The Spirit-level.
Having now seen how the forces of Nature enable us to produce a perfectly perpendicular line, we will see how the same force, though applied in a different manner, enables us to produce a perfectly horizontal line, the intersection of the two lines producing a right angle.
The measuring tool in question is called the Spirit-level, and is represented on the right hand of the accompanying illustration. Its construction is very simple, consisting of a tube, nearly filled with spirit, and having just one bubble of air in it. Now, owing to the force of gravitation, the air-bubble must always be uppermost. Consequently, if the tube be a perfect cylinder, whenever it is held so that the bubble is in the centre, the tube must be horizontal, a hair’s breadth of deviation altering the line. I may here mention that, as far as the principle of the instrument goes, water would serve the purpose as effectively as spirit. But as in cold weather the water might freeze, and so burst the tube, as well as being useless until it was thawed, spirit is always substituted.
This instrument is used for various purposes. Sometimes it is employed for levelling billiard tables, or for ascertaining the exact level of walls and other parts of buildings. Surveyors could scarcely do their work without the Spirit-level, which forms an important part of their chief instrument, the theodolite. Indeed, the new science of land drainage, by which the tough, unproductive clay soil is converted into fertile earth, is entirely dependent on the use of the Spirit-level, which detects the slightest rise or fall in the ground.
A most ingenious modification of the Spirit-level is used by military engineers, and is known by the name of the “Contouring-glass,” a term which requires some explanation.
It is of the utmost importance that a military engineer should be able, whether on foot or on horseback, to ascertain the approximate heights of the various points which he visits, the efficiency or failure of a battery very much depending on the comparative elevation of the spot on which the battery is placed, and that of the place against which its fire is directed. In an unknown country, of which no detailed maps exist, an invading force must of necessity depend on the extemporised surveys of their engineer officers, and one of the most valuable of their devices is the system of Contouring, invented, as far as I know, by the late Colonel Hutchinson, R.E.
The idea is simple enough. A hill is seen, and the engineer makes a sketch of it before he ascends. At the foot he halts, and marks the spot where his foot presses the earth. He then looks in front at a spot exactly on the level of his eye, marks it, and walks to it. He then draws a line across his sketch, at the exact spot on which he is standing, and that is the first “contouring line.” Others follow, until he has reached the top of the hill.
Now, if he can trust himself to look exactly horizontally, he has ascertained the elevation of every part of the hill. He knows the height of his eye from the sole of his foot, and calculates accordingly. Suppose, for example, that it be five feet, and that ten contouring lines are marked, he knows that the entire height is fifty feet, and that each line means an elevation of five feet.
This is a very excellent theory, but one which is not reduced to practice so easily as it looks. There is nothing more deceptive than a contour, especially upon an irregular hill, the invariable mistakes being either greatly to overrate or underrate the height of the contour. When I took my first lesson in this art I caused much amusement to the professor under whom I was studying, by making Shooter’s Hill consist of about seventeen contours. However, as many military students made very much the same mistake, I was not so humiliated as I supposed.
Of course, if a surveying officer be mounted, he takes the contour line as measured from his eye to the ground through the centre of the saddle.
After some practice the eye becomes so much accustomed to the contouring lines that they are taken almost mechanically; but, until this result be gained, an absolute proof is needed, which is furnished by the Contouring-glass—which, by the way, is not a glass at all, after the common acceptation of the word.
It is a simple brass tube about three inches long, not thicker than a man’s little finger, and open throughout. A small spirit-level is fixed on its lower surface, and on the very centre of the upper surface is a tiny steel mirror, which projects downwards like a knife-blade. In order to get a “contour,” the observer looks through the tube, slightly depressing its end. He then gradually raises it, still looking through it. As the tube becomes exactly horizontal the bubble in the spirit-level is reflected in the little mirror, and the object on which the tube is directed is in consequence on a level with the observer’s eye.
At first the management of the contouring-glass is rather tedious; but after a little practice it can be used without pausing for a single step.
Invaluable as is the Spirit-level, with its various modifications, it is nothing but an adaptation of that natural law which causes the bubbles to float on the surface of a stream instead of being submerged below it. We have all seen the multitudinous bubbles of soda-water, or of any effervescing liquid, and have noticed how they are very small when generated, but enlarge quickly, and rise to the surface with a rapidity equal to their enlargement. The same phenomena may be observed in any water-fall, or even in the very familiar and unpoetical operation of pouring beer from a jug into a glass.
The reader will see that in the plumb-rule, the level, and the spirit-level one single principle is employed, namely, the attraction of matter towards the centre of the earth. In the two former instruments this attraction gives a vertical line, and in the latter it gives a horizontal line, but the principle is the same in both.