Thus far science, outside of pure mathematics, is made up of theories which are greatly modified by advancing knowledge, so that they cannot claim in all respects to be eternally established, like the laws of Kepler and the discoveries of Copernicus,--the latter of which were only true in the main fact that the earth revolves around the sun. But even he retained epicycles and excentrics, and could not explain the unequal orbits of planetary motion. In fact he retained many of the errors of Hipparchus and Ptolemy. Much, too, as we are inclined to ridicule the astronomy of the ancients because they made the earth the centre, we should remember that they also resolved the orbits of the heavenly bodies into circular motions, discovered the precession of the equinoxes, and knew also the apparent motions of the planets and their periods. They could predict eclipses of the sun and moon, and knew that the orbit of the sun and planets was through a belt in the heavens, of a few degrees in width, which they called the Zodiac. They did not know, indeed, the difference between real and apparent motion, nor the distance of the sun and stars, nor their relative size and weight, nor the laws of motion, nor the principles of gravitation, nor the nature of the Milky Way, nor the existence of nebulae, nor any of the wonders which the telescope reveals; but in the severity of their mathematical calculations they were quite equal to modern astronomers. If Copernicus revolutionized astronomy by proving the sun to be the centre of motion to our planetary system, Galileo gave it an immense impulse by his discoveries with the telescope. These did not require such marvellous mathematical powers as made Kepler and Newton immortal,--the equals of Ptolemy and Hipparchus in mathematical demonstration,--but only accuracy and perseverance in observations. Doubtless he was a great mathematician, but his fame rests on his observations and the deductions he made from them. These were more easily comprehended, and had an objective value which made him popular: and for these discoveries he was indebted in a great measure to the labors of others,--it was mechanical invention applied to the advancement of science. The utilization of science was reserved to our times; and it is this utilization which makes science such a handmaid to the enrichment of its votaries, and holds it up to worship in our laboratories and schools of technology and mines,--not merely for itself, but also for the substantial fruit it yields.