256 HYDROGRAPHICAL SURVEYING [chap. x. 



take this instead of the object itself. But it must be noted 



that unless this second object is nearly at the level of the 



observer, the angle will still be incorrect. 



Fig. 64 shows a rough field-board, before any shading is 



placed on the sides of the valleys, which will be done with 



the brush before the work is considered completed. 

 Effect of In sketching topography from a distance, passing clouds or 

 Shade ^ rain-squalls frequently throw up into strong relief details that 



would otherwise escape notice. The sun shining on a hill-side 



wiU often reveal details which may be absolutely lost a few 



hours later when the sun has moved round. 

 Baro- The fluctuations of the barometer should be noted hourly on 



^^^^^- board the ship, and allowed for afterwards in calculating 



heights from readings of the aneroid. 



The aneroid should be read at sea-level and at each station, 



the time being noted in each case. 



It is more advantageous to use the aneroid to obtain 



differences of heights at comparatively short intervals than to 



rely on its indications for absolute height. 



