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PART IV. 
ASTRONOMICAL OBSERVATIONS. 
Necessity for Astronomical Observations. 
A traveller merely passing through a tract of country cannot hope 
to make more than a rough map of a belt extending a short distance on 
either side of his path. 
Upon the estimation of the length of his daily march, and of its mean 
direction, his map will mainly depend. 
The degree of accuracy of these two important factors will depend upon 
his experience, upon the trouble he takes to find means of ascertaining 
his speed, and upon his power of estimating the mean value of a courso 
made up probably of an infinite number of windings and deviations. 
When isolated or other well-marked hills exist, he may, however, on 
camping for the night, be able to get a bearing with his compass of an 
elevation at or near his point of departure in the morning, which will 
give a greatly improved value to the direction of his day’s march. 
It is, however, evident, that after a few days, especially in densely- 
wooded country, his position may be very much in error, and hence the 
necessity, if he wishes his map to be in any degree trustworthy, of fixing 
his position from time to time by astronomical observations, by sextant or 
otherwise. 
These have two objects: to obtain latitude and longitude. 
The latitude observations, hereafter described, are comparatively 
simple, and, in the case of latitude by meridian altitude, depend solely 
on the altitude observed. 
Longitude observations are, however, more complicated, and, whatever 
method is employed, with the exception of the moon culminating star 
method, all require accurate local time. This can be found by altitudes 
of the sun or stars at some distance from the meridian, noting the time 
by the watch, and by these observations the error of the watch on local 
time is obtained. 
By repeating the observation in the same spot after the lapse of a few 
