Use of the Zenith Telescope in Determining Latitudes. 197 
1. The latitude of the place is assumed to the nearest two or 
three minutes of are. 
- The zenith distances should be as small as practicable (the 
instruments now used admit of ready access to the zenith in obser- 
vations), and should not be extended beyond twenty-five degrees. 
The differences of zenith distance should be small, and in 
no case exceed a convenient range of the micrometer, say, ten 
minutes of arc, corresponding (in the instruments which we use) 
to about thirteen turns of the micrometer. 
4. The interval of time between the culmination of the stars 
of a pair should not be less than one minute; so as to give time 
deliberately to read the micrometer, and to turn the instrument in 
azimuth, to be prepared for observation ; and should not exceed 
about twenty minutes, to avoid changes in the instruments. 
_ 5. The interval between the pairs should afford time for read- 
ing the micrometer and level, and for setting the instruments for 
the next pair. This will vary with different observers ; but three 
minutes is about the time adopted by most of our observers. 
hen the intervals between pairs are unavoidably long, they are 
filled wp by observing transits for time. 
6. The VI4 magnitude is the least that admits of easy obser- 
vation, with the telescopes which we use hey are by Simms 
f London, have a focal length of forty inches, and object-glasses 
about three inches in diameter ; and we commonly observe with 
? 
the catalogue is to be consulted from the earliest AR which the 
daylight, at the time of beginning the series of observations, per- 
mits, to the latest hour at which it is desirable to observe. Within 
this period of AR, the stars are divided into belts by their declina- 
ions; the breadth of each being equal to that of the field of the 
instrument, and being paired accordingly, the corresponding de- 
clinations north and south of the zenith, as in the following table 
for Mount Independence, near Portland, Maine ; assumed latitude, 
° 45’, and field of the telescope 15’: 
TABLE No. L : 
cs Si Selection of Pairs for Observation with Zenith Telescope. 
in BAG N. or S| Mag. Auth. A.B. N.P.D. 
acy : hot. Be 46° oo! and 4 30! 
; 36 05 46° 27! 14! 
rom ob Vd Seas 46° 34! of!" 
en 2 “ | és I 18 02 57 pod pd AG 
20 : 2 2 
724u |. = vi 3 phe vv; a 46° 30! 14!" 
|_7681 N I a1 56 53 | 46 ‘ 
srevond cack (15! each side of zenith.) 
