Again in Nature we find a parallel, and the coarse Sand-paper of modern Art has long been anticipated in the scale-clad skins of many fishes.
The accompanying illustration is taken from the skin of a Picked Dog-fish found by myself lying dead on the rocks in Bideford Bay. I cut off a piece for transmission to the draftsman, and found that not only did it feel exactly like cutting through a piece of very common sand-paper, but that it blunted the edge of a new knife in exactly the same manner as would have been done by the roughest of sand-paper.
This kind of skin is common to all the shark tribe (including the Dog-fishes, which are but sharks in miniature), and to the Skate, Saw-fish, &c. I have now before me a small, but perfect example of the Saw-fish, the surface of which is covered with flinty scales like those of the Dog-fish, but very much smaller, requiring the aid of a magnifying lens to distinguish them. Even to guess at the number of them is impossible, for they cover the whole of the body, and extend to the very end of the beak, in some places glittering in a strong light as if pounded glass had been sprinkled all over the fish. One of the most interesting points in their structure is the manner in which they reach the rounded jaws, and there become converted into teeth powerful enough to crush the animals on which the fish live. The structure of these jaws will be explained in a future chapter.
Some of the skates and sharks have these scales of great size, so as to show their formation almost without the aid of a magnifying-glass. This is the case with a species of skate, the skin of which is used by the Japanese for wrapping round the handles of their best swords, and which is greatly valued by that nation, the sword being an almost sacred article in the eyes of a Japanese.
There is a well-known museum in which these swords are labelled as having handles of “granulated ivory.” Now, in the first place, there is no such thing as granulated ivory; and, in the next, a mere glance ought to tell the observer that the so-called ivory is a skin of some sort, worked upon the handle while wet, and kept in its place by copper studs. Even the junction of the edges is perceptible, and yet the authorities of the museum in question, although they have been repeatedly corrected, still persist in calling the skate-skin by the absurd title of granulated ivory.
However, if ivory could be granulated, it would certainly look very much like the skate-skin. When examined closely, the scales, whether of Dog-fish, Skate, Shark, or Saw-fish, are seen to resemble hexagonal cones, not coming quite to a point, but truncated, so as to have an hexagonal flattened tip. They are almost of a flinty hardness, especially at their tips, and on inspection of them the observer is not surprised at the use of Dog-fish skin in place of sand-paper.
Perhaps the reader may ask why the Equisetum should be taken as the prototype of the file, and the skin of the Dog-fish as that of sand-paper. The reason is this. The flinty points of the Equisetum are set upon parallel ridges something like those of a file, while the scales of the Dog-fish are without any apparent order, being crowded against each other like the cutting particles upon the sand-paper. That there should not be an order, and that a definite one, is out of the question. But it has not yet been detected by human eyes, and therefore may be practically treated as non-existent.
Tools of Measurement.
In many of the arts, more especially those which belong to engineering and carpentering as a part of architecture, it is absolutely necessary to make sure of a perpendicular line, i.e. a line which, if continued, would reach from any point of the earth’s surface to its exact centre below and its zenith above. Were it not for the power of producing this line, none of the great engineering works of modern or ancient days could have been undertaken.