178 WATER AND FOREST. 



pJates were kept in place by iron girders and the edges of the plates 

 and ends of the girders supported by brickwork on the back and two 

 sides. Trenches were then dug around the block of soil and It was 

 gradually enclosed by walls of brick laid in cement. Below the per- 

 forated iron bottom a zinc funnel was finally fixed, and the drainage 

 water collected and measured in galvanized iron cylinders with gauge- 

 tubes as in the case of the rain-gauge. 



These soil drain-gauges being kept free from vegetation give re- 

 sults not directly applicable for etimating the percolation over a drain- 

 age area more or less covered with vegetation — conditions for which 

 due allowance is made in applying the results. 



The capacity of sand for receiving and transmitting water can be 

 well illustrated by the water supply of The Hague, the capital of the 

 Netherlands. The city is situatea about two miles inland from the 

 North Sea, and has a population of about 190,000. Its domestic water 

 supply is drawn from a tract of uncultivated country lying near the 

 sea and covered with sand dunes like parts of the New Jersey coast. 

 The sand is described as very pure and white. The water is fresh to 

 a depth of sixty-six feet below the sea. It is gathered by infiltration 

 pipes and pumped to its destination. Through the courtesy of Mr. 

 Corey, the United States consul at Amsterdam, I have been able to 

 make a comparison between the rainfall and the percolating water in 

 point of quantity. The average rainfall from 1878 to 1898 was 30 96-100 

 inches per annum, of which about 40 per cent, is estimated as per- 

 colating. 



According to other information, from 30 to 50 per cent, of the rain- 

 fall can be collected. The variation being principally due to the sea- 

 sons in which the rains fall. In summer a loss is stated to take place 

 by reason of vegetation. This draws attention to the point that 

 whether vegetation conserves moisture or not depends very much on the 

 nature of the soil. In the case of sand it impedes the ingress and 

 aids the abstraction of moisture. A sieve would profit nothing as a 

 sieve by planting trees over it. 



CONDUCTIVITY OF FORESTS. 



The water conductivity and capacity of various soils has received 

 much attention at the hands of forest experimenters. 



By conductivity is meant the capacity to transmit water. It In- 

 volves three units: Quantity, distance and time. 



By capacity we understand the quantity of water that a given vol- 



