Fields remote from those of the naturalist are just as instructive. The inventor sets before himself an end with conscious purpose, and then seeks means to reach that end, but at best his methods may be wasteful and imperfect. Nature, with unhasting tread, acting simply through the qualities inherent in her materials, through their singular powers of combination, of mutual adaptability, shows the discoverer results which to understand even in small measure tax his keenest wit, or displays to him structures at times beyond his skill to dissect, much less to imitate. Mechanic art, indeed, is for the most part but a copy of nature, as when the builder repeats the mode in which rocks are found in caves, in ridges at the verge of a cliff, or in the stratifications which underlie a county, all conducing to permanence of form, to resistance against abrading sand or dissolving waters. What ensures the stability of a lighthouse but its repetition of a tree-trunk in its contour? Engines and machines recall the animal body, grinding ore much as teeth grind nuts, lifting water as the heart pumps blood through artery and vein, and repeating in mechanism of brass and steel the dexterity of fingers, the blows of fists. When an inventor builds an engine to drive a huge ship across the sea, he has created a motor vastly larger than his own frame, but much inferior in economy. At a temperature little higher than that of a summer breeze the human mechanism transmutes the energy of fuel into mechanical toil: for the same duty, less efficiently discharged, the steam engine demands a blaze almost fierce enough to melt grate bars of iron. Heat is costly, so that its conservation is an art worth knowing. In the ashes strewn and piled on burning lava nature long ago told us how heat may be secured against dissipation. Other of her garments, as hair and fur, obstruct the escape of heat in a remarkable degree, and so does bark, especially when loosely coherent as in the cork tree. Feathers are also excellent retainers of heat, and have thereby so much profited their wearers, that Ernest Ingersoll holds that the development of feathers has had much to do with advancing birds far above their lowly cousins, the reptiles clad in a scaly vesture. Strength of the Cylinder.