194 Use of the Zenith Telescope in Determining Latitudes. 
The value of the divisions of the level is next found in terms 
the other, heing measured by the micrometer. The advantage 
of the collitnator, from the stillness of the air through which the 
observation is made, is shown in the following comparative re- 
sults, by the two methods: 
NUMBER IL. 
Value of Level Divisions, No.1, by Distant Mark. 
Level. Micrometer. Differ, Differ. | Value of one division of level 
i sev O | E_ /Turns. Divis,| Level. Mier. micr, divisions, 
64:5 | 14:7} 0 o 4:25 19 4°47 
60°3 | 19:0] 0 | 19 4-40 20 454 
56-0} 23-5} 0 } 39 5:50 21 3°82 
50°5}.29°0} Oo 60 
e==0'5862 in micr. divisions. 
E=0:03275 * ts 
Mean value of one division level = 15193 
No. 2 by collimator. 
€=0'0770 
E=o0-0181 oy ee ee ee 
The value of the level divisions is then converted into arc, and 
a table prepared, which, by inspection, shows the correction to 
twice the latitude for an excess of sum of readings of north en 
over sum of south ends, or vice versa; applying this correction 
in north latitude, with the sign N.—S. I constructed a table, 
from which the correction for level could be taken by inspection ; 
the sum of the north end readings being arranged at the top, a? 
the sum of the south end at the side—the correction being found 
on a diagonal line. It has not, however, found favor with the 
observers: but they usually prefer a table of single entry, 1 
which the argument is the difference of the series of north and 
south end readings, and opposite the correction in arc. ; 
ere are two corrections, which it may be as well to notice 
in this place ; viz., for reduction to the meridian and refraction. 
If the line of collimation of the telesccpe is off the meridian, 
the reduction to the meridian employed in altitude and azimuth 
instruments, and in sextant observations, applies. . If the axis, g 
vertical wire, is in the meridian, and the star is observed before 
or after culmination, then the correction is such as is used Wi 
_—_ 
a 
— 
the zenith sector and with the meridian circle, viz.: 7=— 
