67 



Now, the combined effects of these unfavourable condi- 

 tions, natural and artificial, on the surface of the district, 

 for admitting to the strata a supply of the atmospheric water, 

 amount to a considerable deal more than at first sight 

 appears. To these, others also must be added, and which 

 1 shall immediately notice. 



The mean annual fall of rain over the neighbourhood (as 

 ascertained by Mr. Abraham, of Lord- street) for the last 

 seven years, has been from 82 to 36 inches. Now, by taking 

 the maximum fall at 36 inches the question is, how much 

 of this penetrates the superficial obstructions and ultimately 

 lodges in the mass of the strata ? 



During the summer solstice, when the greatest fall of rain 

 occurs, the temperature of the soil is generally high, and the 

 water is also dried up by the rays of the sun ; consequently 

 the rain that falls moderately upon it at that time, is rapidly 

 absorbed, and penetrates but a very little way below the 

 surface, seldom to more than six or seven inches after two or 

 even three days of continuous rain. When it falls heavily, 

 as it does in thunder showers, the surface becomes battered 

 by the violence of the rain to that extent that nearly the 

 whole is suddenly thrown off and discharged by the chan- 

 nels made for that purpose. Evaporation goes rapidly on 

 in various ways, and vegetation absorbs and throws off large 

 quantities of moisture, so that, during the summer months, 

 little if any of the atmospheric water penetrates into the 

 mass of the strata. 



The winter solstice, therefore, must be the only period 

 when the supply to the strata can be given, and that is when 

 the least quantity falls. Now, I have shewn that over nine- 

 tenths of the surface a stiff diluvial clay exists, which suffers 

 water to percolate very slowly through it. The artificial 

 ducts that are now being extensively laid down in the dis- 

 trict for the purpose of drawing off all superfluous or sub- 

 soil water, are conditions that operate powerfully against the 



