194 NINETEENTH REPORT. 



maximum temperatures above 42° during the period in question, and X 

 being the number of such daj's, C is the number of clear days during 

 the period and P is the number of partly cloudy days. In other words, 

 the sum of the maximum temperatures above 42° F. during any period, 

 after 42 has been subtracted from each, is to be increased by 15 for 

 each clear day and 10 for each partly cloudy day during the period. 

 This leaves out of consideration the excess of 1.0° F. in temperature 

 during cloudy weather, which is so small that it may well be disregarded. 



The final formula which is to be brought out with further study, will 

 take into consideration more accurate values for plant temperature, and 

 give proper weight to the effect of wind velocity, humidity, and the 

 caloric and actinic value of sunshine. But the above method will be 

 found to be an improvement over the temperature summation methods 

 which has been so largely used, especially if some system such as 

 Livingston's method of indices is used in connection therewith to correlate 

 plant growth and the effective temperatures thus found. 



In conclusion I would state that these studies are only preliminary to 

 others which I hope to take up further, with the aid of more accurate 

 instruments and methods. It is realized that an enormous amount of 

 research must be carried through before the final goal of establishing 

 exact formula?, as to the relationsliip between climatic conditions and 

 crop production, can be established, and this is only a minute contribution 

 toward the desired end. 



I gratefully acknowledge the valuable suggestions and assistance ren- 

 dered by Dr. E. A. Bessey, Dr. R. P. Hibbard, Prof. C. W. Chapman, 

 and others of the Michigan Agricultural College, and also the aid that 

 Mr. B. B. Whittier, Observer, has given by taking many thermometer 

 readings. 



LITERATURE CITED. 



(Numerals in connection with author's names in the preceding pages, 

 corresjjond with tlie following publications) : 



^Swingle, Walter T., "The Date Palm." Bulletin No. 53, Bureau of 

 Plant Industry, 1904. 



'Smith, J. Warren, "Effect of Weather on the Yield of Potatoes," 

 IDage 222, INIonthly Weather Review, 1915. "Correlation Between Crop 

 Yield and Weatlier, page 792, Montlily Weather Review, 1911. "Effect 

 of Weather on the Yield of Corn," page 78, Monthly Weather Review, 

 1914, and other articles. 



^Abbe, Cleveland, "A First Report on the Relation Between Climates 

 .'.nd Crops." Bulletin No. 36, W. B. No. 342, 1905. 



