286 New Method of Levelling the Avis of a Transit Instrument. 



rived from the transit lamp placed on the bar of the support 

 opposite the one in which the telescope C rests during an ob- 

 servation. 



To make the mathematical axis (or line of rotation) of the 

 transit parallel to the horizon, the line of collimation of the 

 object-glass A, and afterwards that of B, must be rendered 

 parallel to the line of rotation. This will be the case when 

 the line of collimation of the telescope C, placed against either 

 object-glass, preserves during a revolution of the axis of the 

 transit its parallelism to that of the object-glass to which it is 

 dii'ected. The line of collimation of both object-glasses being 

 now parallel to the line of rotation, point the line of collimation 

 of the telescope C exactly at that of the object-glass A, and 

 note the level {g) attached to the former. Then remove the 

 telescope to the Ys of the support E, and point it accurately 

 at the intersection of the cross lines of the object-glass B. In 

 the event of the line of rotation being horizontal, the bubble 

 will revert to its previous position ; otherwise half the devia- 

 tion will be the inclination of the line of rotation. 



As it may be difficult to make the line of collimation of the 

 object-glasses rigorously parallel to the line of rotation, the 

 telescope must be twice pointed at the object-glass ; one ob- 

 servation taking place before.) and the other c^ter the axis of 

 the transit has described half a revolution. The mean of the 

 two readings of the level will then be equivalent to one reading 

 with the cross lines perfectly adjusted. 



In bringing the line of collimation of the telescope of the 

 trarisit to be at right angles to the line of rotation, the latter 

 must be levelled (after the method described) before as well as 

 after the axis is reversed ; otherwise, at least in the case of the 

 pivots being unequal in diameter, the adjustment cannot be 

 accurately effected. 



When the line of collimation of each object-glass has been 

 rendered parallel to the line of rotation, and the telescope C 

 left correctly pointed at the cross lines of the adjacent object- 

 glass, take the axis of the transit instrument out of its Ys, and 

 replace it within them reversed in direction. The other ob- 

 ject-glass will now be brought nearest to the telescope C ; and 

 should the line of collimation of the latter be found strictly 

 parallel to that of the object-glass, it will prove that the pivots 

 are of the same diameter. 



Leeds, August 4, 1831. JoHN NixON. 



Erratum. — Vol. ix. page 428, line 37,^0;- the angular 

 opening of the Y, read half the angular, &c. 



XXXVIII. No- 



