148 
Captain Kater’s description of 
carefully fitted up, it is liable to so many errors from a variety 
of causes, that a single observation with the best level is little 
to be depended upon. 
Thte method of observing by reflection, is perhaps by far 
the most perfect of any that has yet been practised ; but it 
requires a union of favourable circumstances which rarely 
occurs. The fluid generally employed is mercury, which 
reflects a sufficient quantity of light to give a brilliant image, 
but is so easily deranged, that the slightest breath of air, not 
otherwise perceptible, or the distant passing of a carriage, 
the sound of which is scarcely heard-, is sufficient to disturb 
its surface, and to render an observation either impracticable, 
or so erroneous as to be perfectly useless. 
Nevertheless, by taking extreme care to protect the surface 
from wind, observations by reflection from mercury have 
been made with great success at the Royal Observatory at 
Greenwich, by means of the mural circle. But there are 
other objections besides those I have mentioned to all these 
methods. When an instrument with a plumb line is used, 
the observations may be conducted in two ways. In the 
first, the instrument being placed in the meridian, the plumb 
line is brought over a mark which is generally attached to 
the frame work carrying the microscopes. The star is taken 
as many nights in succession as may be thought necessary, 
and the instrument is then turned half round in azimuth, the 
plumb line brought over the same mark, and the star again 
taken, when the mean of the results of the readings in both po- 
sitions of the instrument gives the altitude or zenith distance, 
Here it is presumed, that the refraction remains the same 
