of the moon is the angle which must be added to, or subtracted from, the actual advance of the luminary in the heavens, in order to make its motion equable. Besides the equation of the center of the moon, which represents the first and greatest of her deviations from equable motion, there are many other _equations_, by the application of which her motion is brought nearer and nearer to perfect uniformity. The second of these equations is called the _evection_, the third the _variation_, the fourth the _annual equation_, The motion of the sun as affected by its inequalities is called his _anomaly_, which term denotes inequality. In the History of Astronomy, we find that the inequable motions of the sun, moon, and planets were, in a great measure, reduced to rule and system by the Greeks, by the aid of an hypothesis of circles, revolving, and carrying in their motion other circles which also revolved. This hypothesis introduced many technical terms, as _deferent_, _epicycle_, _eccentric_. In like manner, the theories which have more recently taken the place of the theory of epicycles have introduced other technical terms, as the _elliptical orbit_, the _radius vector_, and the _equable description of areas_ by this radius, which phrases express the true laws of the planetary motions. There is no subject on which theoretical views have been so long and so extensively prevalent as astronomy, and therefore no other science in which there are so many technical terms of the kind we are now considering. But in other subjects also, so far as theories have been established, they have been accompanied by the introduction or fixation of technical terms. Thus, as {266} we have seen in the examination of the foundations of mechanics, the terms _force_ and _inertia_ derive their precise meaning from a recognition of the first law of motion; _accelerating force_ and _composition of motion_ involve the second law; _moving force_, _momentum_, _action_ and _reaction_, are expressions which imply the third law. The term _vis viva_ was introduced to express a general property of moving bodies; and other terms have been introduced for like purposes, as _impetus_ by Smeaton, and _work done_, by other engineers. In the recent writings of several French engineers, the term _travail_ is much employed, to express the work done and the force which does it: this term has been rendered by _labouring force_. The proposition which was termed the _hydrostatic paradox_ had this name in reference to its violating a supposed law of the action of forces. The verb to _gravitate_, and the abstract term _gravitation_, sealed the establishment of Newton's theory of the solar system.