/* = the necessary minimum height of mast in inches measured up to the sup-

porting point of the highest power line. This is determined from the
height of the uppermost postal wire, the allowance of 2 metres between
that and the lowest power line, the sag /„ the separation of the various
power lines from one another, the height of the line supports above the top
of the mast, and any difference in level which may exist between the foot
of the mast and the foot of the telegraph pole.
q = cross-section of power line in square inches.
p = greatest tensile stress in the line in lbs. per square inch.
Hg = pull of the lines acting in any one plane in lbs.
= sum of all the pulls H^ in lbs.
I = distance of the average point of action of the forces H^ from the top of the

mast in inches (+ if above the top of the mast and — if below it).
^k ! = total force in the crossing span or neighbouring span reduced to the height
Z \ of the top of the mast, in lbs.
^n-k = difference of the top pulls in lbs.
R = resultant of Z,j and Z,^ in lbs. for masts at corner points.
•^max. = the pull in lbs. used in the calculation of the mast.
L = height of mast measured to the top of the main corner stays in inches.
Z * X U

b = — - — =; i = deflection of the mast in inches when all the wires in one of

3 X E X J
the neighbouring spans break.
* Note. — See note on p. 185 for explanation of Z^.
