106 
The N.Z. Journal of Science and Technology. 
[Mar. 
The increase in length for half-span 
= s. 
s i = 
x (T 2 — tj 
E 
where E is the elastic modulus of the material. 
Also, from (4), 
X' 
6c, 
x * 
X' 
1 
1 
Sl 2 6c 1 2 6 lc 2 2 c t 2 
whence 
+ ( S 2- S l)' 
Taking successive values of T 2 in the first equation we obtain (s 2 — s\), 
T., 
and then substituting obtain c 2 . Then qm = — 
C 2 
The results are plotted in the upper portion of fig. 7, showing c as 
abscissae and qw as ordinates. From this the lower diagram is prepared, 
showing sag-deflection for various values of qw. 
Methods of Working. 
Generally speaking, at least two-thirds of the work in locating a trans¬ 
mission-line must be done by the engineer or some one competent to run a 
trial line. Hence, unless there is clearing to be done, it is not advisable to 
have a party of more than two—one chainman capable of running trial lines 
in the easier portions, and a reserve chainman—the engineer being free 
when required to prospect ahead of the line. 
Before commencing actual survey the engineer should have a thorough 
knowledge of the country for several miles ahead, and a general knowledge 
very much farther ahead, probably to the end of the survey. In running 
lines across country more often than not you have to work on the blind. 
In a line a mile long you may never be able to see more than 10 chains 
ahead. It is a good test of judgment in such cases to see how close one 
can go to a fixed object. 
Having selected a line, it is advisable to continue it till it becomes 
apparent how it may be improved. This may be done by going back a 
certain distance and angling, or going back the whole distance and off¬ 
setting. If the offsets are long or complicated it will be better to calculate 
the new bearing and run a fresh line. It is remarkable how in hilly or 
more particularly undulating country a very small offsetting will alter the 
whole character of the line. 
It should be borne in mind that the length of a line is not so much a 
matter of how many chains as how many poles and insulators are required. 
When ranging long lines with the prismatic compass, having fixed the 
bearing, the compass should not be depended on to keep the line straight, 
but rather wherever possible the line should be ranged with flagged poles 
or by some conspicuous mark on or near the sky-line. 
In conclusion, the following tables will be found useful in the field for 
testing doubtful points. 
The height of the point of suspension is taken as 31. ft. and the minimum 
clearance allowable 24 ft., leaving for flat country 7 ft. for sag-deflection. 
This corresponds with a span of 6 chains. In Table I and column 2 of 
Table II c is taken as uniform and equal to 4,100. 
