Directive Paths.
An interesting field of ingenuity concerns itself with giving work the right start and a simple path. A tear in a sheet of paper accurately follows the line of a directive crease. Postage stamps, small as they are, we readily detach from one another because perforations give direction to the tearing strain. So the quarryman takes care to cut a V-shaped groove in the rock he is to break, along which groove the break takes its way. A bolt when over-strained will break in the thread, whether this be the smallest section or not, because the thread is a starting point for a parting. A rod of glass is divided with a slight jar, provided that a groove has been filed in its surface. In all this there is shown the importance of avoiding in a casting, or forging, such minute cracks as under severe strain may lead to rupture.