We may ask ourselves what was the natural object from which the Pyramid was copied. The name itself, which is formed from a Greek word signifying fire, shows that a flame was thought to have furnished the idea of this form of building. I cannot, however, but think that the flame had little, if anything, to do with it, and that the real model may be found in the hills which have been formed by Nature. Examples of the Pyramids and the Hills are given in the accompanying illustration. SUBAQUATIC MORTAR.--PAINT AND VARNISH. Having now disposed of the chief points in Architecture, we take some of the subsidiary details. Of late years, when the traffic between different continents has so largely extended itself, and when shipping has increased both in the numbers and dimensions of the vessels, it is absolutely necessary that we should have harbours and docks enlarged and multiplied sufficiently to meet the calls upon them. [Illustration: CADDIS. TEREBELLA. SERPULA. SUBMARINE MORTAR.] Now, it is comparatively easy to construct a building on shore, for all the mortars and cements which are used for the purpose of fastening the stones together are applied when wet, and incorporate themselves with the stones as they dry. But to make a mortar which could be applied while the stones were under water, and would “set” while beneath the surface, was a task not easily to be overcome. Yet it has been done so effectively that at the present day we can build beneath the surface of the water as securely, though not as rapidly, as if the stones had been laid on dry ground. Several such mortars are now known, and, as is so often the case with human inventions, have been anticipated in Nature. We have already seen how the Caddis-worm of the fresh waters can cement together, while under water, the various materials of which its tubular house is formed. The different Sticklebacks perform similar feats, no matter whether they inhabit fresh or salt water. All those who take an interest in the productions of the seashore will have noticed upon our coasts the flexible tube of the Terebella, with its curiously fringed ends. This tube, as any one may see at a glance, is composed of grains of sand and similar materials, fastened strongly together by a kind of cement exuded from the worm, and possessing the property of hardening under water. As on some of our coasts fragments of shell are used for the tube, the worm goes by the popular name of Shell-binder. If one of these worms be taken out of its tube, placed in a vessel with sea-water and a quantity of sand, broken shells, and little pebbles, the mode of building will soon be seen. At the extremity of the head are a number of extremely mobile tentacles, and these are stretched about in all directions, seizing upon the particles of sand and shell, seeming to balance them as if to decide whether they are suitable for the tube, and then fixing them one by one with the cement which has already been mentioned.