It is only within the present century that the vastness of the changes thus beginning to be wrought has become apparent. The scientific achievements of the human intellect no longer occur sporadically: they follow one upon another, like the organized and systematic conquests of a resistless army. Each new discovery becomes at once a powerful implement in the hands of innumerable workers, and each year wins over fresh regions of the universe from the unknown to the known. Our own generation has become so wonted to this unresting march of discovery that we already take it as quite a matter of course. Our minds become easily deadened to its real import, and the examples we cite in illustration of it have an air of triteness. We scarcely need to be reminded that all the advances made in locomotion, from the days of Nebuchadnezzar to those of Andrew Jackson, were as nothing compared to the change that has been wrought within a few years by the introduction of railroads. In these times, when Puck has fulfilled his boast and put a girdle about the earth in forty minutes, we are not yet perhaps in danger of forgetting that a century has not elapsed since he who caught the lightning upon his kite was laid in the grave. Yet the lesson of these facts, as well as of the grandmother's spinning-wheel that stands by the parlour fireside, is well to bear in mind. The change therein exemplified since Penelope plied her distaff is far less than that which has occurred within the memory of living men. The developments of machinery, which have worked such wonders, have greatly altered the political conditions of human society, so that a huge republic like the United States is now as snug and compact and easily manageable as the tiny republic of Switzerland in the eighteenth century. The number of men that can live upon a given area of the earth's surface has been multiplied manifold, and while the mass of human life has thus increased its value has been at the same time enhanced.