Force is generally defined to be “whatever produces or opposes the production of motion in matter.” In this sense, it is a name for the unknown cause of a known effect. It would, however, be more philosophical to give the name, not to the _cause_, of which we are ignorant, but to the _effect_, of which we have sensible evidence. To observe and to classify is the whole business of the natural philosopher. When _causes_ are referred to, it is implied, that effects of the same class arise from the agency of the same cause. However probable this assumption may be, it is altogether unnecessary. All the objects of science, the enlargement of mind, the extension and improvement of knowledge, the facility of its acquisition, are obtained by generalisation alone, and no good can arise from tainting our conclusions with the possible errors of hypotheses. It may be here, once for all, observed, that the phraseology of causation and hypotheses has become so interwoven with the language of science, that it is impossible to avoid the frequent use of it. Thus, we say, “the magnet _attracts_ iron;” the expression _attract_ intimating the cause of the observed effect. In such cases, however, we must be understood to mean the _effect itself_, finding it less inconvenient to continue the use of the received phrases, modifying their signification, than to introduce new ones. Force, when manifested by the mutual approach or cohesion of bodies, is also called _attraction_, and it is variously denominated, according to the circumstances under which it is observed to act. Thus, the force which holds together the atoms of solid bodies is called _cohesive attraction_. The force which draws bodies to the surface of the earth, when placed above it, is called the _attraction of gravitation_. The force which is exhibited by the mutual approach, or adhesion, of the loadstone and iron, is called _magnetic attraction_, and so on. When force is manifested by the motion of bodies from each other, it is called _repulsion_. Thus, if a piece of glass, having been briskly rubbed with a silk handkerchief, touch successively two feathers, these feathers, if brought near each other, will move asunder. This effect is called _repulsion_, and the feathers are said to _repel_ each other. (9.) The influence which forces have upon the form, state, arrangement, and motions of material substances is the principal object of physical science. In its strict sense, MECHANICS is a term of very extensive signification. According to the more popular usage, however, it has been generally applied to that part of physical science which includes the investigation of the phenomena of motion and rest, pressure and other effects developed by the mutual action of solid masses. The consideration of similar phenomena, exhibited in bodies of the liquid form, is consigned to HYDROSTATICS, and that of aeriform fluids to PNEUMATICS. CHAP. II.