PROPERTIES AND FORCES.] 



INTRODUCTION TO PHYSICS. 



body. Some masses may be compressed, and will not regain entirely their previous form ; but a perfect elastic body 

 ia one which will oscillate between its original, a larger, and smaller form eventually, however, regaining its previous 

 size. A common ivory billiard-ball is one of the best illustrations of this class of bodies, and wet clay may be adduced 

 as an instance of those of an opposite kind. All gases are highly elastic, and, owing to atmospheric changes, are 

 perpetually undergoing alteration of bulk. Liquids are sparingly so ; and, in fact, seem to form an exception to these 

 laws ; but, by careful experiments, it has been ascertained that water is elastic ; for, having been compressed by 

 enormous pressure, it will regain its previous volume on the pressure being removed. 



The mind at once recurs to the numerous applications of the elasticity of different materials. The springs of 

 carriages, the buffers of the railway car, the passage of gas through the mains conveying it, the production of wind, 

 the main-springs of clocks and watches, <fec. all are instances wherein elasticity plays an active part. But, perhaps, 

 the most interesting object of this force is involved in the arrangement of the different parts of the human body ; 

 which, by the elasticity of their separate processes, permit any kind of motion to be safely undertaken, that would 

 certainly destroy the construction of the most perfect of human mechanical productions. 



SPECIFIC GRAVITY. A relative property of bodies is that called Specific Gravity ; and the term is employed to 

 express the fact that the same bulk of different substances varies in weight. Thus air weighs about ^th part of its 

 bulk of water ; whilst iron weighs more than seven times as much as an equal bulk of water. The various means used 

 for ascertaining the relative weights, called Specific Gravities, will be described as required in the progress of the work 

 an arrangement which will assist the student in learning the valuable application of the law, more especially in 

 reference to the processes of the chemist, the weight of machinery, <tc. 



CENTRE OF GRAVITY. This term is used to point out that part of a body wherein all the forces acting on it are 

 in equilibrium. In a globe or sphere made of materials of one kind only, the centre of gravity would be at the centre 

 of the body : hence, if the sphere were suspended by that centre, all its parts would hang round it at rest. If, how- 

 ever, we imagine a sphere to be made partly of wood and of lead, the specific gravity of the metal is so much greater 

 than that of the wood, that the lead, if not placed exactly in the centre of the sphere, would have the centre of gravity 

 somewhere in itself. The common toy, the Chinese tumbler, is constructed so that its centre of gravity shall be at 

 the lower part. The centre of gravity of an egg is so placed, that it will, if left to itself, lay sideways on a level 

 surface. If, however, its contents are violently shaken, the yolk will settle to one end, and the egg may easily thus 

 be made to stand upright, because its centre of gravity lias been shifted from its previous position near the centre. 

 The applications of the law will be fully considered under the head of Mechanics : it may, however, be interesting 

 to point out here some CMOS where its action is prominent. In walking, we are continually shifting the centre of 

 gravity of oar bodies : in the construction of railways, the lines are so laid as that, in curves, one rail shall be higher 

 than another, so as to prevent the train running off the line ; and, in loading ships, care is taken that the cargo shall 

 be so distributed as that the centre of gravity of the vessel shall be in the best position to insure speed and safety. 

 When a vessel is required to sail from one port to another without cargo, a quantity of coarse gravel, called " ballast," 

 is east into the hold, which thus maintains the centre of gravity low down in the hull. 



Having thus attempted to give an introductory view of properties and forces to which all matter is subject, we 

 proceed to detail those branches of Science which take cognizance of special forces, qualities, and phenomena ; and in 

 so doing, we shall confine our attention to those only which will be introduced and explained in these volumes, dealing 

 with them in the order in which it is proposed to arrange them. 



It has already been stated that force is required to set matter in motion ; but, beside the forces to which refer- 

 ence has already been made, and which affect only the external condition of matter, there are others which can modify 

 and change it iu so many ways, as to produce results which are of the highest importance in animal and vegetable 

 i\:~- . 



HBAT. The first which we shall notice is the power or agent called Heat one of the most active and necessary 

 in creation. We shall have to examine its effects on matter, in the solid, liquid, and gaseous state ; its production, 

 conveyance or conduction ; and other facts relating to its diffusion and general agency. It may hero be remarked, 

 that the term CALORIC is often used instead of heat, as being more in accordance with scientific phraseology. 

 Matural phenomena, such as Wind, Rain, Dew, Evaporation, <fec., will be dealt with under this head; and the varied 

 applications of heat, and the instruments used for its measurement, such as the Thermometer, will be fully 

 described. 



LIOHT. Under the head of Light, a vast variety of interesting phenomena arrange themselves ; such as colour, 

 reflection, refraction, polarisation, Ac. Photography, as one of the most interesting of the numerous applications of 

 Science, will claim an extended notice ; and ample directions will be given, so us to assist the reader in the pursuit 

 of the art. Optical Instruments, employed in scientific research, and for social and amusing purposes, will be 

 explained, both with respect to their construction and use. Among these will be included, Telescopes, Microscopes 

 of all kinds, the Magic Lantern and Dissolving Views, Cameras, Stereoscopes, Polariscopes, <tc. The production 

 of Artificial Light will also be extensively illustrated. In this branch of philosophical research some most in- 

 teresting discoveries have recently been made by Messrs. Bunsen and Kirchkoff, who, by means of the spectrum 

 analysis, have opened out an entirely now field in Chemical and Optical Science. 



ELECTRICITY. No branch of Science has made, during the present century, such rapid strides as Electricity. 



Its various divisions of Frictional and Voltaic Electricity will be examined, together with numerous natural phe- 

 nomena arising from their agency. Electro-Chemistry, Electrotyping, Plating, and Gilding, naturally fall under 

 this section ; and extended details of these processes will be afforded. 



MAORWISM. To the existence of this force we are indebted for the mariner's compas*, whose variation -will 

 require special notice. Terrestrial Magnetism and Dia-Magnetism will be investigated, and the leading facts and 



