THE SPIRIT-LEVEL. 271 



the lady's yard measure, and in the gardener's or builder's 

 tape, all these being modifications of the same idea. 



OVIPOSITOR OP GALL-FLY. SPRING MEASURE. 



Suppose now that we pass to Nature, so as to ascertain 

 whether any such provisions were in existence before it was 

 imitated, however unconsciously, by man. This certainly was 

 the case with one of the commonest and most insignificant of 

 our insects, the little Gall-fly, belonging to the genus Cynips. 

 It could not lay its eggs without the aid of a very long ovi- 

 positor, and, owing to structural details, it cannot carry that 

 ovipositor in a straight line, as is done by many insects, some 

 of which have already been mentioned. Accordingly, it is 

 coiled up exactly like our measuring tapes, and can be unrolled 

 when needed. The long, protrusible tongues of the Wryneck, 

 Creeper, and Woodpecker are examples of a similar structure, 

 the tendinous portions being coiled round the head when not 

 needed. 



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 



FLOATING BUBBLE. SPIRIT-LEVEL. 



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 



