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Sect. III.] 



HYDEOGRAPHY 



75 



obtcdned with it,* or even with a sextant, by rneasurin 

 the angle subtended by a staff placed at right angles to 

 the observer, and the distance carefully measured between 

 two well-defined marks, one at either end of the staff (such 

 as the clean edore of a sheet of white paper wrapped round 



each end) : the best way of ensuring the staff being 

 right angles nearly, is to place it upright by a plumb' 

 line. Then treating the figure as a right-angled ti-iangle 



and the base the 



the staff will be the perpendicular, 

 distance between your eye and the station, which may be 

 readily computed, as all the angles and a side are 

 known ; but if a micrometer be used, the distance is that 

 between the staff and the ohjeet-glass of the telescope- 

 If neither of these methods be adopted, or if the field of 

 operations be very extensive, a base by sound, though 



r 



much less accurate, may be found convenient. 



19. The measurement of a base by sound, if several 

 trials are made, and the distance be more than a mile, will, 

 in most cases, be sufficiently exact for the above purposes. 

 If possible land a swivel or small gun upon one of your 



stations, and go yourself to the other, the more distant 



the better. Appoint a signal to be shown half or a 

 quarter of a minute before each explosion, in order that 

 the eye may rest between. When the signal is made 

 begin to note the beats,! but not to count until you see 

 the explosion, and then let the next beat be one^ and so 



s be 



done 



count up until you hear the report. Let thi 



several times, and at the end mean the beats, and turn 



filS 



\^t^' 



l.fS 





* See the Book of Tables and Directions supplied with an instrument 

 of this kind, by Rochon. 



f The stop-watch by Mr. Dent is very convenient for this purpose. 



E 2 



