56 
TRANSIT AND SEXTANT OBSERVATIONS AT CAMPS. 
from A to G, if the illuminated end of the axis he east, and from G to A, when reversed. The 
observed interval from a lateral to the middle wire, divided by the secant of the star’s declina¬ 
tion, will give the equatorial interval. The mean of several results should he taken in order to 
insure accuracy. Having the equatorial interval, a catalogue of stars should he constructed; 
giving, according to the declination of each, the true distance in time from each lateral wire to 
the middle wire. Having this list before him, the observer may at once convert the observation 
upon any one wire to what it should have been upon the middle wire itself. And there is also 
this advantage in acquiring thus hy induction the mean of all the wires observed: that, in case 
an error has occurred in one observation, the rest will visibly combine to prove that it ought to 
be rejected. 
The meridional transit observations having been completed, the observer will revolve his 
instrument ninety degrees,* and observe the transit of stars over the prime vertical for latitude. 
For this purpose, from an assumed approximate latitude of the place, should be prepared a 
catalogue of the zenith distances, and times of passage of stars over the first wire, on the prime 
vertical, east and west. To determine the deviation, it will be preferable to select stars that 
cross the prime vertical near the horizon. Only those that cross near the zenith will be used in 
the direct computations for latitude. Set the transit so as to catch the star to be observed upon; 
take four readings of the striding level, direct and reversed; and, should the instrument not be 
accurately placed in the prime vertical, the first wire may be made to bisect the star at its com¬ 
puted time of passage ; the signal being given by the recorder. The illuminated end of axis 
being north, wire A will first thread the star; and afterwards will be accurately noted the times 
of passage over B and C. The motion of the star being slow, there will be time to reset the 
striding level, and have four more readings of level error recorded. The axis of the telescope 
will now be reversed, and four readings again taken with the level. The star will then be 
watched, and the times noted when it passes the same wires, C, B, A, with the illuminated end 
of axis south. With another set of readings for level, the observations upon the east prime 
vertical will be completed. At the same altitude as the last observation east, the telescope will 
be directed west, in time to catch the star upon the first wire; and the times of passage over A, 
B, and C will be noted, with level readings, as before, preceding and following the observations. 
Then reverse the telescope axis and repeat the operation; first with the level, then observing 
the star upon the wires C, B, and A; a final levelling completes the process, which will give 
three results for latitude. Observations should he made on several stars crossing the prime 
vertical near the zenith, and the mean of the results taken for the true latitude. 
Latitude and longitude have been obtained by the preceding methods, with one transit instru¬ 
ment, in a single night. With a sextant and an artificial horizon of mercury, similar results 
may be accomplished as follows: 
Place the artificial horizon where the meridian and prime vertical are visible from thirty 
degrees altitude to the zenith. Dig a slight trench to isolate the cube of earth on which the 
mercury rests. Watch for bright stars to attain nearly equal altitudes of from thirty to sixty 
degrees, near the prime vertical, east and west, upon which to observe for time; and upon the 
meridian, north and south, for latitude. In places between thirty degrees and sixty degrees 
north of the equator, Polaris will always, when not concealed by clouds or mist, be available 
for latitude. Upon a clear night the observer may, therefore, be employed in observing that 
star, whenever compelled to wait for others to arrive in position. He will place a stool south of 
the artificial horizon and move his seat, and the direction of the glass roof till the star appears 
reflected from the mercury. Then, with the sextant clasped in his right hand, and elbow resting 
on his thigh, he will look through the telescope at the reflected image. Keeping the plane of 
the sextant truly vertical, with a sweep of the index with his left hand, twice the angle of ele¬ 
vation of the star, he will bring also into the field of view its image, as reflected from the index 
mirror. These two images being brought by the tangent screw directly in contact, so as to 
* The transit used would have been much more convenient for this purpose, had it rested upon an azimuth circle. 
