Although the great mass of recorded facts must be empirical, and awaiting explanation, such knowledge is of minor value, because it does not admit of safe and extensive inference. Each recorded result informs us exactly what will be experienced again in the same circumstances, but has no bearing upon what will happen in other circumstances.
Overlooked Results of Theory.
We must by no means suppose that, when a scientific truth is in our possession, all its consequences will be foreseen. Deduction is certain and infallible, in the sense that each step in deductive reasoning will lead us to some result, as certain as the law itself. But it does not follow that deduction will lead the reasoner to every result of a law or combination of laws. Whatever road a traveller takes, he is sure to arrive somewhere, but unless he proceeds in a systematic manner, it is 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 travels with a uniform high velocity was proved by Roemer from observations of the eclipses of Jupiter’s satellites. Corrections were thenceforward made in all astronomical observations requiring it, for the difference of absolute time at which an event happened, and that at which it would be seen on the earth. 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 reducing his observations of the fixed stars.
When once the relation between an electric current and a magnet had been detected by Oersted and Faraday, it ought to have been possible for them 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. This peculiar effect was accidentally discovered by Gambey 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 result of the principles of electro-magnetism.9
Many other curious facts might be mentioned which when once noticed were explained as the effects of well-known laws. It was accidentally discovered that the navigation of canals of small depth could be 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 one.10 Giffard’s injector for supplying steam boilers with water by the force of their own steam, was, I believe, accidentally discovered, but no new