INTRODU 



[The present Treatise is intended to furnish introductions to the study of 

 different branches of Natural Philosophy, of the most elementary kind con- 

 sistent with accuracy. Many readers have now, for a considerable time, found 

 an excellent manual of this character in the well-known " Conversations on 

 Natural Philosophy." The Author and Proprietors of that work have, with 

 great liberality, authorized the Committee to use it freely for the purposes of 

 the present Treatise, which accordingly is entirely founded upon it, with hardly 

 any alterations except those which were necessary to adapt it to the form of the 

 publications of the Society.] 



SECTION I. On General Properties of Bodies.' 



THIS Preface being intended as an elementary introduction to the Science 

 of Mechanics, we shall consider our readers as entirely ignorant of natural 

 philosophy, arid endeavour to adapt our explanations to the comprehension 

 of the most uninformed minds. No branch of Natural Philosophy can 

 be understood without some previous knowledge of the general properties 

 of bodies ; we shall therefore begin by taking a survey of these properties. 



There are certain properties which appear to be common to all bodies, 

 and are hence called the essential properties of bodies : these are, Impene- 

 trability, Extension, Figure, Divisibility, Inertia, and Attraction. 



By impenetrability is meant the property which bodies have of occu- 

 pying a certain space, so that, where one body is, another cannot be, 

 without displacing the former; for two bodies cannot exist in the same 

 place at the same time. A liquid may be more easily removed than a 

 solid body ; yet it is not the less substantial, since it is as impossible for 

 a liquid and a solid to occupy the same space at the same time, as for two 

 solid bodies to do so. For instance, if a spoon be put into a glass full of 

 water, the water will flow over to make room for the spoon. 



Air is a fluid differing in its nature from liquids, but no less impene- 

 trable. If we endeavour to fill a phial by plunging it into a basin of 

 water, the air will rush out of the phial in bubbles, in order to make way 

 for the water ; for they cannot both exist in the same space, any more 

 than two hard bodies ; and if we reverse a goblet, and plunge it perpendi- 

 cularly into the water, so that the air will not be able to escape, the water 

 will not fill the goblet ; it rises, it is true, a considerable way into it, 

 because the water compresses or squeezes the air into a small space in the 

 upper part of the goblet ; but, as long as the air remains there, no other 

 body can occupy the same place. 



If a nail be driven into a piece of wood, it will penetrate it, and both 

 the wood and the nail will occupy the same space that the wood alone did 

 before ; but it must be observed, that the nail penetrates between the 

 particles of the wood, by forcing them to make way for it ; for not a single 

 atom of wood remains in the space which the nail occupies ; and if the 

 wood is not increased in size by the addition of the nail, it is because 

 wood is a porous substance, like sponge, the particles of which may be 

 compressed or squeezed closer together ; and it is thus that they make 

 way for the nail. 



