968 ON FORM AND MECHANICAL EFFICIENCY [ch. 



stress, while its upper fibres are undergoing compression. It follows 

 that in some intermediate layer there is a "neutral zone," where 

 the fibres of the wood are subject to no stress of either kind. 



The phenomenon of a compression-member side by side with a tension- 

 member may be illustrated in many simple ways. Ruskin (in Deucalion) 

 describes it in a glacier. He then bids us warm a stick of sealing-wax and 

 bend it in a horseshoe: "you will then see, through a lens of moderate power, 

 the most exquisite facsimile of glacier fissures produced by extension on its 

 convex surface, and as faithful an image of glacier surge produced by com- 

 pression on its concave side." A still more beautiful way of exhibiting the 

 distribution of strain is to use gelatin, into which bubbles of gas have been 

 .introduced with the help of sodium bicarbonate. A bar of such gelatin, when 

 bent into a hoop, shews on the one side the bubbles elongated by tension and 

 on the other those shortened by compression*. 



In like manner a vertical pillar, if unevenly loaded (as for in- 

 stance the shaft of our thigh-bone normally is), will tend to bend, 

 and so to endure compression on its concave, and tensile stress 

 upon its convex side. In many cases it is the business of the 

 engineer to separate out, as far as possible, the pressure-lines from 

 the tension-lines, in order to use separate modes of construction, 

 or even different materials for each. In a suspension-bridge, for 

 instance, a great part of the fabric is subject to tensile strain only, 

 and is built throughout of ropes or wires; but the massive piers 

 at either end of the bridge carry the weight of the whole structure 

 and of its load, and endure all the "compression-strains" which 

 are inherent in the system. Very much the same is the case in 

 that wonderful arrangement of struts and ties which constitute, or 

 complete, the skeleton of an animal. The "skeleton," as we see it 

 in a Museum, is a poor and even, a misleading picture of mechanical 

 efficiency f. From the engineer's point of view, it is a diagram 

 shewing all the compression-lines, but by no means all the 

 tension-lines of the construction; it shews all the struts, but few 

 of the ties, and perhaps we might even say none of the principal 



* Cf. Emil Hatschek, Gestait und Orientirung von Gasblasen in Gelen, KoUoid- 

 Ztschr. XV, pp. 226-234, 1914. 



t In preparing or "macerating" a skeleton, the naturalist nowadays carries 

 on the process till nothing is left but the whitened bones. But the old anatomists, 

 whose object was not the study of "comparative morphology" but the wider 

 theme of comparative physiology, were wont to macerate by easy stages; and in 

 many of their most instructive preparations the ligaments were intentionally left 

 in connection with the bones, and as part of the "skeleton." 



