Relation of Soils to Moisture and Air 49 



outflow of water from tile drains. When the air pres- 

 sure above ground is lessened there is to some extent 

 the same diminution in pressure of the air in the soil, and 

 the water flows more freely from drain or spring, and rises 

 higher in wells. Some years ago late in autunm a corre- 

 spondent of a city paper wrote to the editor that he noticed 

 that just before a rain storm the water in his spring rose, 

 and he wanted to know the reason. The editor, who we 

 suppose had been taught the old fable that the sap runs up 

 the trees in the spring and runs down in the fall, replied 

 that this was easily explained, for it was the sap rimning 

 out of the roots of the trees at that season of the year. 

 Inasmuch as nothing of the sort takes place, as we shall 

 see later in our study of plants, a little more information 

 would have taught the editor that it was simply the release 

 of the atmospheric pressure that caused the water to flow 

 into the spring, just as it causes the mercury to flow into 

 the cup of the barometer. It has been found that in Wis- 

 consin during the summer, from July to September, the 

 surface of the ground water and the rate of percolation 

 into tile drains are subject to daily oscillations in level and 

 changes in the rate of flow, owing to the daily expansion 

 and contraction of the air contained in the upper two or 

 three feet of the soil. "The changes of pressure thus 

 developed react on the capillary water in the soil in the 

 zone where the cavities are nearly or quite filled, forcing 

 the water down and out into drainage channels when the 

 air is expanding, but allowing such as has not been per- 

 manently lost to return again to its normal level when the 

 pressure becomes less with the cooHng and contraction 

 of the soil air." This diurnal rise and fall from the differ- 



