CHAPTER XVII. 
MARITIME OR HYDROGRAPHIC SURVEYING. 
INTRODUCTION. 
803. The object of hydrographic surveying is to fix the positions 
of the deep and shallow points in harbors, rivers, etc., and thus to 
discover and record the contour of the surface under the water, the 
shoals, rocks, channels, and other important features of the locality. 
The surveys on the water are usually based upon previously de 
termined points on the shore. In surveys of any importance, the 
relative positions of prominent points on the shore are first very 
precisely determined by Trigonometric Surveying, Chapter X. 
These form the basis of operations, and afford the means of correct 
ing the results obtained by the less accurate methods employed for 
filling in the details. 
The nature of the work makes special instruments and methods 
necessary. 
In addition to the surveying instruments already described, the 
sextant is much used in hydrographic surveying. When the sex 
tant is used for determining the position of a point, the angles 
are measured between three lines, passing from the required point 
to three known points. The required point is thus determined by 
trilinear co-ordinates, or by the fifth method, as explained in Article 
8, Part I. 
The Sextant. 
804. Principle. The angle subtended at the eye by lines passing 
from it to two distant objects may be measured by so arranging 
two mirrors that one object is looked at directly, and the other 
object is seen by its image reflected from one mirror to the second, 
and from thé second mirror to the eye. If the first mirror be 
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