Nov., 1911.] Climatic Conditions and Plant Growth. 
389 
From the tabulation it is seen that the year 1908 was 1.4 
degrees warmer than 1909; but what is more to the point, the 
warm growing months from April to September inclusive averaged 
2.8 degrees warmer in 1908 than in 1909. This higher temper- 
ature made the drought more disastrous for plant life. 
A porous cup atmometer was operated near Oxford, by 
Professor S. R. Williams, through the months from June to Sep- 
tember, 1908, inclusive. The work was done for Messrs. Burton 
E. Livingston and Forrest Shreve, who have kindly given us the 
figures for use. Without correcting for depth, the figures are 
valuable in showing the atmospheric conditions under which 
vegetation existed here for these months, and in making possible 
comparisons with those obtaining in other portions of the country. 
The porous cup atmometer records the evaporating power of the 
air as this affects the water layer covering the moist clay surface 
of the cup. This surface is in many respects comparable to that 
offered by transpiring foliage under the influence of air conditions. 
The cups were operated during the same months at a large number 
of selected stations in various portions of North America, and 
the results obtained near Oxford may be compared readily with 
those found elsewhere. The average weekly evaporation in the 
vicinity of Oxford was as follows: June, 132 cc. ; July, 182 cc. ; 
August, 211 cc.; September, 212 cc. The evaporation for June 
over the region east of the 100th meridian was somewhat more 
than 100 cc. per week. In July there was a local area covering 
northeastern Ohio, eastern Michigan and a large part of Penn- 
sylvania and New York that showed a weekly average of about 
200 cc. The conditions remained about the same over this area 
dining August. Comparison proves that we were, during all of 
this time, in a region of very dry atmospheric conditions where 
the evaporation was high, compared with records for other portions 
of eastern North America. 
The evaporation from the porous cup atmometer is independent 
of soil moisture and depends upon the atmospheric conditions. 
Therefore, conditions of soil moisture can not be deduced from 
evaporation figures, but must be worked out separately. Our 
study of soil moisture conditions began early in October, 1908, 
when the drought was at its height and the cumulative effect on 
the soil was marked. The study was continued for nine months, 
extending into July, 1909. The results would have been more 
valuable, had the study begun five months earlier. Below is 
given the table of soil moisture: 
