GANNETT.] AZIMUTHS OF POLARIS. 25 
The attention of the observer is directed to the fact that he should 
always use one day of twenty-four hours as the unit when he subtracts 
the time of cuhnination from the time of observation. In any case 
when the time of upper cuhnination, taken from table 1, for the given 
date would be numerically greater than the astronomical time of 
observation, the former time will be taken out for a date one day 
earlier than the date of observation. The surveyor will decide when 
such condition exists by comparing the time given in the table with 
his astronomical time of observation. (See Example 4 and explana- 
tions in footnotes, page 26.) 
The watch time to be used when making observations on Polaris at 
all times except elongation should be as accurate as can be obtained. 
Looking at table 3 near top of page 22, the surveyor will observe 
that for a difference of four minutes in the time argument there is a 
change of about two minutes in azimuth; consequently, to obtain the 
azimuth to the nearest whole minute of arc, the local mean time, upon 
which all depends, should be known within two minutes. When the 
observer uses standard railroad time he will correct the same for the 
difference of longitude between his station and the standard meridian 
for which the time is given at the rate of four minutes of time for 
each degree of the difference in arc. Thus, if the difference in longi- 
tude is 6° 45', the equivalent in time will be twenty-seven minutes. 
The difference of longitude may be taken from a good map. The cor- 
rection will be subtracted from the standard railroad time of observa- 
tion when the surveyor's station is west, or added when east of the 
standard meridian, as the case ma\^ require, to obtain local time. It is 
immaterial where the surveyor obtains the standard time provided he 
gets it right, a result which will be gained most easily by a direct 
personal comparison at a telegraph office. 
If the direction of the meridian is known with an error not greater 
than one-fourth of a degree, the local time can be obtained to the 
nearest minute by observing the sun's transit by the following method, 
suggested by Mr. H. L. Baldwin, jr. 
The transit being in meridian and carefully leveled, place the tele- 
scope so that it will point toward the sun at the time the latter comes 
to the meridian and allow the magniffed image of the sun to fall upon 
a notebook or sheet of white paper about 1 foot distant from eyepiece. 
The telescope should be slightly out of focus (lengthened) to get best 
results, the best focal position to ])e determined by trial. When the 
vertical cross wire bisects the sun's image, note the time by watch. 
This will be the time of apparent noon. To get time of mean noon, 
correct the noted time by adding or subtracting the equation of time, 
taken from the Nautical Almanac "to reduce apparent noon to mean 
noon," or get this from any almanac giving "sun fast" or "sun slow" 
time. 
