INTRODUCTORY DISCOURSE OF THE 



of distinction, but more properly Mechanical Philosophy) investigates the sensible motions of bodies. 

 The second investigates the constitution and qualities of all bodies, and has various names, according to its 

 different objects. It is called Chemittry, if it teach 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 tpeak of the nature of any thing), if it teaches the structure and functions 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 words signifying to speak of animals), if it teach the arrangement or 

 clarification and the habits of the different lower animals ; Botany, (from the Greek word for herbage), 

 including 1'ryetable Phytiology, if it teach the arrangement or classification, the structure and habits of 

 plants; Mineralogy, including Qeoiogy (from the Greek words meaning to speak of the earth), if it teach 

 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 tilings, or the observation of the resemblances and differences 

 of the various animals, plants, and inanimate and ungiowing substances in nature. 



But here we may make two general observations. Thefrst is, that every such distribution of the 

 sciences is necessarily imperfect; for one runs unavoidably into another. Thus, Chemistry shows the 

 qualities of plants with relation to other substances, 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 structure, like Comparative Anatomy. In truth, all 

 arrangement and classifying depends 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 chemical qualities of the other. From hence, in a great measure, follows the 

 tecond 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 Chemistry and Anatomy, especially the latter; and Chemistry 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^ower 

 or force, and it teaches the laws of motion 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 (or 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 onwards, 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 a 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 

 bed 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 mathe- 

 maticians, 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 each 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.* 



This if ei|irrrd mnthrmntirslly by saying, that the oquares of the times are at the cubes of the distances. Mathematical 

 lnxu>K<- is not only the sunplut and most easily understood of any, but the shortest also. 



