286 . DISTRIBUTION OF RAINFALL. 



But between the results of single years there is great irregularity, and, 

 as many think from a study of these records, an increasing irregularity, 

 in the amount of annual rain-fall. There can, however, be nothing more 

 certain than that in many places, there is a great and increasing irregu- 

 larity in the distribution of the rainfall through the several months and 

 seasons of the year, and a tendency to droughts, floods, and uncertain 

 crops.' 



These irregularities have suggested to some, the theory of cycles, and 

 various periods of return have been mentioned, but none definitely 

 proved. Among other theories is one that seeks to show a connection 

 between rainy seasons and the frequency of spots on the sun. But we 

 know with certainty only this fact, that from year to year there is the 

 same amount of exposure of oceans, lakes, and other water surfaces to 

 the sun, and that, in all probability, the amount of water raised by 

 evaporation is about the same year by year. The precipitation of this 

 moisture occurs whenever and wherever the air which contains it be- 

 comes cooled down to the degree of saturation or below, and while some 

 regions, from their prevailing winds, and their mountains or other cir- 

 cumstances of their conformation, are sure to receive copious and often 

 periodical rain-falls, other regions are less favored by these conditions 

 that insure this result, and the amount of rain varies according as the 

 causes operating to bring about precipitation exist or are wanting. 

 Every cause tending to increase the humidity, or to reduce the temper- 

 ature, is, so far as it appears, a cause of rain. We think it is sufiBciently 

 shown by observations in all countries where forests exist that their 

 presence tends to this effect in greater or less degree, and sometimes iu 

 a manner so marked that there can be no mistaking the relation of 

 cause and effect. 



Our space will not permit us to present tables showing the distribu- 

 tion of rain-fall in different regions. Extensive series are already pub- 

 lished and easily accessible ; but unfortunately we have not as yet a 

 single instance in which carefully-conducted experiments have been 

 made to show by actual measurement, by degrees and quantities, the 

 differences that exist in the rain-fall, or in the causes that tend to pro- 

 duce it, within woodland and in the open fields adjacent. Until such 

 stations are established we must depend upon what can be learned 

 from such records iu Europe, and upon accidental circumstances that 

 now and then afford facts of unquestionable meaning, for the rest. 



As for records of floods no year passes without them, and in some 



1 A system of weather records by volunteer observers has been organized in Iowa 

 through the efforts of Dr. Gustavus Hinrichs, professor of Physical Science in the Uni- 

 versity of Iowa, the central station being at Iowa City. About sixty observers have 

 been secured, aud observations are made thrice daily (8 a. m., noon, and 8 p. m.), in- 

 cluding temperature, rain-fall, winds, and clouds. For convenience of reference, the 

 State is divided, by township lines 77-78 aud 89-90, and range lines 14-15 aud 30-31, 

 into 9 districts. The system promises to become one of great practical interest to the 

 State. It has hitherto received no legislative aid, aud depends chiefly upon the efforts 

 of the originator. 



It is thought that a comparison of records, which extend more or less imperfectly 

 through a period of over thirty years, shows a perceptible change of climate, and more 

 frequent droughts. 



The rain-fall shows that in 1876 the lines of equal amount correspond very nearly with 

 those of equal quantity of woodlands, and that those parts of the iState which have a high 

 percentage of forest surface are those that receive the greatest amount of rain-fall. This coin- 

 cidence is of the highest practical interest, and ic matters but little which fact has pre- 

 cedence as the cause, or which is the effect, so lung as their relation remains the same, 

 and one of them is directly under onr control.— {First Annual lieport of the Iowa Heather 

 Stations, p. 50.) 



