438 Royal Society, 



this paper is a material improvement on that of the horizontal float- 

 ing collimator, of which an account was given by the author in the 

 Philosophical Transactions for 1825. Its superiority is derived from 

 its adaptation to the vertical, instead of the horizontal, position ; 

 by which the sources of error arising from the necessity of trans- 

 ferring the instrument to different sides of the observatory, and of 

 taking the float out of the mercury and replacing it at each obser- 

 vation, are wholly obviated. The vertical floating collimator has 

 the further advantage of being adapted for use, not only with a cir- 

 cle, but also with a telescope, either of the refracting or reflecting 

 kind. Such a telescope, furnished with a wire micrometer, and 

 directed to the zenith, becomes a zenith telescope, free from all the 

 objections to which the zenith sector, and the zenith telescope, with 

 a plumb-line, are liable- 



The instrument itself is supported on a square mahogany stand, 

 which slides on two parallel beams, fixed at the upper part of the 

 observatory, in the direction of the meridian, and which has a cir- 

 cular aperture in the centre, having at its edge a projecting rim 

 of iron, to admit of the passage of the telescope. The telescope 

 having a focal length of eight inches, and an aperture of one inch 

 and a quarter, is supported in the vertical position by a bridge, 

 connecting it with a circular iron ring, ten inches and six-tenths 

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

 in a circular iron trough, the central aperture of which is suffici- 

 ently 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 telescope is placed at its lowest end, and its focus is 

 occupied by a diaphragm, composed of two brass plates, each 

 cut so as to form an angle of 135 deg. and placed opposite to each 

 other, so that the angular points are brought to an accurate coin 

 cidence, thus leaving on each side intervening spaces, which form 

 vertical angles of 4-5 deg. each. The telescope below, whether be- 

 longing 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 convenient distance upon one of 

 the beams crossing the observatory ; the light being reflected down- 

 wards by a plane mirror placed in a screen, with a suitable aper- 

 ture immediately above the collimator. The collimator is then to 

 be turned half round in azimuth, the motion being facilitated by 

 rollers, and limited at its 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; together 

 with an account of the precautions to be taken in the employment 

 of the instrument. The time required for completing the determi- 

 nation of the zenith point by its means need not exceed two mi- 

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



observation* 



