140 Reviews, and Notices respecting New Books. 



Sectors ; and the last, Zenith Micrometers, the account of which is 

 contained in §.LXXXI1. The first Zenith Sector, by Graham, is re- 

 markable as the means by which Bradley was led to the discovery 

 of the aberration of light. Also a full account of a complicated 

 Zenith Sector, by Ramsden ; and also of a much more elegant, much 

 more simple, and we have no doubt quite as accurate a one by 

 Troughton. 



In the following Section will be found an account of another of 

 Troughton's instruments, a portable Quadrant. 



In §. LXXXI V. is given the account of Ramsden's Polar instru- 

 ment. 



After an interesting account of the fixed telescopes used at Green- 

 wich, at the suggestion of the Astronomer Royal, for the purpose 

 of settling the question of annua) parallax in the fixed stars, our 

 author, in §.LXXXVII.andsomeof thefollowing Sections, describes 

 the construction and use of another class of instruments, — those 

 employed in Nautical Astronomy. The first which is considered, 

 is the first effective instrument of this kind that was ever contrived: 

 it is generally known under the name of Hadley's Sextant, with 

 which the angle between two objects is measured by means of re- 

 flection from two mirrors, and the inclination of the mirrors is half 

 the required angle. In the Section appropriated to the description 

 of this beautiful instrument, the author, after giving a short account 

 of the original invention, proceeds to give an accurate detail of the 

 instrument in its most perfect form. 



Our author has detailed all the successive improvements made 

 in this instrument, in the account he has given in his Section 

 LXXXVIII. 



But Troughton comes last of all ; and he seems in this instance, as 

 in all others, to have put the finishing stroke of perfection to the 

 construction of these instruments. The whole of §. LXXX1X. is 

 devoted to the description of the Reflecting Circle with three ver- 

 niers. The inventor's own directions for completing the adjust- 

 ments are also added. 



After explaining in the two following Sections how these instru- 

 ments are of use in determining the latitude at sea, and also the 

 time and (by means of it) the longitude; in §. XCII. XCIII. and 

 XCIV. are explained in detail the most accurate methods of ap- 

 proximating to the true longitude, both by sea and land. 



The Sections in which Dr. Pearson explains the process of com- 

 puting the longitude by lunar distances and by occultations are 

 particularly valuable. The details of the latter process are from 

 §. XCV. to §. XCIX., and this forms the most complete treatise on 

 the subject we know of. Our author informs us that in this import- 

 ant and difficult part of his work, he was assisted by M. Mossotti, 

 one of the 40 Members of the Societa Jtaliana, and Professor of 

 Mathematics at Buenos Ayres ; to whom also he has before expressed 

 himself indebted for an ingenious paper (§. XXIX.) on the Errors of 

 the divided Object-glass Micrometer. 



In §. XCIX. we have the methods of computing an occupation 



