Royal Society. £39 



observations of the same kind, for the purpose of verification, and 

 of a nearer approach to accuracy, the whole time required will not 

 be more than five minutes, during which it is not probable that any 

 sensible disturbance can have taken place in the position of the in- 

 strument from changes of temperature. 



Tables are given containing registers of numerous series of ex- 

 periments made, both by the author and by several of his friends, 

 with a view to determine the stability of the instrument, and the 

 degree of reliance that can be placed in the results. In the first 

 series, out of sixty independent determinations of the zenith point, 

 there are twenty-five, the error of each of which does not exceed 

 one-tenth of a second ; thirty-seven under two-tenths ; forty-seven 

 under three-tenths ; fifty-five under four-tenths ; three between four 

 and five-tenths ; and two a little above half a second. But the* author 

 thinks it probable that the greater part of these errors, minute as 

 they are, must be attributed to want of power in the micrometer, 

 which power is directly as the focal length of the object-glass, or 

 mirror, of the telescope to which it is attached, and which neces- 

 sarily limits the precision of which it is capable. 



The author next gives the results of some experiments with a 

 collimator made for Captain Foster, having a float of only five inches 

 in diameter, and with a telescope five inches long : the errors, gene- 

 rally, do not amount to more than two-tenths of a second. 



He then enters into details as to the manner of using the vertical 

 floating collimator in astronomical observations, beginning with the 

 portable azimuth and altitude circle described by the Rev. F.Wol- 

 iaston in his Fasciculus Astronomicus, and applicable to other similar 

 instruments. The new collimator affords, also, the most perfect me- 

 thod of adjusting the line of collimation of a mural circle, or of 

 placing it at right angles to the axis. 



The author next proceeds to describe the method of applying the 

 instrument to the zenith telescope. In comparing the observations 

 made by the zenith sector belonging to the Board of Ordnance, with 

 the zenith telescope used in conjunction with the vertical floating 

 collimator, the mean of errors in the former case was -f 0"*54? and 

 — 0"-75; in the latter, +0"44 and — 0"-66. From observations 

 made on y Draconis, the zenith distance of which at Greenwich is 

 0° 2' 6"-36, and atYork Gate 0° 0' 35"*67, the difference of latitude 

 between the two places was found to be 0° 2' 42"*03, that of 

 Greenwich being 51° 28' 38"-96, and of York Gate 51° 31' 20"*99. 

 The decimals of a second, by the azimuth and altitude circle and 

 horizontal floating collimator were '94; by the same instrument 

 and the vertical floating collimator -76, and by the zenith telescope, 

 and the vertical floating collimator *99 : the mean being -9". 



From the greater degree of precision attainable by the employ- 

 ment of the vertical floating collimator, from the facility of its con- 

 struction and application, and the time saved by using it, the author 

 deems it not unreasonable to infer, that, ere long, the use of the 

 level and plumb-line in celestial observations will be wholly aban- 

 doned. 



LIN- 



