Tite, VATKLA TIONS OF (GEA CIERS. 287 
done once, after which all earlier and future photographs taken 
from these stations can be used to mark accurately on the map 
the glacier’s end at the date they were taken. If vertical angles 
also are measured, and the difference in altitude of the stations 
detenmined, contour lines can) be added to the map. ~ Phis 
method yields the greatest amount of information for the least 
amount of trouble. 
The stations should be selected so as to be as easily acces- 
sible and recognizable as possible; the distance between the 
glacier’s end and the line joining them should be between one- 
seventh and twice their distance apart; nearer the former if the 
g'acier is retreating, nearer the latter if it is advancing. 
In taking the photograph have the camera as level as pos- 
sible, with the plate vertical ; do not use the swing back, and do 
not alter the focus in taking two or more pictures from the same 
station. Hand cameras should rest on some support; a rock 
answers well. The longer the focal length of the lens, the better ; 
negatives or positives on glass yield more accurate results than 
prints, as the paper may become distorted in the manipulation. 
Variations of thickness.—(a) Select a station on the side of 
the glacier’s valley. Markit. Select a direction about at right 
angles to the glacier and record it by compass bearing or by the 
angle it makes with some easily recognizable point. Determine 
by aneroid barometer the difference in level between the station 
and the following points on this line: (1) the sides of the 
slacier; (2!) the highest point of the) glacier; (3) any other 
poimtiss teasiblew) It possible trecord the wditterence in) “level 
between station and the end of the glacier or some base station. 
(0) Observe by aneroid or measure the vertical distance 
between some easily recognizable point in the névé region, such 
as a projecting rock, and the glacier’s surface at its side. If 
possible note difference of level between this station and some 
base station. 
Velocity of flow.—Persons provided with instruments can 
readily determine the velocity of flow by sighting on a line of 
stakes or on some prominent bowlders at intervals of a few days. 
