In many parts of Europe the saying is common, "A western wind brings rain," and it is undeniable that there is a certain connection between rain-storms and a wind from the west. Occasionally, however, rain will put in an appearance from whatever quarter the wind may be blowing. But, as rain is experienced particularly often during westerly winds, the statement above—"a western wind brings rain"—may, with some slight reserve, be permissible.
Almost every one owns a barometer. If, in the summer-time, some little excursion is planned, the mercury in the tube is watched with anxious eye to see whether it will rise, for this is generally regarded as predicting clear and dry weather, whereas the sinking of the mercury points to rain and storm. Until quite recently the true relation between these phenomena was not known to science, and yet it could be safely assumed that there was, in some way, a connection between them. Thus, Otto von Guericke, the inventor of the air-pump, in the year 1660 correctly predicted a storm from a considerable depression that he observed in the water-column of his immense barometer, which measured nineteen Magdeburg ells in length.
If we look back for a moment on these predictions in the field of natural science, we must admit that they can not lay claim to any great degree of accuracy. In Europe, as well as elsewhere, a westerly wind is often accompanied by fair weather; and sometimes it will rain, although the mercury has risen in the barometer.
Predictions like these may hence be made, but with a certain amount of reserve. Some of them, however, are predictions of a superior order—to be discussed hereafter—as in many of them the accompanying conditions are studied, and hypotheses as to the phenomena observed are formulated and discussed in connection with others. The generalizations considered thus far may only be looked upon as probable; there are, however, instances in which the probability can be more fully depended on, although, in these too, the simple method of enumeration is employed.
A case where the probability of the prediction borders on certainty is found in chemistry in enumerating the properties of chemically pure substances. If we say, "All globules of mercury have a grayish-white color, a metallic luster, are opaque," etc., these are assertions true without exception as far as mercury in the perfectly pure state is concerned. If we are dealing with a substance that is chemically pure, we can predict with certainty that it is endowed with certain properties. In fact, we may state it as a natural law that, under all circumstances, substances which have been recognized as identical by the comparison of a series of properties will exhibit other series of like properties. We will call this the law of coexistence of like properties, or, to be brief, the law of coexistence.
The counterpart of this is the law of universal causation. This law, so important a one for predictions, may be thus expressed: under
