Pocketbook of Useful Formulæ and Memoranda
for Civil and Mechanical Engineers

316 Molesworth’s pocket-book Telegraph Construction, by R. S. Brough— continued. By means of these formula the maximum ap^le admissible with an insulator of given strength when employed to support a wire of given weight can be calculated, thus : * L. o- i - 2 sin., . Terminals.—The horizontal strain on a terminal insulator due to the wire is P = t cos. i = t approximately. § m. Strength of Posts. Let P be the resultant strain at right angles to thø post, whether due to change of direction of alignment, wind pressure, terminating of wires, &c., and let h be the height of the centre of pres- sure above the ground line, then the ßtrejigth m required at the ground line must be at least such that m is not < P x h. If x be the factor of safety adopted, then The strength of a post depends (1) on the form * Measure radius = 57'3 feet from bottom of post; then, the number of feet in chord will approximately give the number of degrees in angle.—(W. P. Johnston.)