The principles of science

by William Stanley Jevons

Empirical Observations subsequently Explained.

The second great portion of scientific knowledge consists of facts which have been first learnt in a purely empirical manner, but have afterwards been shown to follow from some law of nature, that is, from some highly probable hypothesis. Facts are said to be explained when they are thus brought into harmony with other facts, or bodies of general knowledge. There are few words more familiarly used in scientific phraseology than this word explanation, and it is necessary to decide exactly what we mean by it, since the question touches the deepest points concerning the nature of science. Like most terms referring to mental actions, the verbs to explain, or to explicate, involve material similes. The action is ex plicis plana reddere, to take out the folds, and render a thing plain or even. Explanation thus renders a thing clearly comprehensible in all its points, so that there is nothing left outstanding or obscure.

Every act of explanation consists in pointing out a resemblance between facts, or in showing that similarity exists between apparently diverse phenomena. This similarity may be of any extent and depth; it may be a general law of nature, which harmonises the motions of all the heavenly bodies by showing that there is a similar force which governs all those motions, or the explanation may involve nothing more than a single identity, as when we explain the appearance of shooting stars by showing that they are identical with portions of a comet. Wherever we detect resemblance, there is a more or less explanation. The mind is disquieted when it meets a novel phenomenon, one which is sui generis; it seeks at once for parallels which may be found in the memory of past sensations. The so-called sulphurous smell which attends a stroke of lightning often excited attention, and it was not explained until the exact similarity of the smell to that of ozone was pointed out. The marks upon a flagstone are explained when they are shown to correspond with the feet of an extinct animal, whose bones are elsewhere found. Explanation, in fact, generally commences by the discovery of some simple resemblance; the theory of the rainbow began as soon as Antonio de Dominis pointed out the resemblance between its colours and those presented by a ray of sunlight passing through a glass globe full of water.

The nature and limits of explanation can only be fully considered, after we have entered upon the subjects of generalisation and analogy. It must suffice to remark, in this place, that the most important process of explanation consists in showing that an observed fact is one case of a general law or tendency. Iron is always found combined with sulphur, when it is in contact with coal, whereas in other parts of the carboniferous strata it always occurs as a carbonate. We explain this empirical fact as being due to the reducing power of carbon and hydrogen, which prevents the iron from combining with oxygen, and leaves it open to the affinity of sulphur. The uniform strength and direction of the trade-winds were long familiar to mariners, before they were explained by Halley on hydrostatical principles. The winds were found to arise from the action of gravity, which causes a heavier body to displace a lighter one, while the direction from east to west was explained as a result of the earth’s rotation. Whatever body in the northern hemisphere changes its latitude, whether it be a bird, or a railway train, or a body of air, must tend towards the right hand. Dove’s law of the winds is that the winds tend to veer in the northern hemisphere in the direction N.E.S.W., and in the southern hemisphere in the direction N.W.S.E. This tendency was shown by him to be the necessary effect of the same conditions which apply to the trade winds. Whenever, then, any fact is connected by resemblance, law, theory, or hypothesis, with other facts, it is explained.

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.‍[445]