3. We come now to consider the dependence of Architecture on mechanical philosophy.

The design of an edifice may be perfect; the estimates and plans may be drawn with the utmost accuracy; and the materials collected may be of the most durable kind: and still, unless the masonry and the carpentry are executed according to nice mechanical principles, the edifice may of itself tumble into ruins. Gravitation is a great law of Nature, which may contribute, according as a building is constructed, to its stability and permanence or to its dilapidation. To render it firm, the centre of gravity of the mass must be directly over the base; and hence the necessity of upright pillars and columns, and of a limited height. The weight of the superstructure must be supported by a sufficient foundation, and points in the building, at which there is a great stress or strain, must be proportionally strengthened. Timbers, which are to support weight, such as the girders of a floor, &c., must be so constructed, that they shall present the necessary support with the least possible material. Arches are to be secured at the points where the pressure accumulates, and composed of materials not likely to crush or yield under great weight.

It is obvious, therefore, that the architect should be able to foresee what pressure will fall upon his foundation; to what degree of strain or stress the different parts of the superstructure will be subject; at what points this strain will accumulate, and by what materials, and by what shape, and what disposition of them, it can be resisted. We can hardly hear these questions, without perceiving that they involve very numerous and difficult principles, to ascertain which has exercised the ingenuity and science of the profoundest mathematicians, as well as of the most accurate experimentalists. Our limits do not allow us to multiply illustrations; but we should do injustice to the subject, if we did not briefly refer to some of the discoveries which have been made respecting the arch, the proper form and adjustment

of columns intended for support, and the limit which has been fixed, by the laws of Nature, to the magnitude of edifices, and to that of their several parts. This will afford us an opportunity of showing, from the architecture of the human body, that the finest efforts of human skill are but humble copies of that of the Creator.

CHAPTER IV.

ARCHITECTURE CONTINUED.

(a.) THE arch was unknown in the architecture of the Egyptians and Persians, and probably in that of the Greeks. We first meet with it in the structures of the Romans, by whom it was employed in bridges and triumphal edifices, though not with much skill, yet on a magnificent scale. We shall understand the principle of the arch, if we conceive a number of pieces of stone or brick, shaped like obtruncated wedges, (that is, not sharp,) joined together by their faces, and all pointing downwards. They must evidently form a curved mass, which, if supported at the extremities, will not only stand, but will be rendered more firm by any weight pressing upon its top, since this weight is made to compress all the parts at once, and nearly in the same degree. The supports on which an arch rests are called piers, or abutments. The whole pressure evidently centres at these points, tending not so much to sink the abutments, as to spread them apart. Hence the care with which the abutment is secured, by anchoring it deep in the earth, or loading it with

* It is maintained, by some, that the arch must have been known to the Egyptians at a very early period, since it is found among the ruins of Thebes, &c. It is doubtful whether, in the instances referred to, the structures are very ancient. The arch seems to have been an Etruscan invention, and to have been employed in the Cloaca Maxima at Rome, as early, many suppose, as the time of the Tarquins.

heavy weights. While it retains its place, any pressure, applied downwards, at the top of the arch, or even on its sides, if it be of the proper form, will only tend to bind the stones, which compose it, more closely together. The advantage which the arch has, for resisting pressure, may be seen in the common watchglass face, which, if plane, would evidently be very liable to break; also, in the superior strength of the round junk bottle, as compared with those which have flat sides and square bottoms; and above all, in the impossibility of breaking an egg, by pressing it endwise between our hands.

Now, it is a striking fact, that those parts in the human body, which are most liable to pressure, are constructed on the principle of the arch. The foot, Fig. 36,

for example, which is required to give a firm and unyielding support to the body, is composed of three arches; one extending from the heel to the toe, and resisting pressure in that or the opposite direction; Another across the foot, and another horizontally, or, as it were, around from the ball of the foot to the heel. These arches are composed of bones, wedged together like the courses of stone in masonry, which, though movable in some positions of the foot, become perfectly immovable, when the weight of the body bears directly over it, and when, of course, a firm foundation is most needed.

So, again, the skull, Fig. 37,-which is peculiarly exposed to injury, from falls, blows, &c., and which, at the same time, covers the most important organ of the body,-forms an arch, or, what is essentially the same,

each other, at the top of the head, and form an arch. This arch lies, at its extremities, on two bones, called temporal bones, which answer as abutments; and it is most interesting to observe the provisions made to prevent the spreading of this arch, which would result from a load or pressure being laid upon the top of the head. In the first place, the lower or temporal bone

laps over the parietal, so that the latter cannot be thrust out horizontally, without breaking it; a contrivance similar to the pinnacles and buttresses in Gothic architecture. In the second place, a bone, called the sphenoid bone, together with the temporal bone, passes across the head, from side to side, just as, in the roof of a house, the tie-beam is used, to prevent the rafters from spreading the walls; or as, during the construction of an arch, the straining-piece is used, to prevent the sides of the arch from being crushed in. It is worthy of observation, too, that those parts of the head, which would strike upon the ground when a man falls, namely, the centre of the forehead, the projecting point of the skull behind, and the lateral centres of the parietal and frontal bones, are strengthened, just as we strengthen those joints in an arch, which are exposed to the most pressure; that is, by increasing their convexity and thickness, and also by ridges of bone on the inside, which correspond with braces used by carpenters at the angles in the centring of a bridge, or the frame of a roof.

(b.) We proceed to notice one or two of the principles which regulate the use of columns in architecture. Perhaps the most curious is that discovered, I believe, by Galileo, which teaches, that, if a given amount of material is to be made into a column of given length, it will be strongest when it takes the shape of a hollow cylinder. The advantage which this form has, in resisting pressure, is twofold: first, a small quantity of matter, when employed as a column, for support, is not so likely to bend, if arranged in a hollow cylinder, as if it were condensed into a solid one, of much smaller diameter. Secondly: it is, under the same circumstances, less likely to break. To illustrate this: when a column, or other piece of timber, bends, and breaks, it is evident, that a twofold effect is produced. On the one side, the particles are crushed into one another; and, on the other, they are torn asunder, like the snapping of a rope: so that, betwixt the portions acting in a manner so different, there is an intermediate neutral