175 
a floating collimator. 
suspended at the side. When the observations have been 
made a sufficient number of times with the plumb line on one 
side, the tube will be turned half round, and the observations 
repeated with the plumb line on the other side. The mean 
of both giving the zenith distance as before. 
In this construction the zenith distance cannot be obtained 
in one evening ; for were the telescope to be turned half 
round after the first observation, so much motion would be 
communicated to the plumb line, that there would not pro- 
bably be time to re-adjust the instrument before the star 
would have passed out of the field of view. 
.As it is highly desirable that the completion of the obser- 
vation sliould not be postponed, I endeavoured to effect 
this in a very fine zenith tube, which was constructed under 
my directions by Mr. Dollond for Colonel Lambton, and in 
another for Sir Thomas Brisbane, by placing the plumb line 
in the centre of motion ; but these various forms are still 
subject to one or other of the inconveniences which have been 
detailed in the preceeding parts of this paper, and which it is 
the object of the floating collimator to remove. 
The accuracy of the instrument I am about to describe, 
will depend upon the goodness of the telescope and of the 
wire micrometer employed. Exclusively of these it is within 
the reach of every observer, as the whole arrangement may 
be completed without difficulty and at a very trifling cost. 
Expence may contribute something in point of convenience, 
but can add nothing to its efficiency. 
To a firm wall, at a sufficient height, let a shelf be fixed 
and supported by a bracket at each end. In the middle of 
this shelf let a circular aperture be made, rather larger than 
the object glass of the telescope. Precisely beneath, and at a 
