NEUTRAL AXIS. The neutral axis of a loaded body is an imagi nary line, separating those portions of the body which are exposed to the action of opposite forces; as for instance, in the cases of girders, or occasionally of columns, there arc some laud= which are exposed to compression, and others which are exposed to become extended, whilst there are lamina: about which those forces of compression and extension are balanced in such a manner as to leave them in a neutral state, so long as the conditions of elasticity of the affected portions are not permanently injured. It had long been known, and has recently been proved by the investigations of Mr. E. that every body, or substance, used in the mechanical operations of building, or of machinery, has its own peculiar laws of elasticity ; and it therefore follows that the position of the neutral axis in any of them must vary in an almost endlrem variety of manners, according to the shape of the particular body experimented upon, or to its physical properties. Thus wood has been ascertained to resist efforts of compression in nearly the same manner that it resists efforts of extension ; whereas, cast iron resets the former class of efforts with a far greater degree of energy than it does the latter class; and wrought iron, again, ie known to compress under smaller efforts than are required to extend it.
Under these circumstances, the position of the neutral axis would vary in beams of either of those materials, if the same form of section be employed ; or, in case it should he required to cause the neutral axis to pass through the same points, it would be necessary to change the form of the body, so as to bring into play the peculiar elastic, pro perties it may possess in such a manner as to allow the sum of the resistances to contraction and expansion to balance one another on either side of the line of the immovable fibres. In rectangular beams of materials of an equal degree of elasticity under any description of effort, that is to say, under efforts either of compression or of exten sion, the neutral axis will be found to pass through the centre of gravity of the loaded beam ; and In all cases the ratio of the moments of the material undergoing compression and extension about the neutral axis must be made equal to the ratio of the forces of extension and compression to secure stability. (RESISTANCE.]