In mining, it is likely to be of great use. In the slate-quarries at Penrhyns it has been found far superior to gunpowder. A huge mass of sixty tons’ weight, for instance, was gently pushed from its firmly compacted bed by the explosion of only eight ounces of cotton, while the slate was not splintered. In other great works it will also be of service. In a cutting on the Syston and Peterborough railway, not far from Stamford, experiments showed the average powers of the gun-cotton to be in the proportion of one to six of gunpowder; so that, in a hard freestone foundation, about five feet thick, and with an entire depth of twenty-eight feet, where six holes were necessary for gunpowder, only one was required for gun-cotton. In all blasting operations, whether in open cuttings, tunnels, or deep mines, a great saving of time, labour, and cost, is thus likely to be effected. CHAPTER VI. Light and its phenomena--Magic pictures--The optical paradox--Chinese metallic mirrors--Effect of an optical instrument on a superstitious mind--Origin of photography--The Talbotype--The Daguerreotype--Sunlight pictures. The cause of those sensations which we refer to the eyes, or that which produces the sense of seeing, is light. The phenomena of vision have always been regarded as among the most interesting branches of natural science. The knowledge of the laws which regulate the phenomena of light, constitutes the science of optics, which explains the cause of many most striking illusions. Magic pictures have been produced, which, when seen in a certain point through a glass, exhibit an object different from that be held by the naked eye. Niceron tells us that he executed at Paris, and deposited in the library of the Minimes of the Place Royale, a picture of this kind; when seen by the naked eye, it represented fifteen portraits of Turkish sultans, but, when viewed through the glass, it was a portrait of Louis XIII. The writer has often seen a singular transformation effected by an ingenious device, called the optical paradox: thus an eagle may be changed into a lion, and a dog into a cat. [Illustration] [Illustration]