170 
Captain Kater's description of 
There appears to have been some advantage gained by 
using a longer float, and it certainly was improved by being 
browned, as previously to that operation, small particles of 
mercury were observed, occasionally to attach themselves to 
the float, which was not the case afterwards. 
It may not perhaps be considered altogether superfluous 
to give in a few words the manner of using the collimator. 
The instrument being placed on the north or south side of 
the observatory with its telescope pointed to the centre of the 
circle and nearly in its plane, it is to be directed, so that the 
wires of the telescope of the circle may be seen through it, 
when reciprocally the cross wires of the collimator will be 
visible through the telescope of the circle, and the collimator 
is to be so placed, that the cross wires may appear in the 
centre of the field of view. The place of the box should then 
be carefully marked, to ensure its being at once restored as 
nearly as possible to the same situation. 
The collimator is then to be removed to the opposite side 
of the observatory, and the same process repeated, the situ- 
ation of the box being here also carefully marked. 
In observing, the star having been taken and the readings 
of the microscopes registered, the telescope is to be depressed 
to the collimator, and the angle formed by the cross wires 
carefully bisected. The collimator is then to be taken to the 
opposite side of the observatory, and the cross wires again 
bisected ; the mean of the readings at the bisections vvill give 
the inclination of the collimator to the horizon, and the difie- 
rence between this and the apparent inclination at either po- 
sition of the collimator will be the correction to be applied to 
the mean of the readings registered at the bisection of the star. 
