Captain Kater's description ^ &c, 
bisected by the micrometer wire at the time of its passing the 
meridian, and the division of the micrometer head -read off 
and registered. The screen being then turned down, the angle 
formed by the crossed wires of the collimator is to be bisected, 
and the reading of the micrometer registered. The collima- 
tor is then to be turned half round, the angle again bisected 
by the micrometer wire, and the reading noted. The mean ot 
these bisections is the place of the zenith, and the difference 
between this and the reading when the star was bisected, is 
the star's zenith distance. , 
rt is evident that the operation for finding the place of the 
zenith may be repeated at pleasure, and consequently that the 
error, if any, in the zenith distance, may be ultimately refer- 
red to inaccurate bisection of the star, or imperfection of the 
screw of the micrometer. 
I may remark, before I conclude, that a telescope,* similar 
to that I have used in the horizontal collimator, may be em- 
ployed as a meridian mark for a transit instrument when a 
distant one cannot be obtained, and that the crossed wires 
afford an excellent object for the adjustment of the line of 
collimation. For this purpose, the telescope must be firmly 
fixed in' the proper position. I attempted some years since 
to effect this by means of a convex lens, having cross wires 
in its focus, but as it did not occur to me to use an eye-glass, 
I was unable to place it in the proper direction, and after 
many unsuccessful' trials I laid it aside. •'p 
* The length of the telescope is here important to accuracy. 
f Since this Paper was written, I have discovered that in the year 1785, such a 
meridian mark was actually used by. Mr. Rittenhouse, who employed for the pur- 
pose the object glass of a telescope thirty-six feet, long, in the focus of which was 
placed a metal plate, having several concentric circles drawn upon it. See Transac- 
tions of the American Philosophical Society, vol. ii. 
