190 
HINTS TO iilAVELLHM 
ivest; if the contrary be the case the telescope must be moved to the easl. 
When the transit of the low star is observed first and the interval shown 
by the watch is less than the difference of the stars’ right ascension, the 
telescope must be moved to the east; if the contrary is the case, the 
telescope must be moved to the west. This must be repeated until 
the sidereal interval, computed from the watch times of transit, and the 
difference of the stars’ right ascensions taken from the ‘ Nautical 
Almanac,’ agree exactly; the telescope will then move in the plane of the 
meridian. Select a star as near the zenith as possible for the -“high 
star,” as when the instrument is truly level the telescope will be on the 
meridian when pointing to the zenith, no matter how much it may differ 
from the meridian when in any other position. 
By Meridian Passage of any Star .—Any star may be used if the local 
time is accurately known, and the time of the star’s meridian passage 
carefully computed (as shown p. 140). The observation is precisely the 
same as for the pole star, but it would be well to take more than one 
star in order to correct any errors that may have been made in observa¬ 
tion or computation. Though the results of such observations as this 
are susceptible of a great degree of precision, yet absolute accuracy must 
not be expected. 
By Stars East and West of the Meridian.— If local time is not accu¬ 
rately known, the true meridian may be found in the following manner:— 
Carefully level the transit theodolite, and set the 360° division as nearly 
true north as you can get it by the attached magnetic needle, then clamp 
the lower plate, and unclamp the vernier plate; select any star at some 
considerable distance east of the meridian, and cover it with the inter¬ 
section of the threads in the diaphragm, clamp the vertical circle , and take 
the reading on the horizontal plate; then, after the necessary interval, 
watch the star until it is again covered with the intersection of the 
threads in the diaphragm west of the meridian, take the reading, and 
then the theodolite will point just as far west of the meridian as it 
originally did to the east, and a point midway between these two horizontal 
readings will be in the true meridian. Care must be taken to keep the 
vertical circle and the lower plate clamped during the interval between 
these two observations. Having thus found the true meridian it can be 
marked as previously directed. Owing to the constant change in the sun’s 
declination it is unsuited for finding the meridian by this method. 
