149 
a floating collimator. 
from night to night, and that the relative positions of the 
different parts of the instrument suffer no change, supposi- 
tions which are too gratuitous to be readily received. 
In the second method, the plumb line being adj^isted as 
before, the star is taken several times when very near the 
meridian, the time at each observation being noted. The in- 
strument is then immediately turned half round in azimuth, 
readjusted if necessary, and the observations repeated, noting 
the time as before. The mean of the results of the readings 
in both positions of the instrument will be the altitude or 
zenith distance of the star at the mean of the obser,ved times. 
The time at which the star .comes to the meridian being 
known, the difference between this and the mean of the 
times of observation gives the horary angle of the star with 
the meridian ; whence the correction is computed which it is 
necessary to apply to the observed zenith distance in order 
to obtain the meridional zenith distance. 
1 know of no objection to this method except the length of 
time required for observing each star, the labour of comput- 
ing the corrections, and the possibility of the instrument suf- 
fering a strain from being turned half round in azimuth. 
The manner of using an instrument furnished with a level, 
is nearly the same as when it is constructed with a plumb 
lime ; but as the level has a scale, the divisions indicated by 
the ends of the bubble are usually read off at the conclusion of 
each observation, and the resulting corrections applied to the 
observed zenith distance. This presupposes the value of 
the divisions of the scale to be well ascertained, and that they 
are equal, which last is not always the case. 
In observing by reflection, the error which might arise 
