PHYSIOGRAPHY. 1. The matter composing our globe presents a great variety of forms; so much so, that it might seem as if there were an endless variety of distinct substances in the world. But it is not so; the number of distinct elements, as we shall see, is comparatively small; and the great variety of nature arises chiefly from the varying action of the three forces by which the elements are gathered and held together in groups or bodies. These forces are Gravitation, Cohesion, and Chemical Affinity. GRAVITATION. 2. No appearance in nature is more familiar than that of objects falling downwards when unsupported. Some invisible force seems to draw or attract them towards the earth. If such an object is prevented from descending by being held in the hand, the invisible agent makes it press against the hand and feel heavy. It is from its giving heaviness (Lat. gravitas) to bodies that the invisible force, whatever it is, is called Gravitation. 3. Every Body has weight, and heavy and light are only comparative terms. Air itself, the very type of lightness, can be shown to have weight. If a hollow glass globe, holding a cubic foot, is emptied of air by the air-pump and weighed, on admitting the air again, it will be found to weigh 537 grains (about 1 ounces avoirdupois) more than before. If instead of air, we admit hydrogen-the lightest substance known-there will still be an increase, but only about th of the former, or 37 grains. 4. Why some Bodies ascend.—That some bodies ascend instead of falling, is no proof that they have not weight. A piece of wood or a cork held below water, mounts up when released. The wood and the cork have both weight—that is, they tend downwards; but the surrounding water is heavier than either, presses in below, and lifts them up. In a similar way, a balloon, which is filled with a gas lighter than air, and smoke, consisting of air rendered lighter by being heated, are forced upwards by the superior weight of the surrounding air. Thus it is the same force-namely, the attraction of the earth-that makes a cascade of water descend, and a column of smoke mount up. It 5. Gravitation universal.-It was long before the attention of men extended beyond such effects of gravitation as those now instanced, where the attraction is between the earth and bodies on or near its surface. was reserved for Newton to raise these seemingly trivial and partial facts into a grand law extending to the whole universe. It has been established by Newton and succeeding philosophers, not merely that the earth draws towards itself all bodies that are near its surface, but that every body draws every other body wherever situated; or, more generally, that every particle of matter in the universe draws every other particle towards itself, and is in its turn drawn towards every other particle. 6. Why this is not readily seen.-If this is true, you might expect that, when two lead balls are suspended near each other, they would be seen to come together, or, at least, to incline towards each other. But no such effect is visible. The reason is that the attractive force of the earth is so overpowering that, by the side of it, the action of small bodies on one another is, in ordinary circumstances, not discernible. But by means of delicate apparatus the attraction of a heavy ball on a light one can be made manifest; and when a plumb-line is hung up near a mountain, it is sensibly drawn from the perpendicular. 7. Attraction according to Mass.-The relative attracting power of bodies depends upon their quantity of matter, or upon their mass, as it is called. This is obvious; if every particle exerts an influence, the more particles a body contains, the sum of their attractions will be the greater. Hence the overwhelming attractive force of the vast globe. 8. Mass and Weight.—It is usual to consider the weight of bodies as a measure of their mass; and when bodies are compared at the same place and under the same circumstances, this is sufficiently correct; but otherwise it is not so. If we suppose a man holding a pound of lead in his hand transported to the surface of the sun, the lead would feel 27 times as heavy as it did on the earth; and yet the mass is the same. There is also a slight difference in the weight of the same bodies when weighed at the poles of our earth and at the equator. Bearing this in mind, we may in ordinary cases take the weight of bodies as the measure of their mass. 9. Specific Gravity.-When we compare bodies of the same uniform substance, their mass must evidently be proportional to their bulk or volume. Three cubic inches of iron have three times the mass of one cubic inch. But a cubic inch of iron has less mass than a cubic inch of lead; in fact, it takes 100 cubic inches of iron to equal in weight or mass 69 cubic inches of lead. The relative weights of bodies when the same volume is taken, are called their Specific Gravities; which furnish an important distinction among substances. 10. In comparing the relative weights of bodies, water is taken as the standard. The weight of a certain measure, say a cubic inch, of pure water is found, and called 1; when a cubic inch of gold is weighed, it is found to be 19 times heavier than the water; the specific gravity of gold is therefore 19. The following table gives the specific gravities of a few of the more important substances: 11. Attraction Mutual and Equal.-The attraction or pull between any two bodies is mutual and equal. If the earth pulls a suspended stone towards it with the force of a pound, the stone is pulling the earth upward with the same force. This is seen in the case of other forces. If, sitting in a boat, you pull another boat towards you by a rope, both boats, if they are equal, will move equally toward each other. Why, then, does not the earth fall towards the stone, as well as the stone towards the earth? The cause why no such motion is seen, lies in the property of matter called inertia. By this property a body at rest resists being set in motion, or, if in motion, resists being stopped; and the amount of the resistance is in proportion to its mass. Hang up two weights, the one of 1 lb., the other of 100 lbs., and note the difference of pressure it requires when you push first the one and then the other, so as to give them a |