 43' of north polar 

 distance. 



NATURE AND DIFFERENT SPECIES OF STARS. It would, 

 of course, be impossible to form any idea of the stars ; 

 but analogy would lead us to imagine that the heat and 

 light which they emit, is in every respect similar to tliat 

 of our own sun ; and some photometrical experiments, 

 which have been made on the latter object, seem to show 

 that, if it were transported to the same distances as 

 some of the stars whose distances are pretty well known, 

 its apparent magnitude would not be so great as that of 

 many which we see around us. From the distance and 

 orbit of 61 Cygui, Bessel was able to arrive at a rough 

 idea of the masses of those stars, which he found were ' 

 not greatly under that of the sun ; and if this were the 

 case with a star of that magnitude, we may conclude 

 that the brighter objects am considerably larger than 

 our luminary. It is certain that reflected light could 

 not reach us from such immense distances ; and we must 

 conclude that, like our own sun, they are self-luminous, 

 although we know nothing of the agency at work to pro- 

 duce those extraordinary effects. It would be rash to 

 imagine that, like it, they are 'all accompanied by a cor- 

 t'ge of planets ; for some of the bitter bodies in the solar 

 system have numerous satellites ; our own earth but one ; 

 and others none whatever. But it would be equally rash 

 to conclude that their heat and light expend themselves 

 in the unprofitable and dark voids of the celestial spaces ; 

 and, in addition to the slight evidence which analogy 

 affords, we have the further proofs of their being the 

 centres of great systems from the remarkable phenomena 

 of double and changeable stars, the latter of which, in a 

 different point of view, may be regarded as evidence of 

 their rotation on their axes, similar to that of our own 

 sun. 



DOUBLE STARS. A few of those curious objects have 

 been known ever since the invention of the telescope ; 

 and in a cursory examination of some of the brighter 

 stars, as Castor, ( Ursie Miuoris, a Herculis, y Virgiuis, 

 an observer could not help detecting the strange appear- 

 ance of two stan close together, and almost blending 

 their light, forming, apparently, but one star to the 

 naked eye. In the middle of the seventeeth centiuy 

 further importance was attached to those curious objects, 

 and several were closely examined, although without any 

 result, for the purpose of determining their annual 

 parallax, for which they offered peculiar advantages. It 

 was then supposed that all these objects were fortuitously 

 or optically double ; one of the components most pro- 

 bably the fainter being situated at a much greater dis- 

 tance from us than the brighter, but both appearing in 

 the same direction. It was for this purpose, likewise, 

 that Sir W. Herschel commenced, in 1779, to apply his 

 powerful telescopes and delicate micrometers to the task 

 of recording their distances and positions in respect to 

 one another. But in looking for one thing, as sometimes 

 happens, another was found ; and it became apparent 

 that the components were not only at the same distance 



