34 



HYDROGRAPHICAL SURVEYING [chap. i. 



detect a small error if the light falls across. They ought, of 

 course, to touch throughout their whole length. 



A steel straight-edge must be kept very clean, and carefully 

 wiped before using, or the paper will soon become very dirty. 



If kept bright, care must be taken that no emery is allowed 

 to touch the chamfered edge, or it will get so sharp in time 

 as to cut the pencil, and even the fingers of the operator. 

 When once clean, rubbing daily with a warm dry soft cloth 

 will keep it so, with an occasional rub of emery in damp 

 weather. 



Straight-edges are now generally supplied nickelled. 



Always to 

 be re- 

 tested 

 when 

 used. 



Points of 

 Measure- 

 ment. 



MEASURING CHAINS. 



To test measuring chains, which are generally 100 feet long, 

 100 feet should be accurately measured by beam compasses 

 along a chalked line on a plank of the upper deck, and marked 

 by nails driven in at intervals of 10 feet. 



Before measuring a base, aU the links of the chains should 

 be examined, and bent ones straightened ; the chains are then 

 compared with this fixed length and the errors noted. The 

 same reference should be made after measuring, and the mean 

 of these errors applied to the distance measured. 



It may be as well to note that, when the chain on com- 

 parison proves to be longer than 100 feet, the surplus is to be 

 added to each length measured, and when it is shorter, sub- 

 tracted. 



The length is to be measured from the outer side of one 

 handle, to the inner side of the other. This is to allow for 

 the necessity of having a pin to put in the ground at each 

 length. 



Each link is a foot long, and every tenth link is marked 

 by a brass label, with as many fingers on it as there are tens 

 of feet from the nearest end. 



PROTRACTORS. 



Protractors of all kinds must be tested for correctness of 



division by the same testing-sheet ruled for the station pointers. 



This is especially necessary in the case of Bullock's pro- 



