234 Mr Freeman, On an altazimuth constructed from [May 1, 



(2) On an altazimuth constructed from the designs of the late 

 Rev. Dr W. Pearson. By A. Freeman, M.A. 



This instrument is described in Dr Pearson's Practical Astro- 

 nomy, vol. II. pp. 464 — 468, and is figured in perspective on Plate 

 xxiil. of that work. It is a portable instrument, inasmuch as it 

 can be taken to pieces and packed in a box 31 h inches long, 16£ 

 broad and 7 deep. It was specially designed by the late Rev. Dr 

 Wm. Pearson, then Rector of South Kilworth, Leicestershire, and 

 Treasurer of the Royal Astronomical Society, and was made in 

 1820 by Fayrer, one of Trough ton's workmen. The telescope 

 has an object-glass by Tulley with a clear aperture of 265 inches 

 and a focal length of 434 inches : it is fitted with a direct and also 

 with a diagonal eye-piece, and is excellent as regards defining 

 power. 



The telescope is mounted so as to be capable of reversion in 

 altitude and azimuth, the plane of the attached circle remaining 

 vertical during the process. The axis of motion of the vertical 

 circle can be adjusted and retained horizontal, when the axis of 

 motion in azimuth has been adjusted to a vertical position : these 

 adjustments are effected by means of the levels, one of which is 

 fixed in the plane of the vertical circle, the other hangs on the 

 axis of that circle, the extremity of this axis remote from the circle 

 being adjustible by opposing screws to perpendicularity with the 

 vertical axis of the instrument. 



The motion in azimuth results from the revolution of a hollow 

 cone, about two feet long, about an upright axis having cylindrical 

 bearings, and fitted securely to the tripod base. 



The chief peculiarity of the instrument is the bracket attached 

 to the revolving cone and supported by a prop. This bracket and 

 the horizontal solid axis which it bears form a counterpoise to the 

 vertical circle and the telescope. 



The vertical circle is supplied with a three branch alidade which 

 revolves about the horizontal axis to which this circle is rigidly 

 fixed by strong radial bars. Each branch of the alidade bears a 

 vernier read by a microscope applied successively to each. The 

 reading of the verniers may also be effected after clamping the 

 alidade to the circle by bringing each in succession opposite to a 

 micro-telescope fitted to the frame of the instrument. 



The three branches of the alidade are moreover connected by a 



