174 
Captain Kater's description of 
float and telescope might suffer derangement, and the direc- 
tion of the ray be consequently changed. 
I may here point out an advantage, and not the least valu- 
able, which this instrument presents, that of enabling the 
observer, by varying the inclination of the float, to bring a 
different part of the arc into use, and thus to check erro- 
neous division of the circle : this may readily be done by 
securely fixing weights to either end of the float.* 
I shall now proceed to give a description of the manner in 
which the floating collimator may be applied to the zenith 
tube. 
The first zenith tube was I believe constructed for Dr. 
Tiarks. It was a telescope hanging in Y s upon pivots pro- 
jecting from each side of the tube near the object end, and 
furnished with a wire micrometer ; to this telescope a plumb 
line was attached. 
When the star was upon the meridian, and of course suffi- 
ciently near the zenith to be seen in the telescope ; it was 
bisected by the micrometer wire, and the divisions regis- 
tered. The telescope was then inverted in the Y s, re-ad- 
justed by means of the plumb line, and the following even- 
ing the star again taken, when the mean of the readings of 
the micrometer gave its zenith distance. 
In the construction of the superb zenith tube 25 feet long, 
now making by Mr. Troughton for the Royal Observatory at 
Greenwich, I understand it is intended that the axis of the 
tube shall be the centre of motion, and the plumb line be 
* The above advantage may be considerably extended by the collimator being so 
constructed as to allow the inclination of the telescope to the float to be varied at 
pleasure. 
