ON THE STRENGTH OF TELEGRAPH POLES. 855- 
D being the diameter ; 
whilst for poles of elliptical section it becomes 
2 
w=K has : : : : : : (4), 
D being the axis in the line of stress, and D, that at right angles to it. 
The value of K, for formule 3 and 4, as deduced from the mean of the 
experiments on new stout creosoted poles, is 765. 
It appears from these experiments that creosoting does not impair 
the strength of red fir, and age has had no apparent influence on its. 
qualities. 
The curves formed by the poles in bending were very perfect.” Rupture 
invariably took place at the ground line, and about half broke by fracture 
of the upper fibres under tension, while the other half buckled in the 
under fibres under compression, showing that nature had proportioned 
them well to their work. 
An important question that arises out of this inquiry is what shall be: 
the proper scantling of timber and the span separating the poles on tele- 
graph lines of varying number of wires. 
In solving this problem we have not only to take into consideration 
the proper factor of safety to be allowed, and the wind pressure to be 
exerted upon the wires and the poles, but the fact that the diminution of 
spans beyond certain limits is impracticable owing to the injurious lower- 
ing of the insulation by the multiplication of supports. 
The factor of safety is 4, and the maximum wind pressure per 
square foot is taken at 18°75 lbs., reducing the effective area in the ratio 
of ten to six, owing to the circular section of the wires. We know very 
little of the average pressure on a long wire supported close to the ground. 
Most records have been made in exposed positions, and at considerable 
heights, whereas the average height of a trunk line of wires is but 20 feet 
above the surface. The President of this Section (Mr. Baker) thinks 
30 ibs. per square foot a fair average to take for telegraph wires when 
very much exposed, but considering the frictional resistance of the ground 
and the obstructions due to hedges, banks, walls, trees, &c., I am in- 
clined to consider this too high for low levels. We have usually taken 
18°75 Ib. per square foot as a fair measure of the wind pressure on our 
wires, and the results of practice very much confirm this view. 
Table No. III. has been drawn up showing the strength of round 
poles one foot long, and varying in diameter from 6 to 13} inches. To 
obtain the strength of any pole it is only necessary to divide the tabulated 
breaking strain by the length of the pole in feet. 
The strength of the timber being satisfactorily settled, and a maximum 
wind pressure and factor of safety having been accepted, the length of 
spans for varying lines is readily calculated. Tables IV. and V. give the 
_ stresses on seventeen and twenty wire lines, so arranged as to fulfil the 
implied conditions. 
The experiments and calculations in this paper were done by Mr. 
Andrew Bell and Mr. John Gavey, of the Post Office Telegraph De- 
partment. 
