302 EFFECTS OF FORESTS UPON CLIMATE: MADEIRA. 



cases led to the planting of pines on the hills, but not enough to have 

 a perceptible influence upon the climate. The period when the forests 

 ^vere destroyed is not known, but there is a tradition that a great fire 

 consumed them not long after occupation,^ and that it smouldered for 

 years before final extinction. Weare wholly without knowledge as to the 

 climatic conditions when the island "was covered with forests, or as to 

 the changes that the clearing wrought, but simply know that the pres- 

 ent conditions have existed for centuries, and probably without mate- 

 rial change. 



But with respect to the agency of trees in condensing moisture, Dr. 

 Graham remarks: 



Wo may consider the tangible influence of forests in this matter, firstly, ^vith regard 

 to the power of directly augmenting the moisture of a climate ; and, secondly, as to 

 the property of absorbing and husbanding water. In this island the great power 

 which trees possess of contributing to the moisture of a country may be well observed 

 upon the mountains. The gentle breezes of the northeast wind almost constantly flow 

 upon the land charged with a certain amount of moisture. During the night the 

 quantity is not large enough to conden«=e into vapor, and the sky therefore remains 

 clear, but in the morning, after several hours of saushine, the moisture is so augmented, 

 chiefly by evapora.tion from the seas, as to become visible as mist or cloud at a cer- 

 tain elevation. » * * The moisture of the ordinary breezes in Madeira, aug- 

 mented as wo have seen (I speak from habitual observations), becomes apparent, as a 

 rule, a little way at sea, before it is actually driven upon the mountains, and the mist 

 invests the hills, whether there be trees or not. In approaching the land, the fleecy 

 masses coalesce and augment in size, and at length for a while rest at an interval of 

 eight or ten feet from the hill-side ; the interval slowly diminishes, and eventually 

 the mist comes in contact with the soil or leaves of trees. * » * Thus far, 

 iu the generation and attraction of mists, trees appear to exert no especial influence, 

 but their power upon mist already formed is great. Where there are no trees tho 

 cloud is driven along, depositing little or no moisture, at length to be again completely 

 vaporized over any heated ground and carried away to the sea ; but trees largely in- 

 tercept mist, and the small component vesicles of water coalesce upon the branches, 

 and fall in drops of water upon the earth.* This I regard as the principal mode in 

 which trees contribute to the water supply of a country. The mists will form whether 

 there be trees or not, but the water, otherwise lost, is strained out and saved by the 

 forest foliage. I believe the process to be purely mechanical, the mere aggregation of 

 small particles into drops ; and, moreover, I have never been able to observe that 

 mist is especially attracted by any particular kind of foliage. 



The mist is, at first, apparently dry, but it slowly increases in moisture and density 

 until, if in motion, it forms drops of water upon tho leaves. When the mist is station- 

 ary little or no deposit occurs. I have watched with much interest for the commence- 

 ment of the dripping, in reference to the supposed pre-eminence of certain kinds of 

 foliage in the power of condensation. The jiine tree invariably begins first, their 

 rough, bush-like clusters of leaves being well adapted to intercept the smallest parti- 

 cles of moisture. The yield of water from this source is very great. The laurels ex- 

 tract water plentifully from mists which are more sensibly damp, and their action 

 in this respect ia more important than that of the conifera?,inasmuch as the dense 

 shade of their broad leaves is subsequently a greater hindrance to the evaporating 

 power of the sun upon the collected water, and the undergrowth is considerable, and 

 highly retentive of moisture. 



1 Discovery followed by settlement in 1419. 



-It was remarked by Saint Pierre, in speaking of the island of Bourbon [Reunion], 

 that the clouds perceptibly deviated from their course to collect around the mountain 

 peaks, from whence they descended into the valleys along the declivities of the forests, 

 which likewise attracted them, and there dissolve in rain, frequently forming rainbows 

 on the verdure of tho trees. He noticed that fields situated in an open place in their 

 vicinity very often suftVred from want of rain, whereas it rains almost the whole year 

 round in tho woods which are not above a gunshot distance. It was by the destruc- 

 tion of a part of the trees that clothed the heights of tho islaud that most of the 

 brooks which watered it were dried up. and now nothing remained but the empty 

 channels. He attributed the diminution of streams in Europe to the same cause, and, 

 applyingtheprinciplesof modern physics to the classic legends of antiquity, he adds, "It 

 is neither among the reeds nor in the depth of the valleys that the Naiads conceal their 

 exhaustless urn, as painters represent them, but at the summit of rocks, crowned with 

 woods, and towering to the heavens." — {Botanical Harmonies Delineated, by J. H. Ber- 

 nadin de Saint Pierre, p. 54.) 



