76 



HYDROGRAPHY. 



[Sect. Ill, 



them into seconds of time by the number of strokes your 

 watch makes in a minute* Then, by multiplying this 

 number of seconds by 1090, and adding one foot for every 

 two degrees of the thermometer above freezing-point^ you 

 will obtain the length of the base in feet. 



To give a direction to the base, the readiest way is to 

 observe the passage of the sun's limbs over the wires of 

 the theodolite nicely levelled, and to note the time by a 

 watch, or to take corresponding altitudes, in order to com- 

 pute the azimuth. If you have only a sextant, astrono- 

 mical bearing will be found convenient and sufficiently 



correct, provided the horizon can be seen at a sufficient 



. By either of these methods the angle between 

 the base and the limb of the sun will be known, and 

 hence the true bearing of the object obtained; or if 

 the magnetic bearing of the object be observed by an 

 azimuth compass, and the variation be determined at the 

 same time, it wdll still be known near enough for the 

 common purposes of navigation. Having arranged the 



gtb of the base, at A and B 



? 



measure angles between the base and all the stations, 



les (if measured with a sextant) 



are as 



nearly 



a; 



possibh 



parallel 



with the horizon, 



ti 



and at all other convenient stations do the same. By 

 his means the relative position of all the stations will 

 be obtained, from any three of which the position of an- 

 other, or of a boat for instance, may be determined by 



measuring 



two contiguous angles between them with 



a sextant. But whenever you have occasion to do this 

 (and in sounding there is no more convenient method of 



fixing the place of the boat), be careful not to select sta- 



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