combination of laws, will occur to a reasoner. Whatever road a traveller takes, he is sure to arrive somewhere, but unless he proceed in a very systematic manner, it is very unlikely that he will reach every place to which a network of roads will conduct him. In like manner there are many phenomena which were virtually within the reach of philosophers by inference from their previous knowledge, but were never discovered until accident or systematic. empirical observation disclosed their existence.

That light is propagated with a certain uniform but very high velocity, was proved by Roemer, by observation of the eclipses of Jupiter's satellites. Corrections could henceforward be made in all astronomical observations requiring it, for the difference of absolute time at which an event happens, and that at which it becomes evident to us. But no person happened to remark that the motion of light compounded with that of the earth in its orbit would occasion a small apparent displacement of the greater part of the heavenly bodies. Fifty years elapsed before Bradley empirically discovered this effect, called by him aberration, when examining his accurate observations of the fixed stars i.

When once the relation between an electric current and a magnet had been detected by Oersted and Faraday, it ought, theoretically speaking, to have been possible for them or any other person to foresee the diverse results which must ensue in different circumstances. If, for instance, a plate of copper were placed beneath an oscillating magnetic needle it should have been seen that the needle would induce currents in the copper, but as this could not take place without a certain reaction against the needle, it ought to have been seen that the needle would come to rest more rapidly than in the absence of the copper. Yet this peculiar effect was accidentally discovered by Gambey

i Laplace, 'Précis de l'histoire de l'Astronomie,' p. 104.

in 1824. Arago acutely inferred from Gambey's experiment that if the copper were set in rotation while the needle was stationary the motion would gradually be communicated to the needle. The phenomenon nevertheless puzzled the whole scientific world, and it required the deductive genius of Faraday to show that it was a necessary result of the principles of electro-magnetism. By an act of deductive reasoning Faraday anticipated that a piece of copper rotating between the poles of a powerful magnet must experience a kind of resistance which will soon bring it to rest, and this effect he proved to exist in a decisive experiment1.

Many other curious facts might be mentioned which when once noticed were explained as the effects of wellknown natural laws. It was accidentally discovered that the navigation of canals of small depth could be greatly facilitated by increasing the speed of the boats, the resistance being actually reduced by this increase of speed, which enables the boat to ride as it were upon its own forced wave. Now mathematical theory might have predicted this result had the right application of the formulæ occurred to any onem. Giffard's injector for supplying steam boilers with water by the force of their own steam, was, I believe, accidentally discovered, but no new principles of mechanics are involved in it, so that it might have been theoretically invented. The same may be said of the curious experiment in which a stream of air or steam issuing from a pipe is made to hold a free disc upon the end of the pipe and thus apparently obstruct its own free outlet. The possession then of a true theory does not by any means imply the foreseeing of all the

k Experimental Researches in Electricity,' 1st Series, pp. 24-44· Paragraphs 81-139.

1 Jamin, 'Cours de Physique,' tom. iii. p. 297.

m Airy, On Tides and Waves,' Encyclopædia Metropolitana, p. 348 *.

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results. The effects of even a few simple laws may be infinitely diverse, and some of the most curious and useful effects may remain undetected until accidental observation brings them to our notice.

Predicted Discoveries.

The most interesting of the four classes of facts or phenomena as specified in p. 157, is probably the third— containing those the occurrence of which has been first predicted by theory, and then verified by observation. There is no more convincing proof of the soundness of scientific knowledge than that it thus confers the gift of foresight. Auguste Comte said that 'Prevision is the test of true theory;' I should say that it is only one test of true theory, but that which is most likely to strike the public attention. Coincidence with fact is the test of true theory, but when the result of theory is announced beforehand, there can be no possible doubt as to the unprejudiced and confident spirit in which the theorist interprets the results of his own theory.

The earliest instance of scientific prophecy is naturally furnished by the science of Astronomy, which was the earliest in development. Herodotus narrates that, in the midst of a battle between the Medes and Lydians, the day was suddenly turned into night, and the event had been foretold by Thales, the Father of Philosophy. A cessation of the combat and a peace confirmed by marriages was the immediate consequence of this happy scientific effort. Much controversy has taken place concerning the exact date of this occurrence, Baily assigning the year 610 B.C., but Sir G. B. Airy has lately decided that the exact day was the 28th of May, 584 B.C.

n Lib. i. cap. 74.

There can be no doubt that this and other predictions of eclipses attributed to ancient philosophers were due to an obscure knowledge of the Metonic Cycle, a period of 6585 days, or 223 lunar months, or about 19 years in which a nearly perfect recurrence of the phases and eclipses of the moon takes place; but if so, Thales must have had access to a long series of astronomical records either those of the Egyptians or the Chaldeans. There is a well known story as to the happy use which Columbus made of the power of predicting eclipses in overawing the islanders of Jamaica who refused him necessary supplies of food for his fleet. He threatened to deprive them of the moon's light. 'His threat was treated at first with indifference, but when the eclipse actually commenced, the barbarians vied with each other in the production of the necessary supplies for the Spanish fleet.'

Exactly the same kind of interest and awe which the ancients experienced at the prediction of eclipses, has been felt in modern times concerning the return of comets. Seneca indeed asserted in most distinct and remarkable terms that comets would be found to revolve in periodic orbits and return to sight. The ancient Chaldeans and the Pythagoreans are also said to have entertained a like opinion. But it was not until the age of Newton and Halley that it became possible to calculate the path of a comet in future years. A great comet appeared in 1682, a few years before the first publication of the Principia,' and Halley showed that its orbit corresponded with those of remarkable comets rudely recorded to have appeared in the years 1531 and 1607. The intervals of time indeed were not quite equal, but Halley conceived the bold idea that this difference might be due to the disturbing power of Jupiter, near which great planet the comet had passed in the interval 1607-1682. He predicted that the comet would return about the end of 1758 or the beginning of

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1759, and though Halley did not live to enjoy the sight, it was actually detected on the night of Christmas-day, 1758. A second return of the comet was witnessed in 1835 nearly at the time anticipated.

In recent times the discovery of Neptune has been the most remarkable instance of prevision in astronomical science. A full account of this discovery may be found in several works, as for instance Herschel's 'Outlines of Astronomy' and 'Grant's History of Physical Astronomy,' Chapters XII and XIII.

Predictions in the Science of Light.

Next after astronomy the science of physical optics has furnished the most beautiful and early instances of the prophetic power of correct theory. These cases are the more striking because they proceed from the profound application of mathematical analysis, and show an insight into the mysterious workings of matter which is surprising to all, but especially to the great majority of men who are unable to comprehend the methods of research employed. By its power of prevision the truth of the undulatory theory of light has been conspicuously proved, and it is especially to be remarked that even Newton received no assistance from his Corpuscular theory in the detection of new experiments. To his followers who embraced that theory we owe little or nothing in the science of light, and even the lofty genius of Laplace did not derive from it a single discovery. As Fresnel himself remarks:

'The assistance to be derived from a good theory is not to be confined to the calculation of the forces when the laws of the phenomena are known. There are certain laws so complicated and so singular, that observation Taylor's Scientific Memoirs,' vol. v. p. 241.