i8 THE POPULAR SCIENCE MONTHLY. 



ization, when these well-ascertained and carefullv-veiified facts are ar- 

 ranged methodically, generalized systematically, and classified logi- 

 cally, 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 un- 

 erring 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. As- 

 tronomy 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 astron- 

 omy 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 nar- 

 rated, in his investigations on light, ])redicttd the fact that the dia- 

 mond was formed of some combustible material from its verv hiiih 

 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 bi- 

 axial crystals there were four points where the refraction of the crys- 

 tal upon an incident ray produced a continuous conical envelope. Dr. 

 Lloyd took a crystal of aragonite, and, following Hamilton's direc- 

 tions, 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 waA' e, reached the same point at the same time, but by differ- 

 ent 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 



