HE correlation of variations in corn yield with fluctuations of climatic factors is more than an academic problem. Many of the human activities prominent in the Corn Belt, rural or urban, are directly or indirectly related to corn. Beginning with the preparation of the seedbed in spring and extending intermittently through the harvesttime of late autumn, the larger part of the Corn Belt farmer's time and activities is applied to corn. Wintertime activities include the feeding of animals with corn. Corn Belt villages exist mainly to serve the corngrower. The cities buy, process, and sell the products of the cornland and serve the corn farmer in many other ways. The average yearly value of the corn crop of the United States is roughly one twenty-fifth the total value of all plant products grown in the world suitable for human or animal food. In recent years corn has occupied yearly about Ioo,ooo,ooo acres' of land in the United States-roughly five times the total area of Indiana. Of these IO10,000,00 acres, more than 60 per cent2 lie in the ten states of which parts are included in the geographical division known as the Corn Belt. On an average, about I9 per cent3 of the area of Iowa is planted to corn. In the heart of the Corn Belt, however, as much as 40 to 50 per cent of the area is in corn yearly. Here no other crop so dominates the landscape. Corn is king. Perhaps no other thing about the corn crop is so interlocked with the weal and woe of the Corn Belt inhabitants as the yearly variation in yield. The corn crop of the United States fluctuates from about one and a half to about three billion bushels.4 The yield per unit area fluctuates notably from year to year. For the state of Indiana, some years give an average per-acre yield more than twice as large as some other years. Iowa, commonly considered a relatively stable state climatically, in some years has an average per-acre yield 300 per cent greater than in others. No other factor affecting the yield of corn from year to year in a given area fluctuates as does weather, or the longer phase of weather, climate. Soil fertility is more or less constant, as is the care given the crop. Varieties are changed slowly. Reed5 has found that in Iowa