METEOROLOGY. — DEW. 491 



is by so much tlie greater as tlie temperature is higher. In very 

 warm climates the dew is so copious as to assist vegetation essen- 

 tially, supplying the place of rain during a great part of the year. 



According to some meteorologists dew is most copious near the 

 sea-board of a country ; very little falls in the interior of great con- 

 tinents, and indeed is said only to be apparent in the vicinity of lakes 

 and rivers.* I cannot agree in any statement of this kind ipade so 

 absolutely. I have never had occasion to see more copious dews 

 than those which occasionally fall in the steppes of San Martin to 

 the east of the eastern Cordilleras, and at a very great distance 

 from the sea ; the dew was so copious that for several nights I found 

 it impossible to employ an artificial-horizon of black glass in order 

 to take the meridian altitude of the stars ; the moment the apparatus 

 was exposed there was such a quantity of water deposited on the 

 surface that it soon gathered into drops and trickled off. I found it 

 necessary to have recourse to mercury to reflect the star under ob- 

 servation. During the clear calm nights the turf of these immense 

 plains receives a considerable quantity of moisture in the form of 

 dew, which materially assists vegetation, and by its evaporation 

 tempers the excessive heat of the ensuing day. In tropical coun- 

 tries the forests contribute to keep down the temperature. In very 

 hot countries it is rare to be out in a cleared spot, when the night 

 is favorable to radiation, without hearing drops of water, produced 

 by the copiousness of the dew, falling continually from the surround- 

 ing trees, so that forests contribute further to produce and to main- 

 tain springs by acting as condensers of the watery vapor dissolved 

 in the air. I might cite a number of observations upon this point 

 which I made in the forest of Cauca. In the bivouac between the 

 4th and 5th of July, 1827, the night was magnificent ; nevertheless, 

 in the forest which began at the distance of a few yards from our 

 encampment, it rained abundantly ; by the light of the unclouded 

 moon we could see the water running from the branches. 



It is possible that the transpiration from the green parts of the 

 trees might have been added to the dew condensed, and so increased 

 the intensity of the phenomenon which I have described ; but I 

 rather incline to believe that the cooling of the leaves by way of 

 radiation had by far the largest share in the production of this dew- 

 rain. It is true that of all the leaves which form the crown of a 

 tree, those whose surface is exposed and radiate freely into space 

 intercept, as would a screen, the radiation of the leaves and branches 

 which are not so exposed ; but, as M. de Humboldt has observed, if 

 the leaves and branches which crown a tree cool directly by emis- 

 sion, those which are situated immediately beneath them by radiating 

 towards the lower parts of the leaves which are already cooled a 

 greater quantity of heat than they receive, their temperature will 

 also necessarily fall, and the cooling will thus be propagated from 

 above downward until the whole mass of the tree feels its effects 

 It is thus that the ambient air circulating through the leaves become* 



• Kaemtz, Meteorology, translated by W. Walker, London- 1844. 



