THE CONTOURING-GLASS. 273 



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 eleva- 

 tion 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 decep- 

 tive than a contour, especially upon an irregular hill, th' 

 invariable mistakes being either greatly to overrate or under 

 rate 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 modifica- 

 tions, it is nothing but an adaptation of that natural law which 

 causes the bubbles to float on the surface of a stream instead of 



T 



