; and, for the same unsupported heights, the cohesion would be about doubled. Evidently, if cohesion, which (to judge from Mr. Airy’s experiments) may vary from one to several hundred pounds per square foot, has to be guessed at in order to determine
, it is plainly better to guess at
at once.
The foregoing equation cannot be regarded as giving very accurate results, mainly because a plane surface of rupture is assumed, whereas, from both theory and observation, this surface is known to be very much curved; besides, the cohesion and friction along the ends of the break have been neglected. However, the hypothesis of a plane surface of rupture, the ends being supposed to be included, gives a greater value to
than the true one, whereas, neglecting the influence of the ends, it tends in the other direction; so that the equation may not err so greatly.
