OF SELBOENE 163 



LETTER XXIX. 



TO THE SAME. 



Selborne, Feb. 7, 1776. 

 DEAR SIR, 



IN heavy fogs, on elevated situations especially, trees are perfect 

 alembics : and no one that has not attended to such matters can 

 imagine how much water one tree will distil in a night's time, 

 by condensing the vapour, which trickles down the twigs and 

 boughs, so as to make the ground below quite in a float. In 

 Newton-lane, in October 1775, on a misty day, a particular oak 

 in leaf dropped so fast that the cart-way stood in puddles and 

 the ruts ran with water, though the ground in general was 

 dusty. 



In some of our smaller islands in the West-Indies, if I mistake 

 not, there are no springs or rivers ; but the people are supplied 

 with that necessary element, water, merely by the dripping of 

 some large tall trees, which, standing in the bosom of a mountain, 

 keep their heads constantly enveloped with fogs and clouds, 

 from which they dispense their kindly never-ceasing moisture ; 

 and so render those districts habitable by condensation alone. 



Trees in leaf have such a vast proportion more of surface 

 than those that are naked, that, in theory, their condensations 

 should greatly exceed those that are stripped of their leaves ; 

 but, as the former imbibe also a great quantity of moisture, it is 

 difficult to say which drip most : but this I know, that deciduous 

 trees that are entwined with much ivy seem to distil the greatest 

 quantity. Ivy-leaves are smooth, and thick, and cold, and there- 

 fore condense very fast ; and besides ever-greens imbibe very 

 little. These facts may furnish the intelligent with hints 

 concerning what sorts of trees they should plant round small 

 ponds that they would wish to be perennial ; and shew them 

 how advantageous some trees are in preference to others. 



Trees perspire profusely, condense largely, and check evapora- 

 tion so much, that woods are always moist : no wonder therefore 

 that they contribute much to pools and streams. 



That trees are great promoters of lakes and rivers appears 

 from a well known fact in North- America ; for, since the woods 

 and forests have been grubbed and cleared, all bodies of water 

 are much diminished ; so that some streams, that were very 



