3*1 



servatory in the direction of the meridian, and which has a circular 

 aperture in the centre, having at its edge a projecting rim of iron to 

 admit of the passage of the telescope. The telescope, of which the 

 focal length is 8 inches, is supported in the vertical position by a 

 bridge connecting it with a circular iron ring, 10 inches and -rVths in 

 diameter, which floats in mercury. The mercury is contained in a 

 circular iron trough, the central aperture of which is sufficiently large 

 to allow of its turning freely round the rim, which rises from the 

 margin of the aperture of the stand. The object-glass of the tele- 

 scope is placed at its lowest end, and its focus is occupied by a dia- 

 phragm, composed of two brass plates, each cut so as to form an 

 angle of 135, and placed opposite to each other, so that the angular 

 points are brought to an accurate coincidence ; thus leaving on each 

 side intervening spaces, which form vertical angles of 45 each. The 

 telescope below, whether belonging to a circle or a zenith telescope, 

 is to be directed so that the image of these angles shall be bisected 

 by the micrometer wire ; for which purpose the diaphragm of the 

 collimator is illuminated by a bull's-eye lantern, placed at a conve- 

 nient distance upon one of the beams crossing the observatory, the 

 light being reflected downwards by a plane mirror placed on a screen 

 with a suitable aperture immediately above the collimator. The col- 

 limator is then to be turned half round in azimuth, the motion being 

 facilitated by rollers, and limited, as to extent, by two catches which 

 receive a projecting wire fixed to the outer circle of the trough. When 

 in this situation, the observation of the diaphragm by the telescope, 

 and the bisection of its angles, are to be repeated, and the mean of the 

 two positions will indicate the exact point of the zenith. 



Minute directions are given by the author for the construction of 

 all the parts of the collimator, and for their proper adjustments ; to- 

 gether with an account of the precautions to be taken in the employ- 

 ment of the instrument. The time required for completing the de- 

 termination of the zenith point by its means, need not exceed two 

 minutes ; and if to this be added the time necessary for a second set 

 of 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 expe- 

 riments, 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 

 60 independent determinations of the zenith point, there are 25, the 

 error of each of which does not exceed -rVth of a second ; 37 under 

 T Vths ; 47 under -rVths; 55 under T Vths ; 3 between 4- and 5 -tenths; 

 and 2 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 at- 

 tributed 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 



