116 
HINTS TO TRAVELLERS. 
I wish to draw attention to the complete system of checks on the 
observations furnished by the above record. In the first place, there 
are two values of the azimuthal or traverse angle R — A and B 4 — A 2 , 
both of which should nearly correspond, and show only trifling differences. 
The subtended angle, or x, which is D divided by the number of 
repetitions, should correspond very closely with d lf d 2 , d 3 , and, as a check 
on the arithmetic, it should agree exactly with the mean of d lf d 2 , d 3 . 
These values are taken out during the progress of the observations, and 
should one of them show even a small discrepancy, the work must be 
condemned and done de novo. Again, A 2 — A 1 and B 4 — B must agree 
very closely. The checks are such that, by examining his record, the 
observer can make certain before proceeding to his next station that he 
has obtained the correct distance. Up to a mile he can detect any error 
made by the signalman in placing the bar at right augles, for it is only 
when exactly set that the black lozenge at the end of the sight-vane 
of the bar appears to him in the middle of the white patch on the bar 
itself. 
The signalman soon learns to place the bar sufficiently near the 
horizontal for practical purposes. An error of 2° of dislevelment, which 
would seldom occur in practice, would only produce an error of about 
three inches in a mile. 
The manner in which this method may be made applicable to other 
classes of survey is shown in Col. Tanner’s paper, published in “ Pro¬ 
ceedings of the Royal Geographical Society,” November, 1891. 
Surveying with the Theodolite. 
{For a description of the instrument, see pp. 23 to 35.) 
Traversing. —There are several methods of traversing with the transit 
theodolite: (1) by making any convenient point zero and measuring all 
angles with reference to it; (2) by making the station last left zero, and 
measuring all angles from it; (3) by making a line joining the second 
and first station zero and measuring all angles from that line. The 
principle involved in each of these is the same, viz., making zero with 
the lower set of screws and measuring all angles with the upper set of 
screws. The distances between each of the stations along the route 
traversed must be measured. 
