42 



J, HOPKINSON HERTFOEDSHIEE RAINFALL, 



period of time, and in the second because the Nash Mills p;anp;e 

 gives the smallest percolation. Both gauges being artificially 

 filled with soil, they must be more apt to let through a greater 

 quantity of water than would percolate through undisturbed soil 

 than a less quantity. Soil, also, must frequently expand and 

 contract, and when contracting in a gauge it must part from the 

 sides of the gauge and so leave around it a space, however small, 

 down which water will pass more freely than through natural soil. 

 In the following table, therefore, in which the average percentage 

 of percolation and evaporation given by the two series of experiments 

 is calculated on the mean rainfall of Hertfordshire, the amount of 

 percolation is probably rather overstated, and that of evaporation 

 rather understated. The summer and winter rainfall are assumed 

 to be equal, 



From Table XV it would appear that evaporation from water 

 is very little greater than it is from the surface of grass-covered 

 soil. At Lea Bridge, for the thirteen years ending 31st March, 

 1873, with a mean annual rainfall of 25-93 inches, of which 

 r2-16ins. fell in the summer, and 13-77 ins. in the winter, the 

 mean annual evaporation was 20*73 ins., or 80 per cent, of 

 the rainfall, the evaporation in summer being 15-91 ins., or 131 

 per cent, of the summer rainfall, and in winter 4-82 ins., or 35 per 

 cent, of the winter rainfall. Thus the evaporation in summer was 

 rather more than three times that in winter ; or, relatively to the 

 rainfall, nearly four times. 



These experiments are perhaps of more value in showing the 

 great difference there is between the amount of evaporation in 

 summer and in winter, than in showing the actual amount of 

 evaporation from the surface of water. That this probably averages 

 nearer 18 inches than 21, seems to be shown by a very careful 

 series of experiments made at Strathfield Turgiss with a gauge now 

 in the possession of Mr. Symons, who is continuing the experiments 

 in London (at Camden Square). 



That there is a very great increase in the amount of percolation 

 through soil when its surface is kept free from vegetation, is shown 

 by Table XVI. This table gives the percolation of rain through 

 5 feet of soil, the surface of which is frequently hoed to keep it 



