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and tracing the slow stages of its progress, we may take its condition at any time as indicating the progress in man's knowledge of the laws of Nature. It has grown with the growth of science and the advance of the human mind.

It was known thousands of years ago that, when fire is applied to water, vapor is formed that is capable of producing mechanical effects, and centuries before the Christian era attempts had been made to use this force in impelling a machine. The revolving Æolipyle of Hero, the Alexandrian, was a toy for a thousand years. During the middle ages many a mechanical genius played with it, but in vain, for there was no knowledge, and consequently there were no valuable results and no progress.

With the modern awakening of inquiry the subject was pursued by many ingenious experimenters, in different countries and at different times, until at the beginning of the eighteenth century so much had been found out about the properties of heat, water, and air, and so many devices had been hit upon to apply them, that it became possible to combine the different elements into a steam machine that could perform work. Newcomen's engine of 1705 embodied the ideas and artifices that had been previously gained, and, though very imperfect, was still available for useful mechanical effects. The steam-engine then first became a fact and a success.

From this time onward, to the patenting of Watt's double-acting engine in 1769, was a period of great activity in improving and developing the machine. Scientific research led the way in working out chemical and physical principles, and inventors were busy in perfecting and combining contrivances which were crowned by James Watt, who, by introducing the principles of separate condensation and double-action, gave to the steam-engine it's modern form and made it available for innumerable applications.

After Watt's great success we have a hundred years of still further improvements and refinements in the mechanism, and an enormous extension of its uses. From a contrivance supposed to be mainly valuable for pumping water, it became a universal motor equally valuable for manufacture and for locomotion on sea and land.

But, though it has been greatly perfected, the steam-engine is confessedly still imperfect. It has been undergoing steady improvement in recent years, and its theory is now so well understood, through the refined elucidations of physical research, that the pathway of future improvement is clearly discerned by sagacious engineers.

The history of the growth of this remarkable mechanism, that has now become so potent in the operations of human society, has an epic interest of grander meaning than can be found in any history of those destructive spasms in society that are sung and celebrated as war. The steam-engine is a triumph of peace, a victory of the pacific and constructive agencies of civilization, a conquest of Nature through the pursuits of science, and a symbol of the rise of modern industrialism and its successful conflict with the malign military spirit by which the world has been scourged through all the past.

Prof. Thurston has therefore chosen a theme of great interest in writing the history of the steam-engine, and it has not suffered in his hands. He has done admirable justice to its large and varied elements. The principles involved in the mechanism at all its successive stages are analyzed and stated with clearness, and the numerous contrivances and constructions by means of which the steam-engine has been built up and adapted to various ends are plainly, perspicuously, and fully described. The characters, circumstances, and labors, of the great men who have had a share in producing it are pleasantly sketched, and the narrative is enriched by anecdotes and personal episodes that relieve and enliven the more serious discussions of the book. Solidly instructive throughout, it is at the same time most agreeable reading, while many of the narrative parts are spirited and exciting.

Nothing further needs to be said to the readers of The Popular Science Monthly respecting the merits of Prof. Thurston's work, as they are already acquainted with them through portions of it which have appeared in these pages during the past year. One feature of the book, however, deserves especial recognition, and that is the elegance and profusion of its illustrations. In this respect no popular work upon the subject has ever appeared that can at all compare