The most brilliant age in the history of science prior to the seventeenth century a.d. was the period of the Hellenistic civilization. Indeed, many of the achievements of the modern age would scarcely have been possible without the discoveries of the scientists of Alexandria, Syracuse, Pergamum, and other great cities of the Hellenistic world. The reasons for the phenomenal development of science in the centuries after the downfall of Alexander's empire are not far to seek. Alexander himself had given some financial encouragement to the progress of research. More important was the stimulus provided for intellectual inquiry by the fusion of Chaldean and Egyptian science with the learning of the Greeks. Possibly a third factor was the new interest in luxury and comfort and the demand for practical knowledge which would enable man to solve the problems of a disordered and unsatisfying existence.

The sciences which received the major attention in the Hellenistic Age were astronomy, mathematics, geography, medicine, and physics. Chemistry as a pure science was practically unknown. Except for the work of Theophrastus, who was the first to recognize the sexuality of plants, the biological sciences were also largely neglected. Neither chemistry nor biology bore any definite relationship to trade or to the forms of industry then in existence and apparently they were not regarded as having much practical value.

The most famous of the earlier astronomers of this time was Aristarchus of Samos (310-230 B.C.), who is sometimes called the "Hellenistic Copernicus." As a result of his discovery that the apparent immobility of the "fixed" stars is due to their vast distance from the earth, he was the first to have any adequate conception of the enormous size of the universe. But his chief title to fame comes from his deduction that the earth and the other planets revolve around the sun. Unfortunately this deduction was not accepted by his successors. It conflicted with the teachings of Aristotle and with the anthropocentric ideas of the Greeks. Besides, it was not in harmony with the beliefs of the Jews and other Orientals who made up so large a percentage of the Hellenistic population. The only other astronomer of much importance in the Hellenistic Age was Hipparchus, who did his most valuable work in Alexandria in the latter half of the second century B.C. His chief contributions were the invention of the astrolabe, the preparation of the best chart of the heavens known to antiquity, the approximately correct calculation of the diameter of the moon and its distance from the earth, and the discovery of the precession of the equinoxes. His fame was eventually overshadowed, however, by the reputation of Ptolemy of Alexandria, the last of the Hellenistic astronomers. Although Ptolemy made few original discoveries, he systematized the work of others. His principal writing, the Almagest, based upon the geocentric theory, was handed down to medieval Europe as the classic summary of ancient astronomy.

Closely allied with astronomy were two other sciences, mathematics and geography. The Hellenistic mathematician of greatest renown was of course Euclid (ca. -323 -ca. 285 b.c), erroneously considered the founder of geometry. Until the middle of the nineteenth century his Elements of Geometry remained the accepted basis for the study of that branch of mathematics. Much of the material in this work was not original but was compiled as a synthesis of the discoveries of others. The most original of the Hellenistic mathematicians was probably Hipparchus, who laid the foundations of both plane and spherical trigonometry. Hellenistic geography owed most of its development to Eratosthenes (ca. 276-ca. 194 B.C.), astronomer, poet, philologist, and librarian of Alexandria. By means of sun dials placed some hundreds of miles apart, he calculated the circumference of the earth with an error of less than 200 miles. He produced the most accurate map that had yet been devised, with the surface of the earth divided into degrees of latitude and longitude. He propounded the theory that all of the oceans are really one, and he was the first to suggest the possibility of reaching India by sailing west. One of his successors divided the earth into the five climatic zones which are still recognized and explained the ebb and flow of the tides as due to the influence of the moon.

Perhaps none of the Hellenistic advances in science surpassed in importance the progress in medicine. Especially significant was the work of Herophilus of Chalcedon, who conducted his researches in Alexandria about the beginning of the third century. Without question he was the greatest anatomist of antiquity and, according to Galen, the first to practice human dissection. Among his most important achievements were a detailed description of the brain, with an attempt to distinguish between the functions of its various parts; the discovery of the significance of the pulse and its use in diagnosing illness; and the discovery that the arteries contain blood alone, not a mixture of blood and air as Aristotle had taught, and that their function is to carry blood from the heart to all parts of the body. The value of this last discovery in laying the basis for a knowledge of the circulation of the blood can hardly be overestimated.

The ablest of the successors of Herophilus was Erasistratus, who nourished in Alexandria about the middle of the third century. He is considered the founder of physiology as a separate science. Not only did he practice dissection, but he is believed to have gained a great deal of his knowledge of bodily functions from vivisection. He discovered the valves of the heart, distinguished between motor and sensory nerves, and taught that the ultimate branches of the arteries and veins are connected. He was the first to reject absolutely the humoral theory of disease and to condemn excessive blood-letting as a method of cure. Unfortunately this theory was revived by Galen, the great encyclopedist of medicine who lived in the Roman Empire in the second century a.d.

Prior to the third century b.c. physics had been a branch of philosophy. It was made a separate experimental science by Archimedes of Syracuse. Archimedes discovered the law of floating bodies or specific gravity and formulated with scientific exactness the principles of the lever, the pulley, and the screw. Among his memorable inventions were the compound pulley, the tubular screw for pumping water, the screw propeller for ships, and the burning lens. Although he has been called the "technical Yankee of antiquity," there is evidence that he set no high value upon his ingenious mechanical contraptions and preferred to devote his time to pure scientific research.

Certain other individuals in the Hellenistic Age were quite willing to give all their attention to applied science. Pre-eminent among them was Hero or Heron of Alexandria, who lived in the last century b.c. The record of inventions credited to him almost passes belief. The list includes a fire engine, a siphon, a force pump, a hydraulic organ, a slot machine, a catapult operated by compressed air, a thermoscope, and even a steam engine. How many of these inventions were really his own is impossible to say, but there appears to be no question that such contrivances were actually in existence in his time or soon thereafter. Nevertheless, the total progress in applied science was comparatively slight, probably for the reason that human labor continued to be so abundant and cheap that it was not worth while to substitute the labor of machines.