CORRESPONDING SOCIETIES. 581 



The few that have been mentioned to me as occurring in the Midland 

 counties all turned out on inquiry to depend obviously on surface 

 drainage, usually from a hollow in a neighbouring hard road. It is thus 

 with the ' meres ' of Derbyshire which appear, from such information as I 

 have been able to procure, to be always fed from adjacent collecting 

 grounds. If any one can furnish an unexceptionable example of a true dew- 

 pond on a chalk-down, or other hill, which is distant a hundred or even 

 fifty miles from the south coast, the news would be welcome. 



Those who believe that dew-ponds in all cases owe their existence to 

 rainfall alone (and among these was that eminent meteorologist, the late 

 G. J. Symons, F.R.S.) bold that, owing to elevation, the temperature of 

 the water in such ponds is very nearly coincident with the dew-point, so 

 that evaporation and condensation balance each other. Whether such a 

 temperature of the water is regularly maintained day and night through- 

 out a season of drought cannot, I believe, be established by existing 

 thermometric observations. I doubt whether it can be established by 

 general reasoning. Before we can accept the view that dew-ponds are 

 replenished by rain alone, we must refute or explain two facts, both of 

 which are supported by strong testimony : 



1. That dew-ponds do not dry up when the low-level ponds of the 

 same district are evaporated. Not only do the dew-ponds replace in some 

 way the loss due to evaporation, but they supply large flocks of sheep. 

 If it is contended that during summer droughts the hill -tops are regularly 

 visited by local clouds which are precipitated as rain, the frequency and 

 substantial yield of such clouds need to be better established than at 

 present. 



2. That dew-ponds cannot in the first instance be filled by rain (see 

 Mr. Clutterbuck's prize essay). This statement, if it can stand inquiry, 

 seems to be decisive against the sufficiency of rainfall. 



On the other hand the restriction of dew-ponds to an area quickly 

 and directly reached by south-west winds blowing from the sea, supports, 

 it would seem, the view that what we call dew-ponds are really rain- 

 ponds. Moisture-laden winds favour cloud-formation and rain, while we 

 have no reason to suppose that they favour dew-formation, but rather 

 the contrary. 



Mr. Clement Reid, F.R.S., of the Geological Survey, sums up his 

 wide experience of dew-ponds in these words : — 



' The conditions that are required for a permanent dew-pond do not 

 seem generally to be understood, failure or success appearing to be the 

 result of chance rather than of any clear comprehension of the principle 

 on which the dew-pond acts. On comparing, at the end of a long 

 drought, the dried-up ponds with those that still contain water, we find 

 that, other things being equal, the best dew-pond has the following 

 characteristics : 



' It is sheltered on the south-west side by an overhanging tree, often 

 only a stunted ivy-covered thorn or oak, or by a bush of holly. Or else 

 the hollow is sufficiently deep for the south bank to cut off much of the 

 sun. The depth or shallowness of the water does not appear to make so 

 great a difi'erence as would be expected. 



' The open downs, even in the middle of summer, receive much heavier 

 dews than would be expected, or than are met with on the lowlands. 



