where either hot air or steam was made to do mechanical work. The one is a primitive type of steam-turbine, the other is the prototype of a class of engine which only after many centuries became practically useful.
Probably astronomy of all the objective sciences was that which the Greeks cultivated most successfully. This is not the occasion on which to relate even in epitomized form the evolution of astronomical knowledge among the ancients. From Thales of Miletus who lived about 600 years b.c., to Ptolemy who flourished at Alexandria, about the 130th year of our own era, the knowledge of the behavior of the heavenly bodies had become increasingly more exact. Thales predicted an eclipse which "came off," if we may apply so irreverent a phrase to such events on May 25, 585 b.c. Not that Ptolemy was the greatest, because one of the latest, of the Grecian astronomers. The discoveries of Hipparchus of Rhodes, in the opinion of modern astronomers are much more important. Not only was the observation of the eccentricity of the lunar orbit made by Hipparchus, but his observations on the motions of the moon became the data which enabled Dr. Halley in the eighteenth century to apply a most delicate test—the acceleration of the mean lunar motion—to Newton's great law of universal gravitation.
But the astronomical system of Pythagoras was actually nearer to the truth than was the Ptolemaic, for, for one thing, it made day and night depend upon the earth's rotation. He postulated a proper motion for the earth, and was thus more correct than Ptolemy. The system of Copernicus was more similar to the earlier than to the later Greek view; and, indeed, it was one of the charges brought by the church against Copernicus that his system was heathen and "Pythagorean." Anaximander made the first map of the heavens.
While we cannot speak of the science of chemistry as having existed in classical ages, since by "chemistry" we mean nothing earlier than the time of Van Helmont, yet every one knows that the Greeks speculated on the ultimate constitution of matter and on the substance of the universe with as much zest as they did on the constitution and nature of mind. The concept of the atom is purely Greek. Doubtless Dalton meant by "atom" something much more definite than did Leucippus or Democritus; but we cannot admit that Dalton's conception of the ultimate structure of matter was, as an intellectual analysis, any more subtle than that other which was the earlier by two thousand years.
Both Thales and Anaximander spoke of a universally distributed, primitive world-stuff, whether moisture, caloric, ether, was not determined, some one thing eternally abiding although its forms were many and evanescent. This does not differ essentially from the modern conception of the all-pervading ether whose properties underlie the forms of grosser matter.
