Now, there is no doubt that by means of a two-foot, or even a six-inch, rule any number of feet might be measured accurately; but, considering the number of junctions that have to be made, it is not likely that any pretence to accuracy could be insured. Then, a rod of forty, or even of twenty, feet in length would be awkward and unmanageable, and the only plan left is to take a string or cord of the requisite length. Even here, however, is a difficulty. The string would not allow of short measurements, such as inches, being written upon it. Let, however, a broad tape of inelastic material be substituted for the string, and all is easy enough. The next plan is to provide for the portability of the tape in question, to insure its reduction into the smallest possible compass, and to be sure that it is not twisted so as to damage its accuracy. These objects are all attained by the ordinary Tape Measure of the present day, which, whether it be a yard measure in a lady’s workbox, or a surveyor’s measuring tape, is a ribbon of comparatively inelastic material, coiled up when not wanted, and capable of being drawn out to its fullest extreme when needed. Putting aside the breadth of the line, and consequently disregarding the liability to twist, we have in the Fishing-reel of the modern angler an exact case in point. So we have in the lady’s yard measure, and in the gardener’s or builder’s tape, all these being modifications of the same idea. [Illustration: OVIPOSITOR OF GALL-FLY.] [Illustration: 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 ovipositor, 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. [Illustration: FLOATING BUBBLE.] [Illustration: SPIRIT-LEVEL.]