Fig. 28.
Fig. 27.
91. Illustrated in Solid Bodies.—We can illustrate the limitation of size in solid masses by Figs. 27 and 28. Suppose that a and b, Fig. 27, are two projections of timber from a post, b being twice as large as a. It is evident that b can not support twice as much weight as a, for gravitation is dragging it downward from its attachment to the upright post with twice the force that it does a. The case is still stronger when, as represented in Fig. 28 (p. 64), the larger timber is twice as long as the smaller. Here d has four times the bulk of c. But it can not support four times as much weight at its end, not only because its own weight presses it downward, but because half of its weight is at a greater distance from the place of attachment than the smaller beam is. Gravitation here operates in opposition to cohesion in such a way that the projecting timber, if carried to a certain size, will fall by its own weight, either breaking in two, or tearing away from its attachment. This tendency is very commonly resisted in buildings and other structures by braces, as represented in Fig. 29. Here the weight of the horizontal timber at some distance on each side of a, is made to press upon the upright post instead of directly downward.
Fig. 29.
92. Farther Illustrations.—The size of bodies, both animate and inanimate, is limited by the Creator in obedience to the principles above developed. This is seen in the fact that there are no animals on the land to compare in size with the monsters of the deep. A whale does very well in the water, because he is buoyed up by that element; but an animal as large as a whale could not well exist on land, because gravitation would act so strongly in opposition to cohesion. At least it would be necessary, in order that he might walk about, or even hold together, that his great bulk should be made up of very firm and tenacious materials. Whenever any thing very large or tall is to be supported, its support is always broad and composed of very cohesive materials. We see this exemplified in the massive trunks of full-grown trees, as compared with the slender trunks of trees of the same kinds in the nursery. We see it exemplified in the fact that the highest mountains are built of the hardest rocks, while the soft chalk formations are confined to those of small size. There is a limit to the height even of the granite mountains in the influence of gravity. If carried much higher than they are, the attraction of the earth, in its opposition to cohesion, would tear them apart in their fissures, or cause the immense weight to crush their foundations. In the moon, where gravitation is less than on the earth (§ 85), mountains can be much higher without these results, and accordingly the telescope shows them to be so. In Jupiter, on the other hand, which is much larger than the earth, the mountains, if there be any, can not rise to any great height, and if there be any living beings as large as we are in that planet they must be made of vastly firmer materials to prevent their being crushed by their own weight.
93. The Above Principles Transgressed by Man.—Man often transgresses these principles in his structures. For example, a building settles because the foundation is not strong enough to bear the superincumbent weight; in other words, the force of gravitation is not sufficiently taken into the account. When a very tall building is erected, the lower portions ought to be made of very cohesive substances. Firm granite is therefore an appropriate material for the lower story of tall brick buildings. At least should the walls of the lower stories of such buildings be made thicker than they ordinarily are, to resist properly the force of gravitation in the weight above. Stores intended to bear much weight on their floors are often built without due regard to the cohesive force required to sustain the weight. Long timbers are sometimes supported only at the ends, when their own weight, to say nothing of what may be brought to press upon them, requires that they should be supported at other points. While in modern buildings the timbers are often too small, in some old buildings the upper timbers are so heavy as to lessen rather than increase the strength of the structure. Especially is this true of the unsightly beams which in some ancient houses we see extending along the ceilings above. Many other examples could be given, but these will suffice.
The practiced eye, in looking at a building, instinctively requires that every part should be seen to be suitably supported. A gallery in a church, therefore, if without pillars or braces from the wall, is displeasing to such an eye, even though there may be really sufficient support provided in the mode of structure. The same can be said of galleries supported by slender iron pillars, especially if they be painted of some light color so as to look as if they were wood rather than iron. For the same reason porticoes without pillars are unsightly. So, too, the eye instinctively looks for a sufficient base to every pillar and pilaster. The concealment of the base in any way, or the substitution of any thing else for it, is an unpleasant anomaly, and yet such anomalies are sometimes seen even in expensive buildings.