Notices respecting New Books. 4>63 



been observed and their orbits computed ; and our knowledge of the 

 sidereal heavens has been greatly extended, not merely in conse- 

 quence of continued observation, but by the more general use of large 

 instruments. Hence the work of 1833, however complete at that 

 time, necessarily leaves the student much behind the point to which 

 this branch of knowledge has actually attained. To supply the de- 

 fects, record the recent discoveries, state the modifications of former 

 views to which they lead, and represent the science as it exists at 

 the present moment, are the objects of the work now before us. It 

 appears under an enlarged form, and with a different title; and though 

 the substance and arrangement of the original treatise have been 

 preserved, the additions and alterations are so extensive and import- 

 ant as to render it rather a new book than a new edition of the 

 former one. 



The present work is divided into four parts. The first embraces 

 what is sometimes called Descriptive Astronomy, beginning with the 

 general notions of the science, and including all the topics usually 

 treated of under this branch of the subject ; — the .shape and size of 

 the earth, the atmosphere, refraction, the theory and use of instru- 

 ments, the apparent and true motions, as well as the appearances 

 and physical nature of the bodies of the solar system. On many 

 of these heads there was little to alter in the original work ; much 

 new matter has however been introduced, and we may instance in 

 particular the chapter on comets, which will be found to be replete 

 with interest. In this we have a condensed account of the remarkable 

 phsenomena exhibited by some comets which have recently appeared ; 

 by Halley's at its return in 1835, when it was observed under very 

 favourable circumstances by the author himself at the Cape ; by the 

 comet of 1843, which approached so near to the surface of the sun, 

 that the intensity of light and radiant heat must have been 47,000 

 times greater than at the surface of the earth, and whose tail at the 

 perihelion passage was whirled round, unbroken, through an angle 

 of 180° in little more than two hours; by Biela's comet, which at 

 its last apparition in 1846 was seen to separate itself into two di- 

 stinct bodies, which, " after thus parting company, continued to 

 journey along, amicably, through an arc of upwards of 70° of their 

 apparent orbit, keeping all the while within the same field of view of 

 the telescope pointed towards them." Some other comets recently 

 observed, which seem to describe elliptic orbits in short periods, 

 are also taken notice of ; and the description of the phaenomena is 

 followed by some most ingenious and highly interesting speculations 

 on the physical nature of those enigmatical bodies. 



The second Part treats of the planetary perturbations ; a subject 

 which from its nature can never be rendered very pojmlar, but which, 

 nevertheless, as is proved in the present instance, may be explained 

 in such a manner that the principal effects on the motion of a system 

 of bodies produced by their reciprocal attraction may be clearly and 

 readily apprehended by a reader having no more than an elementary 

 knowledge of geometry and mechanics. The accurate and minute 

 computation of these effects is quite another thing, and must be left 



