TRANSACTIONS OF THE SECTIONS. 209 



of the water. Well-water is usually much colder and of much move uniform tem- 

 perature than ordinary surface-water. Its coldness is, in part, no doubt due to the 

 Fact that the greatest absorption of rain takes place in the winter months, and the 

 water enters the ground at a cold period and at about the normal temperature of 

 the air. Very careful observations on the Chalk formation south of Croydon show 

 that the month of December is the month which appears principally to contribute 

 to the supply of the springs, and that after a rainfall it takes some time for the 

 springs to rise, depending mostly on the dryness or wetness of the season. A small 

 rainfall in December is almost sure to be followed by low springs in the following 

 year, or a dry December interferes, to a most marked extent, with the contribution 

 of water to the springs ; for although experiments with percolation-gauges show 

 that certain quantities of water percolate at all periods of the year, the measure- 

 ment of wells and gauging the volume of springs show that a large part of the rain 

 falling never influences the quantity of water in the ground in the slightest degree. 

 In the year 1876, between the beginning of May and the end of November, 

 although over twelve inches of rain had fallen at Croydon, the rainfall did not 

 alFect the quantity of water in the subsoil ; but during the whole of this period hoth 

 the volume of water flowing from the springs and the height of the water in the 

 subsoil steadily diminished. During the present year, there has been a gradual 

 subsidence in the water, both with regard to its altitude and volume, since the 

 25th April, although between the beginning of April and the end of July upwards 

 of ten inches of rain had fallen. It is found, upon examination, that the water of 

 the deep wells in the higher parts of the district, notwithstanding the greater depth 

 of the wells, has a lower temperature than the water in the wells in the lower 

 portion of the district that are not so deep. The water of wells which have been 

 sunk into the Upper Greensand has relatively a lower temperature than the water 

 of wells sunk in the chalk. At certain periods the temperature of the water at 

 the surface of the well is somewhat warmer than at the bottom of the well. In 

 other cases the water at the bottom of the well is equal in temperature, or exceeds 

 that of the surface. In all probability, in the cases where the surface-temperature 

 is greater than the bottom-temperature, it is due to the conduction of heat from 

 the warmer air of the well to the surface of the water; and the heat being trans- 

 mitted through the water very slowly downwards, tends to keep the water at the 

 surface at a higher temperature than at the bottom of the well. This is very 

 clearly shown in the case of the temperature of the well at the Water Works at 

 Croydon. The upper water in the well on the 6th December 1876 had a 

 temperature of 64°-5, while at the bottom of the well the temperature was 51°-25. 

 The increase of the surface-temperature in this case was due to the admittance of 

 hot water from the surface-condenser into the well. The temperature ohserved at 

 the bottom of the well was about the ordinary normal temperature. In cases in 

 which the water is excessively cold at the bottom of the well, it is probably due 

 to the inferior motion of the water, the strata being dense, and there being less 

 circulation. In the lower districts, especially where the water rises into the gravel 

 and has very free movement, the temperature of the water is higher than in the 

 chalk. The temperature of the water under the town of Croydon, which is built 

 upon the outflow of the springs into the gravel, is higher than in the wells imme- 

 diately outside the town. The movement of the water from the higher to the 

 lower districts woidd, of itself, tend to raise the temperature. The descent of the 

 water from a high to a low elevation would also tend to compress air and gases 

 held in the interstices of the water, and thus would tend to increase the specific 

 heat. Probably to these two circumstances may be attributed the slight increase 

 in the temperature of the water observed in moving down such valleys as that of 

 Caterham, which has its discharge into the River Wandle at Croydon, and also in 

 other valleys in this neighbourhood. 



