182 WATER AND FOREST. 



the workings of a mine and, as it rose, imprisoned air in some of the 

 mine chambers. The rising water forced this air through the rock 

 in several places — failing to do so at other points. A hydraulic head 

 of forty feet forced the air through two hundred feet of solid coal and 

 the water filled the chamber. Upon pumping out the water it took the 

 air two weeks to find its way back through the two hundred feet of 

 coal and fully destroy the vacuum made by the falling water. On the 

 other hand, slate — with its laminations lying at right angles to the 

 pressure — was perfectly tight and retained the air against a hydraulic 

 head of eighty feet. 



Tests at the German forest stations show that the general effect 

 of forests is to raise the soil temperature during the cold months and 

 lower it during the warm months. As the bulk og rain falls in the 

 cooler months, it follows that the raising of temperature during such 

 times is favorable to increased percolation by reducing the viscosity 

 of the water. A portion of this advantage would be offset by lowering 

 temperatures during the hot months. The surface tension of water is 

 also lowered by increase of temperature, as well as the viscosity. 

 This decreases the tension in the capillaries, causing less resistance 

 to gravitation, and more water passes down on this account. 



The following percolating velocities are quoted in the publications 

 of the agricultural experiment station of the State of Colorado. They 

 were deduced by the Italian Professor Nazzini from data obtained from 

 the filter beds of London, Paris and Berlin. They will serve to il- 

 lustrate the variation of velocity due to texture of material. The 

 velocities refer to that of vertical filtration: 



Minute gravel 86 1-2 feet per hour, t 



Coarse sand 9 1-3 feet per hour. 



Fine sand 1 7-10 feet per hour. 



Sandy soil 8-10 feet per hour. 



Sandy clay 4-10 feet per hour. 



These figures cannot, however, be taken as proper for universal 

 application. Each soil is more or less a law unto itself, and where ex- 

 act results are sought must be studied by itselt. 



There is one great distinction between water flowing freely in 

 open channels or pipes of measured size, and percolating water. The 

 flow of the former is a function of the square root of the head, while 

 water flnding its way through minute passages is found to vary in flow 

 directly with the head and not with the square root thereof. The 



