72 HYJ)ROGRAPHICAL SURVEYING [chap. m. 



when the observer will register the largest angle he observes 

 as the correct one. 



The angle observed should not be smaller than 1°, which 

 with a distance of 20 feet will give a base of over 1,100 feet. 

 It would be better, however, if practicable, to get a base by 

 means of a longer distance, and a larger angle than this, when 

 a very trustworthy result will be obtained ; or to be content 

 with a shorter base, and extend it by angles, as already de- 

 scribed, to a longer working base. 



Measurements must be made on and off the arc, and it would 

 be well to use more than one sextant. 



Small lengths of this kind may also be measured by a micro- 

 meter, but a sextant will give just as good results, and is in a 

 ship always handy. 



No appreciable error wdll be introduced by taking distance 

 — length of pole x cot. angle. 



BASE BY RANGE-FINDER. 



The writer has had no practical experience with this in- 

 strument for surveying purposes, but it is likely to prove 

 useful in a variety of ways, and might be used to obtain a 

 base when it is not convenient to measure it by more accurate 

 methods. 



The 9-feet Barr-Stroud range-finder should give a more 

 reliable result than that obtained by sound. At a distance of 

 about 2 miles, under favourable conditions of light and weather, 

 and using a well-defined object, such as a lighthouse or flag- 

 staff, the probable error of the mean of a number of observa'- 

 tions by a skilful observer may be expected not to exceed 

 1 per cent, of the distance measured. By placing the ship 

 midway between the two base stations, a longer base may be 

 measured without increasing the percentage of error. 



The 9-feet range-finder may be used effectively up to a 

 distance of about 4 miles, but the probable error rapidly 

 increases with the distance. 



The Navigational range-finder, 31 1 inches in length, is port- 

 able, and can be used without a stand. It is unsuitable for the 

 measurement of a base of greater length than about 1,000 



