INTRODUCTION TO MECHANICS. iii 



small compared to the coals which have been consumed within it. In 

 this case, that part of the coals, which one would suppose to be destroyed, 

 evaporates in the form of smoke and vapour, whilst the remainder is 

 reduced to ashes. A body in burning; undergoes, no doubt, very re- 

 markable changes ; it is generally subdivided ; its form and colour altered ; 

 its [extension increased : but the various parts, into which it has been 

 separated by combustion, continue in existence, and retain all the essential 

 properties of bodies. Smoke, indeed, when diffused in the air, becomes 

 invisible, but we must not imagine that what we no longer see no longer 

 exists. Were every particle of matter that becomes invisible annihilated, 

 the world itself would in the course of time be destroyed. The particles of 

 smoke continue still to be particles of matter, as well as when more closely 

 united in the form of coals : they are really as substantial in the one state 

 as in the other, and equally so when, by being diffused in the air, they 

 become invisible. No particle of matter is ever destroyed: this is a 

 principle which must constantly be remembered. Every thing in nature 

 decays and corrupts in the lapse of time. We die, and our bodies moulder 

 to dust : but not a single atom of them is lost ; they serve to nourish the 

 earth, whence, while living, they drew their support. 



It should be observed, that when a body is divided, its surface or ex- 

 terior part is augmented. If an apple be cut in two, in addition to the 

 round surface, there will be two flat surfaces ; divide the halves of the 

 apple into quarters, and two more surfaces will be produced. 



Inertia, the next essential property of matter, expresses the resistance 

 which inactive matter makes to a change of state. Bodies appear to be 

 not only incapable of changing their actual state, whether it be of motion 

 or of rest, but to be^endowed with a power of resisting such a change. It 

 requires force to put a body which is at rest in motion ; an exertion of 

 strength is also requisite to stop a body which is already in motion. The 

 resistance of a body to a change of state is, in either case, called its inertia. 

 In playing at cricket, for instance, considerable strength is required to 

 give a rapid motion to the ball ; and in catching it we feel the resistance 

 it makes to being stopped. Inert matter is as incapable of stopping of 

 itself, as it is of putting itself into motion. When the ball ceases to move, 

 therefore, it must be stopped by some other cause or power, which we 

 shall presently explain. 



The last property which appears to be common to all bodies is 

 attraction, under which general name we may include all the properties 

 by which one atom of matter acts on another, so as to make the latter 

 approach or continue near the former. Bodies consist of infinitely small 

 particles of matter, each of which possesses the power of attracting or 

 drawing towards it, and uniting with any other particle sufficiently near to 

 be within the influence of its attraction. This power cannot be recog- 

 nized in minute particles, except when they are in contact, or at least 

 appear to be so : it then makes them stick or adhere together, and is 

 hence called the attraction of cohesion. Without this power, solid bodies 

 would fall in pieces, or rather crumble to atoms ; although we are so 

 much accustomed to see bodies firm and solid, that it seldom occurs to 

 us that any power is requisite to unite the particles of which they are 

 composed. 



The attraction of cohesion exists also in liquids : it is this power which 

 holds a drop of water suspended at the end of the finger, and keeps 

 the minute watery particles of which it is composed united. But as this 

 power is stronger in proportion as the particles of bodies are more 



