Royal Astronomical Society. 399 



toreals in Planetary Observations. By the Rev. Richard Sheep- 

 shanks. 



Mr. Snow's observations admitted of being so grouped as to fur- 

 nish four sets of comparisons in right ascension and five sets in de- 

 clination. The effects of parallax and refraction were computed by 

 the formulae used at Greenwich (Greenwich Observations, 1836, 

 pages lxiv. and lxv.). The right ascension of the star was taken 

 from Lord Wrottesley's Catalogue, the declination from the Astro- 

 nomical Society's Catalogue, and the semidiameter of Venus from 

 Mr. Snow's Observations ; and thus the right ascension and declina- 

 tion of the pJanet were obtained for the Ashurst sidereal times of 

 observation and compared with the places interpolated from the 

 Nautical Almanac for the same times. The resulting corrections to 

 be applied to the right ascensions and declinations of the Nautical 

 Almanac are as follow : — 



The mean epoch is about 8 h 30 m Greenwich mean solar time. 



The author remarks generally with respect to the treatment of 

 such observations, that they may be boldly grouped without sensible 

 error, so as to make one reduction serve for a considerable number 

 of observations ; and that to ensure the greatest facility for group- 

 ing, the observations of one element (if both cannot be made simul- 

 taneously) should be repeated several times as rapidly as possible 

 alternately with similar sets of observations of the other element. 



"With respect to the value of such observations, the results above 

 given will show that an equatoreal, when thus used, is no mean rival 

 to meridian instruments. The star can be subsequently determined 

 with any required degree of accuracy, and the observations can be 

 made with as great freedom from constant error with an equatoreal 

 as in the meridian. In this latter respect, indeed, the power of re- 

 petition gives to the equatoreal a great superiority, and may be made 

 to counterbalance the disadvantages arising from want of steadiness. 

 The last-named quality can, however, in most instances, be obtained 

 in as great a degree as is requisite. The hour-circle being firmly 

 clamped, if the instrument be well balanced, sudden changes can 

 arise only from careless handling. 



The supposed uncertainty and instability of the adjustments are 

 probably the principal obstacle to the free use of equatoreals in En- 

 gland ; but the author considers that most equatoreals can be ad- 

 justed very nearly, and that when ordinary care has been taken, the 

 position remains sufficiently permanent ; and it is certain that when 

 rationally used, the effect of any unavoidable derangement is so nearly 

 annihilated as to be quite insensible. The difficulty of performing 



