only displaced by the immortal "Principia" of Newton. It commences with the doctrine that the earth is globular and fixed in space, it describes the construction of a table of chords, and instruments for observing the solstices, it deduces the obliquity of the ecliptic, it finds terrestrial latitudes by the gnomon, describes climates, shows how ordinary may be converted into sidereal time, gives reasons for preferring the tropical to the sidereal year, furnishes the solar theory on the principle of the sun's orbit being a simple eccentric, explains the equation of time, advances to the discussion of the motions of the moon, treats of the first inequality, of her eclipses, and the motion of her nodes. It then gives Ptolemy's own great discovery—that which has made his name immortal—the discovery of the moon's evection or second inequality, reducing it to the epicyclic theory. It attempts the determination of the distances of the sun and moon from the earth—with, however, only partial success. It considers the precession of the equinoxes, the discovery of Hipparchus, the full period of which is twenty-five thousand years. It gives a catalogue of 1,022 stars, treats of the nature of the milky-way, and discusses in the most masterly manner the motions of the planets. This point constitutes another of Ptolemy's claims to scientific fame. His determination of the planetary orbits was accomplished by comparing his own observations with those of former astronomers, among them the observations of Timocharis on the planet Venus.

INVENTION OF THE STEAM-ENGINE. In the Museum of Alexandria, Ctesibius invented the fire-engine. His pupil, Hero, improved it by giving it two cylinders. There, too, the first steam-engine worked. This also was the invention of Hero, and was a reaction engine, on the principle of the eolipile. The silence of the halls of Serapis was broken by the water-clocks of Ctesibius and Apollonius, which drop by drop measured time. When the Roman calendar had fallen into such confusion that it had become absolutely necessary to rectify it, Julius Caesar brought Sosigenes the astronomer from Alexandria. By his advice the lunar year was abolished, the civil year regulated entirely by the sun, and the Julian calendar introduced.