of Bessel's Astronomical Observations. i'29 



of these changes consequently renders necessary the reading 

 off of the level at every observation. An arm on the other 

 end of the axis, with a collar through which this end of the 

 axis passes, is firmly clamped to the axis by means of a screw, 

 and then serves for bringing the telescope into its proper 

 position. The artist has thu contrived that there should be 

 no strain whatever on the circumference of the two circles, 

 from which in other cases change of figure and considerable 

 errors have arisen. The illumination of the wires is through 

 the axis, which is perforated at the extremity furthest from 

 the circle. The necessary changes of its intensity are ob- 

 tained by a diaphragm attached to each pillar, the greater or 

 less opening of which allows a proportional quantity of light to 

 pass. A considerable increase of light has been effected 

 since November 1820, by conical metal tubes polished in the 

 inside and fixed in the perforated hollow of the pillar. This 

 contrivance has been before adopted in the observatories of 

 Munich and Gbttingen. 



In placing this highly perfect instrument, I have endea- 

 voured to avoid every circumstance known to me by former 

 experience, which might tend to produce unsteadiness; — the 

 manner which I have used having been found to answer the 

 v purpose, I think that I ought to describe it. An excavation 

 of six feet deep, ten feet long from east to west, and seven 

 feet broad from north to south, was lined with a wall ; in the 

 middle of this hollow there was built a parallelopiped three 

 feet and a quarter high, of broken rocks, forming a mass of 

 solid masonry, leaving between its vertical sides and the sur- 

 rounding wall an empty space of six inches in breadth. The 

 two granite pillars (six feet long, two feet broad, fifteen inches 

 thick) which were formerly the supports of the transit, were 

 placed on this parallelopiped, in the direction of the meri- 

 dian, the centres being four feet distant and walled round. 

 On them was placed a slab of stone carefully cut, six feet 

 long, four feet broad, and one foot thick, which was as firmly 

 as possible attached to the pillars by means of cement and 

 iron cramps. The upper surface ot this slab is six inches 

 below the floor ; on it are placed without any other essential 

 fastening the new pillars, six feet four inches high, 20-5 inches 

 broad, and 1 5 inches thick, of very hard perfectly uniform 

 sandstone of a very fine grain, which on account of the uni- 

 formity of its grain I have preferred to granite. 



The manner of placing the supports secures them against 

 the effects of the freezing and thawing of the ground, which 

 only act on the surrounding wall, that goes deeper than the 

 frost ever penetrates into the ground: they nre likewise pro- 

 tected 



