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PART I.*

OF ATTRACTION AND REPULSION,

AND THEIR APPLICATION AS MOVING POWERS; INCLUDING THE LAWS OF PNEUMATICS AND HEAT.

9. EXCLUDING the vital energy, then, there are two great powers which are (one or other, or both) concerned in producing all the motions and changes which we see going on around us-ATTRACTION and REPULSION.

10. As the latter, Repulsion, is called into action in an unusual degree in bodies which are heated, while its power seems to diminish in proportion as they are cooled, it has generally been regarded as identical with the influence which gives rise to the phenomena of heat.

SECTION I.

ATTRACTION.

11. THE universal influence, Attraction, which operates in drawing bodies and the particles of bodies together,

* The history and description of the Steam-Engine commence at paragraph 168. The previous part consists of a popular account of the laws of Heat and Pneumatics, and the Chemistry of Air, Fuel, Water and Iron.

and retaining them in contact, is of several kinds,* of which two chiefly must be attended to in the consideration of force or motion :-First, that which forms bodies into coherent masses, acting between their minute particles only when in contact (at insensible distances), called the attraction of cohesion, attraction of aggregation, or simply cohesion; illustrated by the firmness with which the particles of a piece of iron or marble adhere to each other: and, Second, that which brings and retains bodies near to each other, acting at sensible or apparent (indeed at all possible) distances, called the attraction of gravitation, or simply gravitation, illustrated by a stone falling to the ground when left in the air unsupported.

12. Probably the phenomena of every kind which consist in a drawing together or holding together of bodies, are the result of one fundamental power. But it is convenient to subdivide them, and to make distinctions between the different effects produced.

CHAPTER I.

ATTRACTION OF COHESION.

13. When we attempt to break a piece of wood, stone, glass, ice, or any other solid, we find that its particles are firmly bound to each other, and that the exertion of

* We here omit chemical attraction or affinity, electric attraction, and magnetic attraction. The first, acting between the particles of different bodies, unites them together, gives rise to new varieties of bodies, and to the phenomena of combination and decomposition; but is not a source of visible motion. The two latter give rise to distinct motion; but the moving power exerted has hitherto been considered

a considerable force is necessary before we can effect a separation. The force which binds the particles so firmly together, and which must be overcome by some superior force before we can break the solid, is spoken of as the ATTRACTION OF AGGREGATION, or ATTRACTION OF COHESION.

14. It is particularly to this form of attraction that the repulsive influence is opposed, as we see in water, which, when cooled (see note to paragraph 114), becomes ice, in which cohesion predominates, and the particles are firmly bound to each other, so as to form a solid; while the ice, when heated, again becomes water, in which the cohesive attraction is neutralised or overcome, and the particles are loosened, so as to be movable upon each other.

Application of Attraction of Cohesion as a Moving Power.

15. This force has never been used as a source of motion, except, perhaps, in the following remarkable instances, in which it was happily applied for that purpose: -The walls of a building in Paris had declined from the perpendicular, and were in danger of falling outwards, from the pressure of a heavy roof. By the following plan, suggested by M. Molard, they were restored to the upright position. A number of iron bars were stretched across the upper part of the building, passing freely through the walls. The bars were heated, in consequence of which they increased in length (57); and parts of the bars, at first within the walls, were now exterior to them. In this state the bars were fixed to the walls. They were then allowed to cool; when

unfit for use as a mechanical force, working through too short a distance, and not being easily procured. Attempts have lately been made, however, to render electro-magnetism efficient for this purpose.

cooled, they returned to their former size, and, being firmly fixed to the walls, necessarily pulled them inwards (towards each other); the contraction of the bars taking place gradually, but with great force. By repeating this process several times, the walls were restored to the perpendicular. Here the repulsive influence, repelling the particles of the bar, made it longer. When the bar had cooled, some power drew the particles back to their former distances. This force is considered the same as that which binds the particles of a solid so firmly together the attraction of cohesion.

16. The same means were used to save from destruction Armagh Cathedral, in Ireland, by restoring to the perpendicular the pillars, which were considerably inclined, and on the stability of which the whole structure depended; and have been applied with success for a similar purpose in Market Weston church, in Lancashire. The wall to be restored had declined about 19 inches from the perpendicular, and weighed upwards of 200 tons. These are very interesting and striking illustrations of the application of scientific knowledge to practical purposes, and of the truth of the fine saying-knowledge is power.

17. Though this force is not, in ordinary cases, made use of as a moving power-by giving materials rigidity, and strength, and firmness, so as to bear pulls, strains, thrusts, and pressure of every kind without yielding, it is an essential element in giving effect to other moving powers. Cast-iron pillars, chain piers, iron cables, steam-engines, suspension-bridges, are striking instances of the power of the cohesive attraction. The great

force of the cohesive attraction is well illustrated by the following table, showing the loads required to break (i. e. overcome the cohesion of) a prism, or cylinder, of one

square inch transverse section, of the following bodies, if

[blocks in formation]

The cohesive attraction, and friction (arising from the roughness of the surfaces of bodies), are the sources of that resistance, without which we could not have any control over motion, or power of regulating it.

CHAPTER II.

ATTRACTION OF GRAVITATION, OR GRAVITY.

18. The peculiar feature of those cases of attraction which are classed under "gravity," is, that the substances drawn towards each other are at distances apparent to our senses; or, if in contact, yet not so near as to be within the sphere of action of the cohesive attraction. A stone falling to the ground is an example of the attraction of gravitation. It is retained there with a certain force, and cannot be lifted without applying force the attraction between the earth and the stone is the force which retains it there. But it does not stick to the ground in the same way in which its particles adhere to each other; therefore, although, to our sense of vision, apparently in close contact with the ground, it is not so near as to be within reach of the cohesive attraction; not so close as the particles of the stone are to each other.

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