Page:Miscellaneous Papers on Mechanical Subjects.djvu/35

Rh may be afterwards exposed. Hence, we cannot lay down any rule for choosing the diameter of the screw bolt required for a given purpose. Practical men can judge of the proper size with considerable nicety, but they have no means of ascertaining it with absolute precision.

If the diameter be given, and it be required to find the proper thread, the nature of the question is not essentially altered. The amount neither of power, nor of strength (nor any other condition), is thereby determined. A certain limit is assigned, but within that limit the proportions of strength and power, &c., may vary indefinitely, according to the actual formation of the thread.

There are three essential characters belonging to the screw thread, viz., pitch, depth, and form. Each of these may be indefinitely modified independently of the others, and any change will more or less affect the several conditions of power, strength, and durability.—The mechanical power of the screw depends on the pitch, which for a given diameter determines the angle of the inclined plane, and on the form of thread which regulates the direction in which the force applied will act.—The strength of the screw in the thread varies with each of the three characters; in the centre part, being as the area, it is little affected, except by change of depth.—The durability of