ment than to be uninfluenced by any prejudice or theory in correctly recording the facts observed and allowing to them their proper weight. He who does not do so will almost always be able to obtain facts in support of an opinion however erroneous. Thus the belief still exists with great force in the majority of uneducated persons, that the moon has great influence over the weather. The changes of the moon, full, new and half moon, occur four times in every month, and it is supposed that any change may influence the weather at least on the day preceding or following that of its occurrence. There will thus be twelve days out of every 28 on which any change of weather would be attributed to the moon, so that during the year many changes will probably be thus recorded as favourable to the opinion. The uneducated observer is struck with these instances and remembers them carefully, but he fails to observe, or at least to remember, that changes of weather often occur also when there is no change of the moon at all. The question could only be decided by a long course of careful and unbiassed observation in which all facts favourable or unfavourable should be equally recorded. All observations which have been published negative the idea that there can be any such influence as the vulgar mind attributes to the

moon.

But it would at the same time be an error to suppose that the best observer or experimentalist is he who holds no previous opinions or theories on the subject he investigates. On the contrary, the great experimentalist is he who ever has a theory or even a crowd of theories or ideas upon his mind, but is always putting them to the test of experience and dismissing those which are false. The number of things which can be observed and experimented on are infinite, and if we merely set to work to record facts without any distinct purpose, our records will have

no value.

We must have some opinion or some theory to direct our choice of experiments, and it is more probable that we hit upon the truth in this way than merely by haphazard. But the great requisite of the true philosopher is that he be perfectly unbiassed and abandon every opinion as soon as facts inconsistent with it are observed.

It has been well said by the celebrated Turgot, that "the first thing is to invent a system; the second thing is to be disgusted with it;" that is to say, we ought to have some idea of the truth we seek, but should immediately put it to a severe trial as if we were inclined to distrust and dislike it rather than be biassed in its favour. Few men probably have entertained more false theories than Kepler and Faraday; few men have discovered or established truths of greater certainty and importance. Faraday has himself said that

"The world little knows how many of the thoughts and theories which have passed through the mind of a scientific investigator, have been crushed in silence and secrecy by his own severe criticism and adverse examination; that in the most successful instances not a tenth of the suggestions, the hopes, the wishes, the preliminary conclusions have been realized *."

The student is strongly recommended to read Sir J. Herschel's Preliminary Discourse on the Study of Natural Philosophy (Lardner's Cabinet Cyclopædia), especially Part II. Chaps. 4 to 7, concerning Observation, Experiment, and the Inductive Processes generally.

*Modern Culture, edited by Youmans, p. 222. [Macmillan and Co.]

LESSON XXVIII.

METHODS OF INDUCTION.

WE have now to consider such methods as can be laid down for the purpose of guiding us in the search for general truths or laws of nature among the facts obtained by observation and experiment. Induction consists in inferring from particulars to generals, or detecting a general truth among its particular occurrences. But in physical science the truths to be discovered generally relate to the connection of cause and effect, and we usually call them laws of causation or natural laws. By the Cause of an event we mean the circumstances which must have preceded in order that the event should happen. Nor is it generally possible to say that an event has one single cause and no more. There are usually many different things, conditions or circumstances necessary to the production of an effect, and all of them must be considered causes or necessary parts of the cause. Thus the cause of the loud explosion in a gun is not simply the pulling of the trigger, which is only the last apparent cause or occasion of the explosion; the qualities of the powder; the proper form of the barrel; the existence of some resisting charge; the proper arrangement of the percussion cap and powder; the existence of a surrounding atmosphere, are among the circumstances necessary to the loud report of a gun: any of them being absent it would not have occurred.

The cause of the boiling of water again is not merely the application of heat up to a certain degree of tempera

ture, but the possibility also of the escape of the vapour when it acquires a certain pressure. The freezing of water similarly does not depend merely upon the withdrawal of heat below the temperature of o° Centigrade. It is the work of Induction then to detect those circumstances which uniformly will produce any given effect; and as soon as these circumstances become known, we have a law or uniformity of nature of greater or less generality.

In this and the following Lessons I shall often have to use, in addition to cause and effect, the words antecedent and consequent, and the reader ought to notice their meanings. By an antecedent we mean any thing, condition, or circumstance which exists before or, it may be, at the same time with an event or phenomenon. By a consequent we mean any thing, or circumstance, event, or phenomenon, which is different from any of the antecedents and follows after their conjunction or putting together. It does not follow that an antecedent is a cause, because the effect might have happened without it. Thus the sun's light may be an antecedent to the burning of a house, but not the cause, because the house would burn equally well in the night. A necessary or indispensable antecedent is however identical with a cause, being that without which the effect would not take place.

The word phenomenon will also be often used. It means simply anything which appears, and is therefore observed by the senses; the derivation of the word from the Greek word pawóμevov, that which appears, exactly corresponds to its logical use.

The first method of Induction is that which Mr Mill has aptly called the Method of agreement. It depends upon the rule that "If two or more instances of the phenomenon under investigation have only one circumstance in common, the circumstance in which alone all the in

stances agree, is the cause (or effect) of the given phenomenon." The meaning of this First Canon of inductive inquiry might, I think, be more briefly expressed by saying that the sole invariable antecedent of a phenomenon is probably its cause.

To apply this method we must collect as many instances of the phenomenon as possible, and.compare together their antecedents. Among these the causes will lie, but if we notice that certain antecedents are present or absent without appearing to affect the result, we conclude that they cannot be necessary antecedents. Hence it is the one antecedent or group of antecedents always present, when the effect follows, that we consider the cause. For example, bright prismatic colours are seen on bubbles, on films of tar floating upon water, on thin plates of mica, as also on cracks in glass, or between two pieces of glass pressed together. On examining all such cases they seem to agree in nothing but the presence of a very thin layer or plate, and it appears to make no appreciable difference of what kind of matter, solid, liquid, or gaseous, the plate is made. Hence we conclude that such colours are caused merely by the thinness of the plates, and this conclusion is proved true by the theory of the interference of light. Sir David Brewster beautifully proved in a similar way that the colours seen upon Mother-of-pearl are not caused by the nature of the substance, but by the form of the surface. He took impressions of the Motherof-pearl in wax, and found that although the substance was entirely different the colours were exactly the same. And it was afterwards found that if a plate of metal had a surface marked by very fine close grooves, it would have iridescent colours like those of Mother-of-pearl. Hence it is evident that the form of the surface, which is the only invariable antecedent or condition requisite for the production of the colours, must be their cause,