294! Royal Astronomical Society. 



exemplified in surveys of considerable extent and difficulty, in ■which 

 the editor has been engaged either professionally or as an amateur. 



An excellent account is given of the construction and use of the 

 chief instruments employed in astronomy and geodetic surveying, and 

 a collection of valuable geodetic tables concludes the work. 



We recommend this valuable work to all persons who take an in- 

 terest in the subject on which it treats, as the only one in our lan- 

 guage from which all necessary information may be obtained. 



XXXIX. Proceedings of Learned Societies. 



ROYAL ASTRONOMICAL SOCIETY. 



[Continued from vol. xxxiv. p. 543.] 



June 8, rj^HE Astronomer Royal resigned the Chair to the Rev. 

 1849. A. R. Sheepshanks, and then delivered to the Society 

 an oral statement, illustrated by models, " On Instruments adapted 

 to the Measure of small Meridional Zenith Distances." 



The Astronomer Royal remarked that there were three distinct 

 ranges of observation, for which three distinct classes of instruments 

 had been employed. The first and most comprehensive included 

 the observations for zenith distances throughout the whole extent of 

 the meridian, from the north horizon to the south horizon ; to which 

 observations the mural circle and similar instruments M^ere adapted. 

 It was evident that constructions adapted to secure this extent of 

 observations in all their generality could scarcely be expected to 

 avail themselves of the advantages peculiar to observations confined 

 to one part of the heavens ; and accordingly, in nearly every in- 

 stance in which observations on a more limited arc of the meridian 

 were sufficient, instruments of the other classes had been employed. 

 The second class of instruments might be included under the name 

 of zenith sector. With this instrument, by limiting the range o 

 observation to six or seven degrees on each side of the zenith, a 

 part of the arc of meridian was embraced which sufficed for most 

 geodetical purposes ; that part, both from the smaller proportion of 

 obscuration by clouds and from the smaller uncertainty attending 

 the calculation of refraction, was better in an astronomical point of 

 view than the rest; the instrument also might, with great porta- 

 bility, be made of large dimensions, with long telescope and long 

 plumb-line, two advantages which were highly valued in the last 

 and in the beginning of the present centuries. Generally, its use 

 had been confined to geodesy ; although it must not be forgotten 

 that one of the most celebrated specimens of it, namely, Bradley's 

 zenith sector, was constructed solely for astronomical observations 

 at a fixed observatory ; still, however, that instrument had in prac- 

 tice been confined to the observation of a single star passing the 

 meridian within two minutes of the zenith, and it might thus be 

 considered as included in the third class. The third class included 

 those instruments specially adapted to the observation of stars pass- 



