171 
a floating collimator. 
For example, let the mean of the readings of the bisection 
of the cross wires when the collimator is to the south of the 
instrument be i .so" of altitude, and when it is to the north 
8'. 40''. The mean of these readings 8'. 5", is the true incli- 
nation of the collimator to the horizon, and the difference be- 
tween this and I'.so" (o'. 35'') must be added to all altitudes 
taken to the south, or subtracted from those to the north of 
the zenith. 
The instrument I have described may be called the hori- 
zontal collimator., but another and in most respects a prefer- 
able arrangement may be employed, similar to that suggested 
by Professor Bessel. The telescope may be firmly fixed in 
a position perpendicular to the float, and I should then name 
it the vertical collimator.^ This must be placed directly under 
the telescope of the circle ; and though not in a convenient 
position for observing, it yet possesses the very great advan- 
tage of obviating the necessity for carrying the collimator 
from one* side of the observatory to the other, nothing more 
being requisite than to turn the float half round in azimuth, 
and to take the readings of the microscopes when the angle 
formed by the cross wires is bisected in each position of the 
collimator, the mean of which will be the place of the zenith 
point. 
This is the construction which appears best calculated for 
a public observatory ; but in addition, it would perhaps be 
adviseable that it should be furnished with a horizontal colli- 
mator, having a float of increased length. It is intended that 
. ♦ The float of the vertical collimator should be circular, and an opening be made 
in the bottom of the tube of its telescope to throw light on its cross wires by means 
of an inclined plane mirror. 
