92 Mr. Nixon on the Measurement of the 



In most of the measurements the range of the bubble was 

 confined between points of their scales made use of in the 

 subsequent operations, but the result was not materially dif- 

 ferent from that obtained by trials on nearly the whole extent 

 of the scales. 



Fortin's Level. — The value of its divisions was variously 

 ascertained: 1st, Precisely as just described, the tube being 

 laid upon the under surface of the horizontal bar of one index 

 of the sector; to which it was attached, in one case by a string, 

 and in another by a little glue. — It was fixed parallel to the 

 telescope of the sector with the surface of the fluid within the 

 tube equally bisected longitudinally by the line passing through 

 the middle of its divisions. (When transferred to the proof- 

 frame, (the cross-level of the latter being at its mark,) before 

 the tube was fastened within its Ys, it was made to have 

 the surface of the contained fluid bisected as just described.) 

 Another method of finding the value of the divisions when 

 the tube was attached to the proof-frame, was to place it upon 

 the rings of the sector, and compare the run of its bubble at 

 different variations of inclination of the sector, with that of 

 the levels of the latter. This method is free from every ob- 

 jection to which the other is liable, except that of regulating 

 a large instrument by a smaller and inferior one. The re- 

 sults were as follows : 



March 5th, 1830. Fortin's level-tube attached by strings 

 to index bar of sector. 



Arc 3' 7"-5 1° = 0"-60 Temp. 43° 

 3 -10 1° = -60 — 43 



April 1830. The level-frame fastened at its ends, by rib- 

 bons, to the cylindrical rings of the sector, and compared with 

 the right-hand level of the latter ; 



Arc 1° = 0"-65 Temp. 47° 



April 1830. The frame glued to the inside of the vertical 

 arches of the sector, and compared with the left-hand level 

 of the latter; 



Arc 1° = 0"-63 Temp. 52° 



April 1830. The brass sockets of the level glued to the 

 index bar of the sector, and a ribbon passed tightly over the 

 middle of the glass tube and the index-bar. The run of the 

 bubble amounted to 1721 divisions. 



Arc 17' 17" l"" = 0"'60 Temp. 60° 

 April 1830. Repeated with the ribbon cut off*, which caused 

 the bubble to run immediately exactly \", from one end towards 

 the middle of the tube. The result was unsatisfactory, from 



the 



