32 PHENOMENA OF VEGETATION. 



and the water flowing from its foliage uninterruptedly, drop by drop, 

 was collected in cisterns. 



I have met with a similar account of a tree to the east of Tocat in 

 Asia-Minor, visited by the Rev. Mr Van Lennep, and described by 

 him. Mr Lennep is said to be well-known as a competent observer, ' 



In the case mentioned first there is the explicit statement that 

 " Every morning the sea breeze drove a cloud towards the wonderful 

 tree." And there is nothing in any of the statements I have seen 

 relative to these phenomena inconsistent with the supposition that 

 they may have been produced by the deposition of moisture on the 

 surface of the leaves, irrespective of the source whence the moisture 

 was obtained. 



Dr Wells, in an essay on Dew, — a little work which, according to 

 Sir John Herschel, deserves to be considered as a model of experi- 

 mental enquiry, — was the iirst to place in a clear light the nature of 

 the process of dew formation. According to his views, as epitomised 

 by Sir John Herschel, " The chief facts to be accounted for are these : 



1. Dew (as distinguished from small rain or moisture produced by 

 visible fog) is never deposited except on a surface colder than the aii". 



2. It is never deposited in cloudy weather ; and so strict is its con- 

 nection with a clear sky that its deposition is immediately suspended 

 whenever any considerable cloud passes the zenith of the place of 

 observation. 3. It is never copiously deposited in a place screened 

 or sheltered from a clear vieio of the sky, even if the screen be of very 

 thin material, such as muslin or paper suspended over it. 4. It is 

 most copiously deposited on all such leaves as are good radiants and 

 had conductors of heat, such as grass, paper, glass, wood, &c., but 

 little or not at all on had radiants, such as polished metals, which are 

 also good conductors. And lastly, it is never deposited if there is 

 much wind. All these circumstances, as Dr Wells has shown, point 

 to the escape of heat from the bodies exposed by radiatio7i out into 

 space, or into the upper and colder regions of the air, faster than it 

 can be restored by counter-radiation, or by conduction, from contact 

 with the warm air — or with solid substances — the wind acting in this 

 respect with great efficiency by continually removing the air in con- 

 tact. Hoar frost differs only from dew by being frozen in the 

 moment of deposition, and therefore accreting in crystalline spiculse." 



It may have been observed that tufts of grass and other herbage 

 are often covered with dew-drops in the early morning, or with hoar 

 frost if it be winter, though the ground be not so ; and it may have 

 been observed that heavy drops of water fall from a tree in a fog 



