THE ENGLISH LAKE DISTRICT. 113 



Evaporation. — It may not be uninteresting to learn the annual amount of 

 Evaporation in the immediate neighbourhood of a locality distinguished by so 

 prodigious a rain fall. The Tables shew that a comparatively small part of the 

 vapour requisite for the production of the enormous depth of water annually 

 deposited in the Lake District, is formed over or from the terraqueous surface 

 constituting the locality. 



The chief supply comes from those tropical regions where an unclouded and 

 almost vertical sun is continually lifting up the waters of the ocean into the at- 

 mosphere in an invisible form ; the warm vesicular vapour thus generated by 

 evaporation rises to a great height, and is transported by currents into our 

 Northern climate, where it descends as rain, and maintains the existence of our 

 lakes, rivers, and springs — while the large amount of sensible heat liberated 

 during the condensation and conversion of the vapour elevates the temperature 

 of the mountain valleys above that of the adjacent plains in the colder months, 

 and no doubt tends materially to modify the climate at all seasons. A cubic 

 inch of water will form a cubic foot of steam of the same apparent temperature 

 as the water ; but during the process of conversion it has absorbed 1000 degrees 

 of heat from the surrounding atmospheric space — of which the thermometer gives 

 no indication till the vapour is reconverted into water, when the latent caloric is 

 slowly evolved in a sensible form. The amount of perceptible heat yielded up 

 annually in the formation of 141 inches of rain at Seathwaite, is almost in- 

 credible, and the extent to which the climate of this naturally inhospitable region 

 is ameliorated- by this natural provision for equalizing temperature, must be pro- 

 portionally great. The average depth of water raised by evaporation at White- 

 haven in the 12 years between 1842 and 1853, is 29-664 inches, and this quan- 

 tity may be accepted as the liquid equivalent of the average amount of vapour 

 thrown off by our lakes in the course of a year. The real amount yielded by the 

 entire area of the district is of course considerably less. The rain fall in the 

 Lake country varies from 67 to 141 inches per annum. Of the 141 inches pre- 

 cipitated at the head of Borrowdale, probably at most 25 inches is supplied on the 

 spot in a vesicular form ; so that the mass of vapour annually imported from lower 

 latitudes must be equal to 116 perpendicular inches of water — nearly the whole of 

 which finds its way back again unchanged to its original source — the ocean. 



The waters of the ocean cover nearly three-fourths of the surface of the 

 globe ; and, of the 38 millions of miles of dry land in existence, 28 millions be- 

 long to the Northern hemisphere. It is computed that the depth of water raised 

 by evaporation from the entire surface of the ocean is about 4 feet per annum ; 

 while, in tropical seas, the amount converted into vapour cannot be less than 

 a quarter of an inch daily all the year round, or say 90 inches annually. In the 

 month of May, the liquid equivalent of evaporation from our lakes occasionally 

 amounts to 0-25 inch per diem for several days together, and in extremely dry 



VOL. XXI. PART I. 2 H 



