272 NATUBE'S TEACHINGS. 



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 " Con- 

 touring-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 



