Page:Popular Science Monthly Volume 3.djvu/497

Rh have noticed, that the varying warmth on which the processes of vegetation depend, corresponds with the varying diurnal course of the sun. So soon as this was noticed, and so soon as the periodic nature of the sun's varying motions had been ascertained, men had acquired in effect the power of predicting that, at particular times or seasons, the weather on the whole would be warmer than at other seasons. In other words, so soon as men had recognized the period we call the year, they could predict that one half of each year would be warmer than the other half. Simple as this fact may seem, it is important to notice it as the beginning of weather-prediction; for, as will presently appear, it has an important bearing on the more complex questions at present involved in the prognostication of weather-changes.

It became manifest, almost as soon as this discovery had been made, that the weather of particular days, or even of weeks and longer periods, could not, by its means, be predicted. A week in summer may be cold, and a week in winter may be warm; nor, so far as is even yet known, is there a single part of any year the temperature of which can be certainly depended upon, at least within the temperate zone. In certain tropical regions there are tolerably constant weather variations; but, so far is this from being the case in the temperate zones of either hemisphere, that it is impossible to affirm certainly, even that during a week or fortnight at any given summer season there will be one hot day, or that during a corresponding period in winter there will be one day of cold weather.

It became manifest also, at an early epoch, that terrestrial conditions must be intimately involved in all questions of weather, since the year in different countries in the same latitudes presents different features. Such differences are of two kinds—those which have a tendency to be constant, and those which are in their nature variable. For example, the annual weather, in Canadian regions having the same range of latitude as Great Britain, differs always to a very marked degree, though not always to the same degree, from that which prevails in this country; here, then, we have a case of a constant difference due unquestionably to terrestrial relations. Again, when we have a hot or dry summer in this country, warm or damp weather may prevail in other countries in the same latitudes, and vice versa; differences of this kind are ordinarily variable, and in the present position of weather-science are regarded as accidental.