

AIDS OF THE NEWTONIAN PERIOD. '273 



collection of such at Uraniburg was by far the 

 finest which had ever existed. He endeavoured to 

 give steadiness to the frame, and accuracy to the 

 divisions of his instruments. His Mural Quadrant 

 was well adapted for this purpose ; its radius was 

 five cubits : it is clear, that as we enlarge the 

 instrument we are enabled te measure smaller arcs. 

 On this principle many large gnomons were erected. 

 Cassini's celebrated one in the church of St. Petro- 

 nius at Bologna, was eighty-three feet (French) high. 

 But this mode of obtaining accuracy was soon aban- 

 doned for better methods. Three great improvements 

 were introduced about the same time. The applica- 

 tion of the Micrometer to the telescope, by Huy- 

 ghens, Malvasia, and Auzout ; the application of the 

 Telescope to the astronomical quadrant, and the fix- 

 ation of the center of its field by a Cross of fine wires 

 placed in the focus by Gascoigne, and afterwards by 

 Picard. We may judge how great was the improve- 

 ment which these contrivances introduced into the 

 art of observing, by finding that Hevelius refused to 

 adopt them because they would make all the old 

 observations of no value. He had spent a laborious 

 and active life in the exercise of the old methods, 

 and could not bear to think that all the treasures 

 which he had accumulated had lost their worth by 

 the discovery of a new mine of richer ore (u). 



The apparent place of the object in the instru- 

 ment being so precisely determined by the new 

 methods, the exact Division of the arc into degrees 



VOL. II. T 



