They soon reached the Noiret farm; and there in fact, under a shed along the wall of the barn, were piled up pieces of timber roughly squared and blackened by damp. Eugène marked a certain number of them with his knife, leaving those that were crooked, knotty, or cross-grained.
“What is a cross-grained piece of timber?” asked Paul.
“Cross-grained timbers are those whose fibres form a spiral round the heart. You can understand how the fibres of the wood not being vertical, and forming spirals more or less complete, lose their resisting property; these fibres, on account of the circuit they make,—and which is not a regular one,—become disjoined, and leave deep cracks between them. These timbers, therefore, are rejected as defective, as are also those whose heart is unsound, or which have what they call soft rings; that is, diseased parts between their layers—a sort of interior ulcers which not only deprive the wood of its homogeneity of resistance but develop decay around them. It often happens that these soft rings are not observed, and that timbers which appear very sound fall rapidly to dust. And as these diseases are frequent or rare according to the soils in which the wood grew, it is essential to know whence the timbers employed in buildings came. One forest will produce oak admirable in appearance, but which rapidly decays; another furnishes timber that is always sound. Generally, timber grown on light and dry soils is good; the produce of damp, clayey ground is bad.
“You will have these cross-grained and crooked timbers put on one side; they will do to make the centres for the cellars; they are fit for nothing else, unless it be for firewood. As to these fir poles, they will serve for scaffolding.”
It was late, so the cousins asked for breakfast at the farm. While they were laying the table, Paul said, “I want to know how you use the theodolite.”
Fig. 20.
“In the case of an operation such as we have just been performing, it is the simplest thing in the world. I asked Branchu to send my instrument to the château, that I might not be troubled with it all the morning; but there is no need to have it here to show how to use it. You know that the theodolite consists of a graduated circle, divided into 360 degrees. This circle, movable on its centre, is furnished with an air-bubble level, and a telescope above, both of which turn horizontally on a pivot in the centre of the circle. The level and the centre of the telescope are perfectly parallel to the plane of the circle. This is placed upon a stand with three legs, and the circle is first fixed horizontally by means of three regulating screws, and by turning the level. The air-bubble must be always at the centre, to whatever degree of the circle the tube is directed. This being done, and the feet being placed at the point marked on the ground—verifying the position by means of a plumb-line passing through the centre of the plate—the glass is directed towards a fixed point where a ‘sight’ is placed. The glass of the telescope is crossed by two hairs at right angles, which mark its centre. The intersection of the two hairs must fall on the point on which the telescope is directed. But previously the indicator or vernier below the telescope is set to the zero of the circle. It is therefore the entire instrument that has been turned. If, then, for example, we wish to construct a right angle on the line joining the point where you are standing with the first sight, you turn the glass till its indicator stands at 90 degrees (the quarter of the circle). You send a man with another sight in the direction of the glass, and have this sight carried to right or left until its centre is exactly on the line of the vertical hair of the glass. You have this sight fixed. It is then certain that the line drawn from the point where you stand in the direction of the second sight forms a right angle with the first base line, since two diameters cutting a circle divided into 360 degrees at right angles give 90 degrees for each quarter of the circle. By the help of this instrument, having previously indicated on the plan of a building whose foundations you are laying out, the angles which certain lines, starting from any point, form with each other, you can transfer these angles to the ground. Suppose you have to lay the foundations of a semi-circular portico. Having fixed the centre, and traced the semi-circle on the ground, placing the theodolite on this centre, you will be able to direct lines that will cut this circumference at regular intervals, and which would mark, for instance, the centres of the columns or pillars. Since from point A to point B you have 180 degrees (Fig. [20]), you will divide these 180 degrees into as many parts as you choose on the circle of the theodolite, and the centre of the glass will give you, at a great distance, the same divisions on the semi-circular portico. In the same way as the theodolite serves the purpose of laying the foundations of a building, it enables us to take the bearings of a tract of country. For suppose the base, E F, to be a known length which you have measured: placing your instrument at E, you direct the glass on a point C,—a tree, a steeple, or a pole; you have then the number of degrees on the circle comprised by the angle C E F. You transfer this angle to your paper, then moving the instrument to the point F, you direct it thence on this same point C; you obtain similarly the angle C F E, which, transferred to the paper, gives you exactly the position of the point C, and the unknown distance from E to C and from F to C; then either of these lengths will serve you for a base in its turn, and operating from the point C and the point F, and sighting a fourth point D, you know the lengths C D and F D. Thus you can operate over a whole country; this is what is called triangulation, the first operation required for getting a map of the country. But we are getting into another region of knowledge. Let us go to breakfast!”
CHAPTER VIII.
PAUL REFLECTS.
The omelette au jambon despatched, Paul remained silent.