Now that we have traced in outline the vain attempts at physical propulsion, let us turn to take a view of the evolution of that other invention whose advent alone delayed the practical utility of the paddle-wheel to boats. Who shall say how it was that steam came first to be regarded as a means of giving power? In certain parts of the world, where geysers and boiling springs existed, man must naturally have been struck by the elastic force which steam possessed. An intellect which had any leaning to the side of practical economy must have reasoned that here was a valuable force running to waste, which might have been employed in the service of mankind, just as the swift-running rivers could be made to turn the water-wheels. But, as we said just now, steam was not wanted yet, for human labour was too cheap to bother about it; and we might remark incidentally that it was owing to this same cheapness that the galley, or rowing craft, was encouraged for many centuries in the Mediterranean, to the partial exclusion and great discouragement of the big sailing ship. Indeed, slavery, or abundance of cheap, compulsory labour, has been the means of holding back the progress of the world. Had the big sailing ships come at an earlier date the far-off countries would have been discovered much sooner, and the study of the properties of steam--or some other means as the equivalent of physical power--would have been regarded with a greater enthusiasm. Perhaps it would be more accurate to speak of the re-discovery of steam than of its invention: for as early as 130 B.C. Hero, of Alexandria, had written a treatise on “Pneumatics,” and described a light ball supported by a jet of steam which came out of a pipe into a cup, much as one sees in the rural fairs of to-day the same idea used when the force of water raises a light ball for the bucolic rifleman to shoot at. Hero also referred to the “aeolipile,” which was a hollow ball mounted on its axis between two pivots, one of which was hollow and acted as a steam pipe. Two nozzles formed part of the ball and were fitted at right angles to the pivots on which the ball revolved, and owing to the reaction caused by the escape of the steam from the jets touching the ball the latter was made to revolve. This is well illustrated in the plate facing page 18.