to its different objects. It is called Chymistry if it teaches the properties of bodies with respect to heat, mixture with one another, weight, taste, appearance, and so forth; Anatomy and Animal Physiology (from the Greek word signifying to speak of the nature of anything) if it teaches the structure and function of living bodies, especially the human; for, when it shows those of other animals, we term it Comparative Anatomy; Medicine if it teaches the nature of diseases, and the means of preventing them and of restoring health; Zoology (from the Greek word signifying to speak of animals) if it teaches the arrangement or classification and the habits of the different lower animals; Botany (from the Greek word for herbage), including Vegetable Physiology, if it teaches the arrangement or classification, the structure and habits of plants; Mineralogy, including Geology (from the Greek words meaning to speak of the earth), if it teaches the arrangement of minerals, the structure of the masses in which they are found, and of the earth composed of those masses. The term Natural History is given to the three last branches taken together, but chiefly as far as they teach the classification of different things, or the observation of the resemblances and differences of the various animals, plants, and inanimate and ungrowing substances in nature.

But here we may make two general observations. The first is, that every such distribution of the sciences is necessarily imperfect; for one runs unavoidably into another. Thus Chymistry shows the qualities of plants with relation to other substances

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and to each other; and Botany does not overlook those same qualities, though its chief object be arrangement. So Mineralogy, though principally conversant with classifying metals and earths, yet regards also their qualities in respect of heat and mixture. So, too, Zoology, besides arranging animals, describes their structures like Comparative Anatomy. In truth, all arrangement and classifying depend upon noting the things in which the objects agree and differ; and among those things in which animals, plants, and minerals agree or differ, must be considered the anatomical qualities of the one, and the chymical qualities of the other. From hence, in a great measure, follows the second observation, namely, that the sciences mutually assist each other. We have seen how Arithmetic and Algebra aid Geometry, and how both the purely Mathematical Sciences aid Mechanical Philosophy. Mechanical Philosophy, in like manner, assists, though in the present state of our knowledge not very considerably, both Chymistry and Anatomy, especially the latter; and Chymistry very greatly assists both Physiology, Medicine, and all the branches of Natural History.

The first great head, then, of Natural Science, is Mechanical Philosophy; and it consists of various subdivisions, each forming a science of great importance. The most essential of these, and which is indeed fundamental, and applicable to all the rest, is called Dynamics, from the Greek word signifying power or force, and it teaches the laws of motion in in all its varieties. The case of the stone thrown

forward, which we have already mentioned more than once, is an example. Another, of a more general nature, but more difficult to trace, far more important in its consequences, and of which, indeed, the former is only one particular case, relates to the motions of all bodies which are attracted (influenced or drawn) by any power towards a certain point, while they are, at the same time, driven forward by some push given to them at first and forcing them onward, at the same time that they are drawn towards the point. The line in which a body moves while so drawn and so driven, depends upon the force it is pushed with, the direction it is pushed in, and the kind of power that draws it towards the point; but at present we are chiefly to regard the latter circumstance, the attraction towards the point. If this attraction be uniform, that is, the same at all distances from the point, the body will move in a circle if one direction be given to the forward push. The case with which we are best acquainted is when the force decreases as the squares of the distances from the centre or point of attraction increase; that is, when the force is four times less at twice the distance, nine times less at thrice the distance, sixteen times less at four times the distance, and so on. A force of this kind acting on the body will make it move in an oval, a parabola, or an hyperbola, according to the amount or direction of the impulse, or forward push originally given; and there is one proportion of that force, which, if directed perpendicularly to the line in which the central force draws the body, will make it move round in a circle, as if it were &

stone tied to a string and whirled round the hand. The most usual proportions in nature are those which determine bodies to move in an oval or ellipse, the curve described by means of a cord fixed at both ends, in the way already explained. In this case, the point of attraction, the point towards which the body is drawn, will be nearer one end of the ellipse than the other, and the time the body will take to go round, compared with the time any other body would take, moving at a different distance from the same point of attraction, but drawn towards that point with a force which bears the same proportion to the distance, will bear a certain proportion, discovered by mathematicians, to the average distances of the two bodies from the point of common attraction. If you multiply the numbers expressing the times of going round, each by itself, the products will be to one another in the proportion of the average distances multiplied by itself, and that product again by the distance. Thus, if one body take two hours, and is five yards distant, the other, being ten yards off, will take something less than five hours and forty minutes."

Now this is one of the most important truths in the whole compass of science; for it does so happen, that the force with which bodies fall towards the earth, or what is called their gravity (the power that draws or attracts them towards the earth), varies with the distance from the Earth's centre, ex

* This is expressed mathematically by saying that the squares of the times are as the cubes of the distances. Mathematical language is not only the simplest and most easily understood of any, but the shortest also.

actly in the proportion of the squares, lessening as the distance increases; at two diameters from the Earth's centre it is four times less than at one; at three diameters, nine times less; and so forth. It goes on lessening, but never is destroyed, even at the greatest distances to which we can reach by our observations, and there can be no doubt of its extending indefinitely beyond. But by astronomical observations made upon the motion of the heavenly bodies, upon that of the moon, for instance, it is proved that her movement is slower and quicker at different parts of her course, in the same manner as a body's motion on the earth would be slower and quicker, according to its distance from the point it was drawn towards, provided it was drawn by a force acting in the proportion to the squares of the distance, which we have frequently mentioned; and the proportion of the time to the distance is also observed to agree with the rule above referred to. Therefore, she is shown to be attracted towards the Earth by a force that varies according to the same proportion in which gravity varies; and she must consequently move in an ellipse round the Earth, which is placed in a point nearer the one end than the other of that curve. In like manner, it is shown that the Earth moves round the Sun in the same curve line, and is drawn towards the Sun by a similar force; and that all the other planets in their courses, at various distances, follow the same rule, moving in ellipses, and being drawn towards the Sun by the same kind of power. Three of them have moons like the Earth, only more numerous; for Ju