Page:Popular Science Monthly Volume 7.djvu/28

 18, when these well-ascertained and carefully-verified facts are arranged methodically, generalized systematically, and classified logically, so as to deduce and elucidate from them the laws that regulate their rule and order. Lastly, we have the stage of prophecy, when these laws are so applied that events can be predicted to occur with unerring accuracy. Astronomy is said to be the only science which has thoroughly reached the last stage. Other sciences are in various stages of growth. Electricity in some branches has reached the third stage, but in many branches it is still in its infantine period. Astronomy predicts eclipses, transits, occultations, for any period in the future, and the "Nautical Almanac" is the most wonderful example of prescient knowledge: a sailor may go away for a five years' cruise, and yet in this book he will find every event in the motion of the planets, the movements of the tides, the rotation of the moon, the eclipses of the sun, etc., faithfully and unerringly foretold. But astronomy has produced greater wonders than these. The planet Uranus was found to suffer from some slight disturbances in her path round the sun. Adams in England and Leverrier in France simultaneously and independently, from the known laws of gravity, predicted the existence and position of another unknown planet. Galle, of Berlin, directed by Leverrier, found the planet in the spot indicated, and it was called Neptune.

Newton, the grandest scientific man the world has perhaps ever seen, and the founder of the laws that led to the prophecy just narrated, in his investigations on light, predicted the fact that the diamond was formed of some combustible material—from its very high index of refraction. The combustion of diamond is now an ordinary, though expensive, lecture experiment. Light has given us one or two other scientific prophecies. Poisson, from theory, pronounced that, in the case of an opaque circular disk, the illumination of the centre of the shadow caused by diffraction at the edge of the disk would be precisely the same if the disk were altogether absent. Arago proved this to be true. Again, Sir William Hamilton predicted that in biaxial crystals there were four points where the refraction of the crystal upon an incident ray produced a continuous conical envelope. Dr. Lloyd took a crystal of aragonite, and, following Hamilton's directions, discovered what the mathematician had predicted.

Whewell predicted from theory that there must be a certain point in the North Sea, midway between Lowestoft and the coast of Holland, where there was no rise or fall of the water, because the crest or high-water mark of the tidal wave, and the trough or low-water mark of the same wawe, reached the same point at the same time, but by different routes. Captain Hewett, R. N., found that it was so.

Electricity has its prophets. Faraday, examining Sir Charles Wheatstone's beautiful experiment on the velocity of electricity by means of a rotating mirror, said: "If the two ends of the wire in