"It is possible--is it not?--for you to imagine a continuous volley of iron balls passing near you in one line, in a horizontal direction, with considerable velocity. Suppose that a pane of glass were to be gradually moved so that a corner of it would be struck by one of the balls; then the entire sheet of glass would be shivered by the concussion, even though the bullet struck but a single spot of glass, the point of contact covering only a small area. Imagine now that the velocity of the volley of bullets be increased a thousand fold; then a plate of glass thrust into their track would be smoothly cut, as though with a file that would gnaw its way without producing a single radiating fracture. A person standing near the volley would now hear a deep purr or growling sound, caused by the friction between the bullets and the air. Increase gradually the rapidity of their motion, and this growl would become more acute, passing from a deep, low murmur, into one less grave, and as the velocity increased, the tone would become sharper, and at last piercingly shrill. Increase now the rapidity of the train of bullets again, and again the notes would decrease in turn, passing back again successively through the several keys that had preceded, and finally would reach the low growl which first struck the ear, and with a further increase of speed silence would ensue, silence evermore, regardless of increasing velocity.[10] From these hundreds of miles in a second at which the volley is now passing, let the rapidity be augmented a thousand times, reaching in their flight into millions of miles each second, and to the eye, from the point where the sound disappeared, as the velocity increased, a dim redness would appear, a glow just perceptible, indicating to the sense of sight, by a continuous line, the track of the moving missiles. To all appearance, the line would be as uniform as an illuminated pencil mark, even though the several integral bullets of the trail might be separated one from another by miles of space. Let a pane of glass now be thrust across their track, and from the point of contact a shower of sparks would fly, and the edges of glass close to either side of the orifice would be shown, on withdrawing the glass, to have been fused. Conceive now that the velocity of the bullets be doubled and trebled, again and again, the line of red light becomes brighter, then brilliant, and finally as the velocity increases, at a certain point pure white results, and to man's sense the trail would now be a continuous something, as solid as a bar of metal if at a white heat, and (even if the bullets were a thousand miles apart) man could not bring proof of their separate existence to his senses. That portion of a pane of glass or other substance, even steel or adamant, which should cross its track now would simply melt away, the portion excised and carried out of that pathway neither showing itself as scintillations, nor as fragments of matter. The solid