PBEVALBNCB OF EVAPORATION. 59 



forests exercise a considerable influence in diminishing the evapora- 

 tion of moisture from the soil ; but, in connection with the con- 

 sideration of this, there should be taken into consideration the 

 general phenomena of evaporation from forests. 



M. Cdzanne in his Suite to Etude snr les Torrents des Mantes Alpes, 

 byM. Surell, remarks that "evaporation is for modem physical science 

 a phenomenon comparatively simple and well understood. It is known, 

 for each degree of temperature, what is the tension and what is the 

 measure of saturation of the air. But looked at from a meteorological 

 point of view the pheuomenon is one of the most obscure; and the 

 limited number of observations which have been made cannot admit 

 of their being compared together. 



" What, in a given place, is the depth of water which is carried off 

 in one hour by evaporation? This varies according as the evaporating 

 surface specified may be drenched or only damp, be stagnant or 

 running water, be in the sunshine or shade, according as it may be 

 summer or winter, according as the air may be saturated or not, or 

 be more or less saturated, and according as the air may be at the 

 time windy or calm. 



" Two adjacent basins, placed in the same meteorological circum- 

 stances, evaporate unequal quantities of water if they be larger or 

 smaller, if their sides be more or less elevated, if they be sheltered 

 from the wind or not, etc. 



" When it is said that the annual evaporation at Rome is 2'462 

 millimetres, at Marseilles 2289, at Paris 0698, at London 0754, at 

 Toulouse 0"649, at Copenhagen 0'209,* all that is meant to be said is 

 that in the circumttances in which the observers were placed they 

 have determined these results. But the mean annual evaporations 

 fumisbed for each place are far from having the same scientific value 

 as the mean annual rainfalls, which present on the contrary a specific 

 character. It may always be affirmed with certainty that evaporation 

 goes on decreasing from the equator to the pole, where, however, it is 

 far from being nil, even in the greatest frost jt that it is more feeble 

 near the coast than in the interior of a country ; aud more feeble also 

 under the equatorial wind from the south-east than under the /wiar 



• Vall&s : Tnondaticns, p. 23. 



t Hayes : La Mcr Hire du Pole, p. 224. Wet linen, exposed to the air, 



dried at the lowest ttnipeiatnieB ; and a sheet of ice, easpeuded 



by » thread, evaporated away by little and little. 



