152 
Captain Kater's description oj 
readings on the circle at each position of the telescope is the 
zenith point. This may be obtained in a manner most inde- 
pendant of all other corrections, if the level be fixed nearly 
at right angles to the axis of the telescope, suspending it in 
the zenith, and then repeating the observation after the tele- 
scope has been turned on its axis: This method, which pre- 
supposes only an arrangement very easily made in the slit of 
the observatory, or upon the pillars, will give a very exact 
result, since the cross wires appear at all times well defined 
and without motion, which is not always the case in the com- 
parative observations of stars by a zenith sector and the circle 
itself.'' 
By this method, the necessity of observing out of the meri- 
dian, or of waiting till another night for the completion of 
the observation is obviated, and the zenith point may be im- 
mediately determined with as much accuracy as can be 
attained by means of a level. But though this is by far the 
best mode of employing a level that has ever yet been de- 
vised, it is still subject to the objections which have been 
urged against that instrument. 
It would require a level of very great delicacy and extent 
of scale to indicate the fraction of a second ; and such a one 
would be readily deranged by a small inequality of tempera- 
ture, or by the unavoidable elasticity of the parts necessary 
for its adjustment. 
In the course of some former enquiries, I made many 
experiments to ascertain the degree of reliance that might 
be placed on the position of a body floating upon the sur- 
face of mercury, and fully satisfied myself that it might be 
so contrived as to have always when at rest the same 
