382 Notices respecting New Books. 



are executed with extreme care and accuracy, and with the view of 

 enabling; future astronomers, by comparing the present representa- 

 tions with the actual appearances of the objects some ages hence, to 

 determine whether they remair. permanently the same, or are gra- 

 dually undergoing changes in .leir physical condition. 



Chap. I. Of the Nebula of .lie Southern Hemisphere. — The subject 

 of this chapter is a catalogue of the nebulae and clusters of stars, 

 containing the reduced results of all the observations of each object 

 which occurred in the regular course of sweeping, in which either 

 the place of the object was taken, oj- any particular in its appear- 

 ance or physical character noted, without any selection of good, or 

 suppression of discordant observations ivhatever. In its arrangement 

 aiid construction it is similar in every particular to the catalogue of 

 Northern Nebulae and Clusters printed in the Philosophical Trans- 

 actions for 1833, and is reduced to the same epoch, namely the be- 

 ginning of 1830. The Introduction contains general observations 

 respecting the mode of observing and registering the observations, 

 the explanation of the abbreviations, &c. The whole number of 

 nebulae and clusters is 1708. Of these 89 are identical with ob- 

 jects formerly observed by Sir John himself at Slough ; 135 are con- 

 tained in Sir William Herschel's catalogues ; 9 are Messier's, and 

 206 have been identified (though with considerable doubt in many 

 cases) with objects observed by Mr. Dunlop. The remaining 1269 

 are a new acquisition to astronomy. 



In the " Remarks on the Catalogue," Sir John first considers the 

 degree of precision which may be ascribed to the place of an object 

 determined by a single observation ; a great number of them having 

 been observed only once. As it is found that the cases are compa- 

 ratively rare in which two observations of the same object differ by a 

 whole minute in NPD, or by a quantity in RA, which when con- 

 verted into space would give the same amount of discordance in the 

 direction of the parallel, he infers that, with the exception of large 

 clusters having no centrally situated remarkable star, and large ill- 

 defined or irregularly' shaped nebulae, the error of a single obser- 

 vation (taking the error at half the discordance) is in general within 

 half a minute of a degree. As to errors of reading and reduction, 

 it is presumed, from the care that was taken and the re-examina- 

 tions made, that they are very unfrequent. The reductions were 

 carefully re-examined whenever there appeared the smallest ground 

 for suspicion. 



He then proceeds to explain the figures and drawings to which 

 reference is made in the catalogue, and to give detailed descriptions 

 of some of the more remarkable objects ; discussing, incidentally, 

 the presumptions of a change of state afforded by comparison of their 

 present appearances with former descriptions and drawings. He 

 also describes the method he followed in laying down the mono- 

 graphs, and determining the positions of the stars visible in the ne- 

 bulae. The usual process was as follows. The differences of RA 

 and NPD of the principal stars were talcen with the equatorial mi- 

 crometer. Adopting these as a basis of projection, the more conspi- 



