ON THE STRENGTH OF TELEGRAPH POLES. 853 
On the Strength of Telegraph Poles. 
By W. H. Preece, F.R.S., M.Inst.C.k. 
[A communication ordered by the General Committee to be printed in extenso 
among the Reports. | 
THE resistance of timber to rupture has frequently been experimentally 
investigated both at home and abroad, but almost invariably with the 
view of determining the dimensions or scantling of beams, trusses, and 
framed structures. Few experiments have been made upon naturally 
grown trees with a view of utilising them in their native condition. 
Fincham’s! experiments on the relative qualities of timber used for 
the masts of ships were made upon cut pieces, 4 feet in length, and 3 
Square inches in sectional area. The strength of square balks was inves- 
tigated by Messrs. Tredgold, Barlow, and others, the results giving a 
constant of 1,341 for red pine in the formula 
4 
w= Ke BO ag Ra RE Rs laa 
where w = breaking weight in lbs. at end of bean. 
b = breadth in inches 
d = depth 3 
1 = length As 
K=a constant dependent on the character of the 
timber. 
The formula for round timber based on the same value of the con- 
stant K becomes 
¥ being the radius. 
The value of the constant quoted above was obtained by experiments 
on small sections of timber one and two inches square only. Wisely 
distrusting results obtained in this manner, Mr. Edwin Clark, during the 
construction of the Britannia tubular bridge, caused trials to be made 
with beams of red pine 12 inches square, which gave a constant of 810 
only, whilst experiments on balks of Baltic fir made by the Mersey Dock 
Board gave results varying between 771 and 873. “Mr. Gavey, during 
certain investigations made in Bristol in 1876, obtained a constant of 804 
_ from square sections cut out of Norway round telegraph poles. 
Telegraph poles in England now consist principally of native grown 
unhewn Norwegian or Swedish red fir preserved with creosote. 
They have acquired their pre-eminence by a species of natural selec- 
tion, after extended trials of cut Memel timber, native grown larch, and 
preserved Scotch fir. The Scandinavian red fir is now almost exclu- 
Sively employed. It seems to have been provided by nature specially for 
telegraphic purposes. 
* Papers on Naval Architecture, vol. i. pp. 53-4. 
