Page:Popular Science Monthly Volume 40.djvu/826

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SPECULATIONS concerning changes of climate have an interest that never flags. It rarely happens in the succession of seasons that two of an identical character come in succession; and any more than usually marked variation easily prompts the fancy that some modification in the character of the climate is impending.

The subject of climatology is a difficult one. The data for the proper study of it have hardly begun to be collected. We are embarrassed when we undertake to define climate and what marks to accept as its characteristics. Hann and Humboldt define it as comprising the whole of the meteorological phenomena characterizing the state of the atmosphere at any place, particularly as they affect our organs or have an influence on animal or vegetable life. The general character of the conditions can not be determined by the observations of one year, for they are liable to be contradicted by those of the next year; nor by those of any short term of years, for a similar reason. A period must be taken long enough to furnish the data for composing a type; and the more the years vary, as between one another, the longer must the period be. Many factors enter into the composition of a climate and form complicated combinations, all of which must be unraveled so as to give each factor its true force and position; and then the determination of their relative importance affords another source of embarrassment. Temperature and moisture are accepted as the most important factors, and temperature as the dominant one; and the climate is deduced by considering the average mean temperature for a term of years. Equal yearly averages do not, however, signify identical climate. A place where the summer heat and the winter cold are extreme has not the same climate as one where the range is relatively narrow, though the yearly averages may be the same in both. Hence we need separate determinations of summer and winter averages. The combinations of conditions of temperature and moisture may be endless, while the averages of either may be hardly disturbed. These facts make it hard to compare climates even when they are steady for long periods. In the capricious climates of our temperate latitudes a just determination and comparison form a baffling task.

Observations, more or less systematic, with instruments, have been made of climatological features for about a hundred years, but on a general co-operative plan they have been carried on imperfectly for less than a third of that time, or about the period