LESSON THE FIRST.
"If you please, Paul, we will take our lessons walking, and for a good reason." This arrangement was quite satisfactory to Paul, who was certainly not accustomed to this mode of teaching at the Lyceum. The prospect of a course of lessons de livered, re-produced in writing by the pupil, and corrected indoors, had not seemed to him at the first blush quite to harmonize with the idea which a youth of sixteen forms of hours consecrated to recreation; and although after his first attempts architecture seemed to him a very noble study, and he was proud enough to think that his plan was perhaps at this moment being inspected by his sister Marie and her husband, yet, at the moment he was directing his steps towards his cousin's apartment, he had looked with a somewhat longing eye at the fine old trees in the park, and the brilliant green of the meadows between their dark trunks. A sigh of satisfaction escaped him as he tripped down the steps.
"Let us proceed leisurely towards that part of the estate where we are to build the house,"said his cousin, as soon as they were outside;"a knowledge of the ground is in dispensable to the architect's further progress. There are, as you know, several kinds of soils; some resisting, others soft and compressible in various degrees. Rocks form the firmest foundation - one on which we may build with con fidence - provided they have not been excavated or dis turbed. The name of virgin soil is given to that which presents• itself in the condition in which geological phe nomena have placed it; that of made ground' to soil which has been disturbed or deposited by man, or pro duced by vegetation, or brought to the spot by the sudden violence of torrents. As a general rule, we should give an exclusive preference to virgin soils; yet even some of these must be mistrusted, as I shall explain to you directly.
"We must then endeavour to distinguish a virgin soil from `made' or disturbed ground; and to do so, some acquaintance with elementary geology is indispensable. Thus, the, crystalliform rocks, granites, gneiss, and crys talline schists remain in the condition in which the cooling of the globe and the upheavals of its crust have placed them. The sandstones, the calcareous rocks, the marls, the gravels, even the clays deposited by water under an enormous pressure, are stratified - that is to say, deposited in layers, like the courses of a building, and present an excellent foundation, The hill there on the right, in whose direction your sister's wood extends, presents, as you see from this point, escarpments laid bare by the waters of the brook we are going to cross; observe that the stone, which seems denuded, presents itself in almost horizontal layers. It is an oolitic limestone, excellent for building, and on which you may confidently rely as a foundation also. In these strata, therefore, we may excavate cellars, and make use of what we have taken from the excavations to raise the walls. Here we are walking on sandy clays, intermingled with millstone grit. This also forms a good and incom pressible foundation. It is otherwise with pure clays; not that they are compressible, but, if they are not secured - if, for instance, they lie on a declivity - they are liable to slip in consequence of the infiltration of water between their layers, and the house built on them goes down with them.
And thus you may sometimes see whole villages built on clayey declivities, descending into the valley. Great at tention, therefore, must be paid to the method in which you build in clays, if you would avoid these dangers. Sometimes also, when they are greatly compressed by a heavy building, the clays sink down under the weight, and rise proportionally at a little distance, in see-saw fashion. Marine sands, pure, fine or gravelly, are well adapted to receive foundations, because the sand settles naturally, however slightly moistened it may be. To such a degree is this the case, that we can form an artificial foundation if needful by depositing good beds of sea-sand on a ques tionable soil, and moistening these beds thoroughly. The finer the sand is and the freer from clay the better, for its small, hard, equal grains leave only very slight intervals between them and touch on several points. If the weight compresses the layer of sand, and forces it to settle down, the settling down is regular, and consequently harmless. The building settles thus to the extent of some fractions of an inch, according to its weight; but it does not dislocate, because it settles uniformly. The alluvial deposits formed by slowly-flowing waters, such as rivers or lakes, also com pose good foundations, because the layers of gravel or mud have been gradually deposited, and are closely heaped together by the liquid that transported them. It is quite otherwise with marshy soils, for the water, having no cur rent, has allowed vegetables to grow in its bed. These vegetables on dying are annually replaced by others. Suc cessive layers of detritus are then formed under very trifling pressure, leaving between them innumerable cavi ties, just like a heap of rotten hay. These deposits are called peat-bogs. Nothing can be safely placed on these deposits, for they sink down under the lightest burden. Stop ! here we are near the stream, at a point which exhibits this phenomenon. Stamp on this closely-turfed soil. You perceive that the ground sounds hollow, and shakes beneath the shock. Sometimes these peat-beds reach to such a depth, through the accumulation of vege table detritus, that the bottom can scarcely be reached. If you build upon these, your construction will gradually sink, often unequally, on account of the inclination of the sub-soil, so that the building will lean to one side. It is thus that at Pisa and at Bologna, in Italy, there are towers which inclined thus while they were being built, until the turf was completely compressed under their weight. When these soils occur, the turf must be removed, the rock or gravel must be reached, or piles must be driven in very close to each other, until they can be forced no deeper. Then, on the heads of these piles is.placed what is called a raft, a kind of wooden framing, between the spaces of which concrete is poured, and on which the first courses of masonry are placed. Whole cities are built thus. Venice and Amsterdam rest only upon forests of piles driven in mud, which is spongy, because it was formed under a shallow sheet of water which had not power to com press it.