LESSON THE SECOND.
Paul was greatly pleased with the method adopted by his cousin for giving him the first notions of building. In the evening he presented as his day's work a fair transcription of all that his teacher had explained to him on the ground. He even illustrated his text by some pretty good diagrams. The corrections were quickly made after dinner. But next day the incessant rain prevented them from going out, and Eugene decided that the second lecture should be given in the house."We shall have illus trations enough before us; the chateau itself will supply them. We will go through it from cellar to attic, and study its materials and methods of construction - to criti cize them if they are bad, or to take note of them if they are good."When teacher and pupil had gone down into E 2 the cellars, Eugene began by saying,"Look how damp this cellar wall on the side of the courtyard is; and see how the mortar in the joints of the stones has fallen, owing to two causes : - first, in building these walls, the precaution was not taken of cementing them on the outside, so as to make the water in the ground run down to the bottom; second, the mortar employed in the building was not made with hydraulic lime. There are two principal xinds of lime : fat or rich lime and hydraulic lime. The first is obtained by burning the compact limestones usually found at the top of the beds; it is called fat because when slaked it is glutinous and sticks to the tool with which it is mixed; this lime, on being immersed in water, swells and sends forth a dense vapour, as yOu may have observed, and mixed with sand is slow in setting. Employed above ground, mortars made with this lime become at length very hard, but retain more or less for a time a certain plasticity. These mortars, however, as they are slow in setting, are readily softened by water, and cannot then ever harden. Hydraulic limes, on the other hand (obtained by burning the lias limestones), when mixed with sand, soon become very hard, and set all the better for being in a damp place. Hence this lime is called hydraulic, because it is employed for all masonry-work under water. In default of Has limestones, artificial hydraulic limes are made, by grinding a certain proportion of clay with a lime stone suitable for making ordinary lime. Hydraulic lime is tested by slacking - that is to say, mixing it with water; when it slakes with the production of very little vapour.
"It is with hydraulic lime that concretes, of which I spoke to you yesterday, are made. The mortar being prepared, a certain proportion of hard gravel, about the size of eggs, is mingled with it; the whole is well mixed and thrown into the excavations, where it is rammed with wooden ram mers. If the lime is good and the concrete well made, it forms a veritable rock, similar to the con glomerates or pudding-stone of natural formation. As, when set, water penetrates with difficulty through these concretes, they prevent that percolation of subjacent water to which cellars made in wet grounds are liable.
"If the wall you see there had been built with mortar made with hydraulic lime, it would have been sound, and the mortar joints would have been as hard as the stone itself. You will easily understand that when the water has gradually softened and liquefied the mortar in the beds and joints at the base of a wall, the stones which compose it settle, and all the rest of the building suffers. That is why the front of the house, towards the court, presents a considerable number of cracks, that are filled in from time to time, but of course with no result in doing away with the cause of the mischief.
"You observe that the cellar wall which receives- the arch of the vault is very thick, much thicker than is the wall of the ground floor. The latter is scarcely 2 feet thick, whereas this is full 3 feet. This additional thick ness is given to the inside principally to receive the springing of the vault. A sketch will enable you to under stand the reason of this arrangement. Let A (Fig. 9) be the thickness of the wall of a house on the ground floor - a thickness oft foot 8 inches if cellars are wanted beneath the ground floor; the floor line being at B and the outside ground line at C, it will be well first to indicate the floor line by' a projection, - a greater thickness given to this wall on the outside, say of 2 inches. At A, then, the wall will have a thickness of i foot 10 inches. Your cellar arch being drawn at D, we must reserve a resting-place of at least 8 inches, to receive the first arch-stones of the spring of the vault; then it is well to give on the side next the ground a greater projection, to make a good footing for the plinth; this projection being 2 inches, we shall have at F a thickness of 2 feet and at G 2 feet 8 inches at least, as it will not do for the wall which rises to bear on the oblique beds of the vault, otherwise it would not have a solid foot ing, and would be weakened or reduced in thickness by this arch, which would penetrate it, as we see in the draw ing 1. But come here into this other cellar, which belongs to the oldest part of the chateau, and is built with good stones. The builder did not wish to lose space within, and as he built with worked stone he sought to economise material. What, then, did he do? (Fig io.) He gave his cellar wall only the thickness of that of the ground floor; at regular distances he put massive corbels 2 feet above the floor; upon these corbels he carried arches projecting io inches, and on these arches, which replace the extra thick ness or counter-wall of which I spoke to you just now, he carried his vaulting arch. This perspective sketch will enable us readily to understand this method of construc tion. The upper wall thus leaves the vault perfectly free and rises plumb over its lower face.