582 KEPORT— 1900. 



Thick sea-mists often cling to their tops for several hours after sunrise, 

 •while the plains below are already dry and sunny. It is only by noticing 

 the large amount of moisture intercepted and dripping from the over- 

 hanging boughs as the sea-mist drifts slowly past that one can realise 

 how prolific a source this must be. The amount condensed in this way 

 might be tested by a rain-gauge of wide aperture. 



' When one of these ponds is examined in the middle of a hot summer 

 day it would appear that the few inches of water in it could only last 

 a week. But in early morning or towards evening, or whenever a sea- 

 mist drifts in, there is a continuous drip from the smooth leaves of the 

 overhanging tree. There appears also to be a considerable amount of 

 condensation on the surface of the water itself, though roads adjoining 

 may be quite dry and dusty. In fact, whenever dew is on the grass, the 

 dew-pond is receiving moisture ; and this moisture, owing to the shade of 

 the overhanging tree, is partly preserved throughout the day ; so that 

 sheep or cattle may drink daily from a small shallow pond which receives 

 no rain, and yet the pond be not exhausted unless the nights are excep- 

 tionally dry.' 



We now want observations by the thermometer. We should be glad 

 to know the temperature of the water of the pond at various depths, as 

 taken hourly through a summer night ; the temperature of the surface 

 of the ground (free from grass) a few feet away from the pond ; the tem- 

 perature of the air at various small heights above the hill-top, as well as 

 in a neighbouring valley ; and also the relative humidity of the air as 

 determined by wet and dry bulbs. If a small party would make such 

 observations during a few clear still nights in hot summer weather, some 

 blanks in our knowledge would be filled up at once. We should be glad 

 to know how the level of a dew-pond varies during a spell of sultry 

 weather. Observations on the clouds and mists of the summits of the 

 downs are also much to be desired. Day observations would have their 

 use too, and should not be omitted. 



Is it too much to ask that residents in the south-eastern counties will 

 investigate these matters for us ? A party of a dozen meteorologists 

 who would choose a time of settled hot weather, and give a week to the 

 inquiry, would throw much light upon a question of considerable scientific 

 and practical importance. 



While so many data are wanting, it would be unwise to express a 

 confident opinion as to the relative importance of rain and dew in the 

 supply of water to the ponds of the chalk downs. No scientific man will 

 ■willingly decide any question on indirect evidence if direct evidence can 

 be got. 



In making inquiries about dew-ponds I have been helped by many 

 friends and correspondents, among others by Mr. W. Whitaker, F.R.S., 

 Mr. Clement Reid, F.R.S., Mr. John Hopkinson, Mr. T. W Shore, 

 Mr. F. J. Bennett, Mr. John Brigg, M.P., Professor G. S. Brady, F R.S., 

 and the Rev. Henry Green. 



Bibliography of Detc-ponds. 



Hales, Stephen . . 'Statical Essays,' vol. i. Experiment XIX. pp. 52-57. 



Second edition, London, 8vo. 1731. 

 White. Gilbert . . ' The Natural History and Antiquities of Selborne,' Letter 



XXIX. (to Ban-ington). London, 4to. 1789. 



