612 THE AMERICAN JOURNAL OF SOCIOLOGY
varies accordingly. This principle is applicable not only to the purely mechanical forces, but also to the properties of inorganic, organic, and superorganic bodies, in so far as these properties result in the modification of the state of repose or of movement of the elementary parts. The sociologist, while studying social statics and dynamics, must always take into account that every force applied to a body always produces three different effects : First, a certain increase of the total velocity. Second, a certain total deviation.
Third, an instantaneous increase of velocity, or acceleration. In sociology, as well as in mechanics and physics and else- where, matter and force are abstract concepts of properties which alone are known to us. The essence of force and matter is confused with their effects. Through the latter alone, force and matter exist for us. It is by the effects alone that we are able to measure them, to compare them from the point of view of the third effect, acceleration. If different forces are necessary in order to produce the same acceleration in the movement of different bodies, the conclusion will be that these bodies offer an unequal resistance. Mass may then be considered as the measure of the resistance to the movement. As to the law action and reaction are equal, it implies that in nature no force exists in isolation. For every force whatever there always corresponds another force which is equal to it, and which has an opposite effect.
SECTION III. ASTRONOMICAL LIMITS.
The astronomical bodies have limited structures, limited movements, and influences which are correspondingly limited. Our globe is far from being homogeneous. Its mass is differen- tiated into parts whose frontiers are quite distinct. Its center is in a melted state. A solid envelope formed of superposed layers follows. This superficial crust, which is of relatively slight thickness, since it equals scarcely a thousandth of the terrestrial diameter, is the inclosed field upon which all organic beings vegetate and live. This limiting envelope is the result of relationships between the inorganic terrestrial aggregate and the external factors, astronomical, atmospheric, climatic, etc. The
