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. 94. =Attraction of Natural Philosophy and of Chemistry.=--The attraction of which I have treated in this and the previous chapters is that which belongs to Natural Philosophy, in distinction from that of Chemistry. Its effects are only mechanical, while the attraction of Chemistry goes beyond this, and affects the _composition_ of substances. For example, the attraction between the two gases, oxygen and hydrogen, which makes them unite to form water, belongs to Chemistry; while that which makes the particles of water cohere is in the province of Natural Philosophy. CHAPTER VI. CENTRE OF GRAVITY. [Illustration: Fig. 30.] [Illustration: Fig. 31. Fig. 32.]