34«i Mr. Nixon on the Measurement of the 



close to that of the telescope. The levels were then noted 

 before and after the pai'allelism of the lines of collimation had 

 been elfected. 



When the line of collimation of the proof object-glass had 

 been made parallel to, or more correctly, level with that of the 

 telescope by unscrewing it within its cell, the cross wires of the 

 cylinder intersected each other at 

 E, about I' to the xacst of C, or point 

 in which they were situated pre- 

 viously to reversing the cylinder ; 

 but on turning the cylinder half 

 round within its Ys, the intersec- 

 tion was removed to D, about 1 1' 

 east of C. The axis of the (re- 

 versed) cylinder, situated at F, 

 must therefore have been about 

 6' out of its previous direction ; a 

 consequence either of the Ys being 

 of an irregular figure, or of the cylinder not being straight. 

 Two systems of cross wires, one for each object-glass, would 

 certainly have been preferable. 



Having discovered that the tube containing the additional 

 object-glass did not fit sufficiently tight within the cylinder, 

 beyond which it projected considerably, the evil was remedied 

 by applying glue to fix it. Still there existed some degree 

 of doubt whether the tube was not sensibly deflected by its 

 own weight. 



The average of twenty-five measurements in April 1829, 

 and January and February 1830, with the room constantly at 

 45° Fahr., made the instrumental error between 12"'0 and 

 I6"*6, mean 13"*8. Those of 1829 were made before the 

 present object-glass of the cylinder was substituted for the 

 original one of greater focal length, on a plank 5'5 feet long, 

 5 inches broad, but only 3 inches thick. The average of 

 these (four) measurements was 14'"'1. 



By the Eleventh Method varied. — With an efficient micro- 

 meter, a more simple or accurate method than the one just 

 described could scarcely be devised. However, as the error 

 was of necessity to be determined by the spirit-level, the mea- 

 surements were repeated with such improvements in the ap- 

 paratus and variations of the method as promised the most 

 successful results. 



Two iron brackets, driven, at a distance of 30 inches from 

 each other, into a seam of the stone wall forming the east side 

 of the room, and from which they projected nearly a foot, 



supported 



