MANIPULATION OF THE TELESCOPIC ALIDADE 
127 
30° 00' when the telescope is level. Among topographers it is 
customary, in reading vertical angles, first to level the instru- 
ment and set the vernier at 30° 00', and second to turn the tele- 
scope down or up for the reading on the distant object. It is 
then necessary to record only one angle reading — that made 
after the cross-hair is set on the target; a reading less than 30° 
00' indicates an angle of elevation, one greater than 30° 00' an 
angle of depression if the arc is graduated from right to left. 
This procedure is not recommended for petroleum geologists, 
however, because of inherent differences in the work of these 
two classes of alidade-users. In the topographer’s party the 
lowest-paid man is ordinarily holding the rod on the station to 
which the sight is being taken. It is of little consequence 
whether he remains there four minutes or two. When he is 
moving on to the next station the topographer’s time is occupied 
with sketching contours; he has no idle moments. In the petro- 
leum geologist’s party, the reverse is the case. The highest- 
paid man ordinarily holds the rod; the amount of work the party 
can do in a day is in inverse ratio to the length of time he is 
kept idle while the instrument man makes observations. While 
he is moving on to the next station, the selection of which will 
ordinarily require 10 to 20 minutes, the instrument man has 
nothing to do except compute his results — a task which if neces- 
sary may be done later in ^The office.” Moreover, the geol- 
ogist’s plane table is customarily lighter and smaller than that 
used by the topographer; it is seldom possible to get it in a 
precisely level attitude. Therefore it would be necessary to 
level the telescope and set the vernier after the geologist has 
occupied the fore sight station and while he is waiting for the 
observations to be made. The procedure of the instrument man 
should be planned explicitly to minimize the length of time the 
rodman is kept at a station. Just as much of instrument work 
as possible should be done after the rodman has been waved 
on.” With this in mind, the instrument man signals the rod- 
man as soon as the cross hair is set on the selected mark on the 
rod; after the rodman has departed he reads and records the 
vernier, levels the telescope, reads and records the new position 
