460 POPULAR SCIENCE MONTHLY 



forget; are not prejudiced. When such instrumental records, scat- 

 tered 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 tem- 

 perature, 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 tempera- 

 ture 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 indi- 

 cates 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 indefiniteky. 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 care- 

 ful 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 character- 

 istics and with considerable uniformity over large areas. These varia- 

 tions 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 



