Page:Popular Science Monthly Volume 69.djvu/464

460 forget; are not prejudiced. When such instrumental records, scattered though they are, and difficult as it is to draw general conclusions from them, are carefully examined, from the time when they were first kept, which in a few cases goes back about one hundred and fifty years, there is found no evidence of any progressive change in temperature, or in the amount of rain and snow. Apparent signs of a permanent increase or decrease in one or another element have been fairly easy to explain as due to the method of exposing the thermometer or of setting up the rain gauge. Little care was formerly taken in the construction and location of meteorological instruments. They were usually in cities, and as these cities grew, the temperature of the air was somewhat affected. The rain-gauges were poorly exposed on roofs or in court-yards. The building of a fence or a wall near the thermometer, or the growth of a tree over a rain-gauge, was enough, in many cases, to explain any observed change in the mean temperature or rainfall. Even when the most accurate instrumental records are available, care must be taken to interpret them correctly. Thus, if a rainfall or snowfall record of several years at some station indicates an apparent increase or decrease in the amount of precipitation, it does not necessarily follow that this means a permanent, progressive change in climate, which is to continue indefinitely. It may simply mean that there have been a few years of somewhat more precipitation, and that a period of somewhat less precipitation is to follow.

For the United States, Schott, some twenty years ago, made a careful study of all the older records of temperature and rainfall, including snow, from Maine to California, and found nothing which led to the view of a progressive change in any one direction. There was evidence of slight variations of temperature, occurring with the same characteristics and with considerable uniformity over large areas. These variations have the characteristics of irregular waves, representing slightly warmer and slightly cooler periods, but during the fluctuations the temperature differed by only a degree or two on one side or the other of the mean. Obviously, this is too slight a range to be of any general or practical interest, and in any case, these oscillations give no evidence of a continuous change toward a warmer or a cooler climate. Schott found that these waves of higher and lower temperature followed one another at intervals of about twenty-two years on the Atlantic coast. In the interior the intervals were about seven years. The records of the closing of rivers, the Hudson, for example, to navigation, show no permanent change in the dates for the last hundred years or so.

It has been well pointed out that if a list were carefully compiled of heavy snowstorms, of droughts, of floods, of severe cold, of mild winters, of heavy rains, and of other similar meteorologic phenomena