an 

 Aneroid. 



36 HYDROCxRAPHICAL SURVEYING [cHAr. i. 



the dial as greatly to facilitate the reading to two places of 

 decimals. 



Index An important point for delicate reading is the construction 



Needle. ^^ ^j^^ index needle. This should be very thin towards the 

 point, and turned with its edge at right angles to the plane 

 of the dial. In this position it should admit of very accurate 

 reading, and moreover assists the observer to hold the instru- 

 ment at right angles to his line of sight, and thereby to avoid 

 parallactic errors. For this reason the point, though as thin 

 and fine as possible one way, should be tolerably wide in the 

 other, so as to show plainly by its apparent increase of width 

 when it is being looked at from any direction but at right 

 angles to the dial. 



The point of the index needle should cover about half the 

 graduation of the arc. If it is too short to reach to it, or so 

 long as to project over it, it is not so easy to read accurately. 



Reading In reading, the best position for the aneroid to be held is 

 upright, on a level with the eye, the index being vertical. 

 Read with one eye only, or parallax will creep in. Tap gently 

 each time before reading, and turn the instrument flat, and 

 then vertical again for a second reading, to prevent mistakes, 

 tapping as before. In whatever position, however, the instru- 

 ment is read the first time, it must be always held for all other 

 readings on that day, the reason being that the weights of the 

 different parts in such a delicately made little instrument have 

 considerable influence on its free movement, and that this 

 influence must be so disposed as to act in the same manner at 

 each reading. 



Care of The surveyor wiU of course never let the pocket aneroid 



out of his own possession, and will place it about his person 

 in such a manner as to minimise chances of shocks or blows 

 in scrambling through rough country, getting wet in rainy 

 w^eather, or tumbhng out of the pocket. To prevent the latter 

 accident, always use a lanyard. The instrument should always 

 be carried in its case. 



If a small aneroid is not in regular use, the delicate internal 

 parts, especially the chain, are liable to stick from oxydization, 

 when at length taken up a height. It is therefore convenient, 

 if an air-pump be on board, to place the instrument under the re- 

 ceiver from time to time, so as to keep all working parts in order. 



Aneroid. 



