ASTRONOMICAL TRANSIT OBSERVATIONS AT CAMPS. 
55 
between this time and the right ascension of the star for the night, will give the approximate 
error of the chronometer. Apply this error to the right ascension of the first known circum¬ 
polar star that approaches the meridian. The estimated latitude of the place, plus or minus 
the star’s polar distance, according as the star happens to he above or below the pole, will give 
the altitude at which the vertical circle should he set, in order to find the star sought. This 
should he done several minutes before the computed time of the star’s meridian passage. If the 
star does not then appear in the telescope, sight along the upper edge of the tube; and if it can 
he seen with the naked eye, one may judge whether the instrument needs to he moved in azi¬ 
muth east or west. This motion should, if possible, he communicated by the micrometer screw 
attached to the Y. The star at length having been brought into the field of the telescope, the 
recorder, who watches the chronometer, states how many minutes are wanting to the time of 
computed meridian passage. The observer then turns the azimuth micrometer screw until the 
middle wire of the telescope is in advance of the star’s place, about equal to the distance over 
which he imagines the star will move during the time specified. Now, if able, while awaiting 
the signal from the recorder, who repeats the distance from the meridian in minutes of time by 
the chronometer, the observer, by the aid of the striding level, makes the axis of the instrument 
horizontal. He then places his eye to the telescope; and, as the star approaches the meridian, 
turns the azimuth micrometer screw; and, as the recorder repeats successively, “two minutes,” 
“one minute,” “thirty seconds,” “fifteen seconds,” brings the middle wire closer to the star; 
and when the recorder cries “time,” makes the middle wire bisect the star. The transit is now 
probably very nearly in the meridian. To test this, observe the passage of the next two known 
stars that pass the meridian; one high, near the zenith ; the other differing thirty or forty 
degrees from the first in declination. If nearly equal differences he found between the observed 
times of passage of these stars over the middle wire, and their right ascensions, respectively— 
that is, if the difference between the AR. and time of passage of the first star over the middle 
wire agrees, within half a second or less, with the difference of AR. and chronometer time of the 
second star’s transit—the instrument is sufficiently near the meridian for the night’s work. The 
adjustments should not, therefore, after this he disturbed until a complete set of observations 
has been obtained. This set consists in a record of the times of transit of every Nautical 
Almanac, or well known star that approaches the meridian, until one high and one low star, or 
two circumpolar stars, one above and one below the pole, have been observed for deviation of 
instrument, and about five near the equator for time. A record of the readings of the level 
should frequently be made for data to correct for inclination of axis. Should the moon appear, 
her bright limb will be observed when tangent to the wires; and the culminators of the Nauti¬ 
cal Almanac will be added to the list. The error in collimation of the optical axis is supposed 
to be small, before the commencement of the observation. To insure this, the telescope, when 
first set up and levelled, may be directed to some distant clearly defined point, and so adjusted 
that the middle wire may bisect and thread the object. The axis then being reversed, and the 
telescope again turned to the point, the apparent lateral distance of the wire from it is equal to 
twice the collimation error; which may be nearly corrected by the screws that hold the dia¬ 
phragm. The residual error should be nicely determined, at leisure, after the night’s usual 
observations have been completed. Polaris, or some other close circumpolar star, should be 
observed upon the first three wires, “A,” “B,” “C,” with illuminated end of axis east; then 
reverse the axis in the Y’s, and observe the star’s passage over the same three wires “ C,” “ B,” 
“A,” the error in level having been recorded for both positions of the axis. This operation 
will give data for the determination of error in collimation; for which, unless very small, the 
results for time should be corrected. The distance from each lateral to the middle wire, called 
the equatorial interval, should be obtained on first commencing the use of the instrument. Cir¬ 
cumpolar stars are supposed best for this determination, because a small error in the time of 
noting the observation does not appreciably affect the result. Upon a clear night, and with a 
steady instrument, Polaris or some other star should be observed upon each wire consecutively, 
