is the power that will effect all these changes?" This it is the very object of Mr. Etzler's volume to show. At present, he would merely remind us that there are innumerable and immeasurable powers already existing in nature, unimproved on a large scale, or for generous and universal ends, amply sufficient for these purposes. He would only indicate their existence, as a surveyor makes known the existence of a water-power on any stream; but for their application he refers us to a sequel to this book, called the "Mechanical System." A few of the most obvious and familiar of these powers are, the Wind, the Tide, the Waves, the Sunshine. Let us consider their value.
First, there is the power of the Wind, constantly exerted over the globe. It appears from observation of a sailing-vessel, and from scientific tables, that the average power of the wind is equal to that of one horse for every one hundred square feet. We do not attach much value to this statement of the comparative power of the wind and horse, for no common ground is mentioned on which they can be compared. Undoubtedly, each is incomparably excellent in its way, and every general comparison made for such practical purposes as are contemplated, which gives a preference to the one, must be made with some unfairness to the other. The scientific tables are, for the most part, true only in a tabular sense. We suspect that a loaded wagon, with a light sail, ten feet square, would not have been blown so far by the end of the year, under equal circumstances, as a common racer or dray horse would have drawn it. And how many crazy structures on our globe's surface, of the same dimensions, would wait for dry-rot if the traces of one horse were hitched to them, even to their windward side? Plainly,
