Notices respecting New Books. 65 



as an outline figure of a transit-circle or equatorial is contained in 

 the book. The descriptions, it is true, though brief, notice all the 

 essential points, and are perfectly clear if trie reader has seen the 

 instruments; but as many persons are interested in the principles of 

 astronomy who have not access to an observatory, we think figures 

 should have been inserted. The best figures and the fullest descrip- 

 tions are only makeshifts ; but though we cannot do as much as 

 might be wished, that is no reason for not doing as much as we can. 



The treatise is said to be one of practical astronomy. It must be 

 understood that the practice contemplated is that of a fixed observa- 

 tory. In other words, nautical and geodetical astronomy are not 

 included, though they might with great propriety form part of a 

 treatise of practical astronomy. We are inclined to think that 

 Mr. Main should have observed his own limitation with either more 

 or less strictness. We have failed to find any account, however brief, 

 of the zenith sector, and suppose it was omitted as not being an 

 instrument used in a fixed observatory ; yet it is mentioned in con- 

 nexion with two totally distinct subjects (pp. 160, 166). And, 

 further, in the chapter on the determination of geographical lati- 

 tude no mention is made of the method depending on the use of 

 that instrument. On the other hand, Mr. Main has noticed the 

 principal problems of sextant astronomy in one part or other of his 

 book, and has investigated the appropriate trigonometrical formulae. 

 We think it is very much to be regretted that, having given so much 

 of this latter subject, he has not given more. The step which sepa- 

 rates the general formula from the numerical result in any particular 

 case is one which the average student would find a difficulty in taking. 

 This is a fact which the student is often unwilling to believe until 

 he is induced to make the attempt, and it is for many reasons desi- 

 rable that he should make the attempt — an opinion in which we 

 believe Mr. Main shares (pp. 103 (note) and 117). It must be 

 remembered that a student who learns astronomy without access to 

 instruments (and this is the commonest case) is in danger of regard- 

 ing his text-book as a mere collection of geometrical problems, and, 

 in consequence, of resting content with a vague notion of his subject, 

 such as a student of chemistry- would form of his subject if he never 

 made an experiment. We believe that there are few things more 

 calculated to remedy this inconvenience than the actual working 

 out of a few numerical cases of determinations of latitude and local 

 time, starting from the data which would be actually supplied by the 

 sextant and chronometer. All that is wanting for this purpose in 

 Mr. Main's book could be supplied by two or three pages of text, 

 and as many of unworked examples. 



Mr. Main has very wisely omitted many problems or propositions 

 which have a sort of traditional right to a place in works of this kind, 

 and has but rarely given an unpractical solution of a problem merely 

 on account of its neatness. We are inclined to think that a few 

 more omissions might be made with advantage, and one or two of 

 his solutions suppressed ; instances occur in the chapters on Time 

 and Refraction. 



Phil. Mag. S. 4. Vol. 27. No. 179. Jan. 1864. F 



