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principles. The size of a circle is undetermined ; Mr. Bessel ob- 

 tained excellent results with one of two feet ; and the largest which 

 was ever constructed, the Dublin 8 feet circle, gives probably the 

 most accurate conclusion : the lesser iire less influenced by irregu- 

 larities of temperature and by flexure ; the larger have their divi- 

 sions more visible, and admit more powerful telescopes. This last 

 consideration seems indeed to present a limit, for in the present state 

 of the English glass-houses neither good crown or flint glass can 

 easily be obtained, and a telescope longer tiian 5 feet can scarcely 

 be got perfect. As to reading off' divisions, I find that on the de- 

 clination circle of our equatorial, (42 inches,) successive readings 

 where the dot is good seldom differ more than y of a second, and I 

 found the same in a few trials of the great theodolite used in the 

 Trigonometrical Survey : linear divisions are, I suppose, equally dis- 

 tinct^ 



That construction which admits of being reversed in azimuth, 

 seems at first sight to posssess many advantages over the mural cir- 

 cle ; the power of eliminating the error of collimalion, and those 

 which proceed from oblique illumination of the limb and inacuracy 

 of the plumb-line or level, is of great importance ; nor is it less that 

 many observations may be taken on the same day. On the other 

 band, mural circles are much steadier, and much more expeditious 

 in Uieir use ; and the happy invention of the collimator seems to re- 

 move all the objections that press upon them. This gives an in- 

 variable object of known altitude, by observing which, before or after 

 the passage of a star, any disturbance of the microscopes is instant- 

 ly detected,* and it seems as accurate as level, plumb-line, or even 



• I find Che Collimator a singularly useful appendage to the equatorial ; its superiority to the 

 level is quite striking. A small repealing equatorial with its polar axis formed into a telescope, 



