254 



HYDROGRAPHICAL SURVEYING [chap. x. 



Topo- send tw o men with 10-foot pole end-boards connected by a 

 Large 30-foot wire-cord to points which it is desired to fix witliin a 

 Scales. radius of half a mile or so. The distance of each point is deter- 

 mined by measuring with sextant or theodolite the angle 

 subtended by the 30-foot cord stretched taut between the two 

 men, one of whom stands fast, and the other walks slowly back- 

 wards and forwards on the arc of a circle until the maximum 

 angle is obtained. The height is found from the angle of 

 elevation and distance, and can be taken out of the height 

 tables supplied for work in the field. 



When the angle of elevation or depression is large, it must be 

 remembered that the distance found by the 30-foot cord 

 method is the hypothenuse, and must be reduced to the hori- 

 zontal distance, wliich is readily effected by means of the scale 

 for that purpose on Craven's protractor. 



Red and blue pencils are useful for topography. With the 

 blue we show streams, and the red is used for marking roads. 

 With only a black-lead pencil, the markings of these details 

 are apt to get confused with the contour lines to express the 

 hills. 



Much topography can be done with the pocket sextant and 

 compass only, the latter being only used, however, when three 

 objects to fix by cannot be got. The magnetic meridian, or 

 several magnetic meridians, must be ruled on to the rough 

 board, to permit the use of bearings. When the only objects 

 available are much above us or below us, correct angles cannot 

 be got with the sextant, and though we allow ourselves a 

 certain amount of latitude in our angles for the purpose of 

 topography, it wall often be necessary to take a small theodolite 

 for the purpose. A pocket sextant can be taken as well, and 

 the theodolite, which requires more time to set up and 

 arrange, only be used when the sextant angles Avill be too 

 erroneous. 



If we have a theodolite, we must take advantage of good 

 opportunities to get a series of elevations and depressions for 

 heights. 



In taking angles with a sextant to objects on different 

 levels, try to find some natural mark which is exactly above 

 or below, as the case may be, the object the farthest from 

 your level, and nearly on a level with the other object, and 



Red and 



Blue 



Pencils. 



Pocket 

 Sextant 

 and 

 Compass. 



Diffi- 

 culties 

 with 

 Sextant 

 Ajigles. 



