164 Probable Effects of Vegetation on (Jtimate. 



of water followed almost immediately, — in some cases the coffee-plants 

 dying in consequence ; the jungle was allowed to grow again when 

 water returned, the springs were opened, and the rivulets and streams 

 flowed afresh as formerly. Around Ahmednuggur, springs shaded 

 by trees have invariably been observed to dry up almost immediately 

 on the trees being removed. Having seen the result of the destruc- 

 tion of trees in diminishing the fall of rain, we come now to the con- 

 verse state of matters, so as to establish the proposition by both 

 varieties of proof. Unfortunately our evidences on this side of the 

 question are much less numerous than those on the other, though 

 equally uniform and pertinent, the propensity to remove or destroy 

 being much more prevalent and active than to establish forests. 

 The St Helena Almanac for 1848 gives particulars of the increase 

 of the fall of rain within the last few years, attributable to the in- 

 crease of wood : within the present century, the fall has nearly 

 doubled. The plantations seem to have performed another service 

 to the island. Formerly heavy floods, caused by sudden torrents of 

 rain, were almost periodical, and frequently very destructive : for 

 the last nine years they have been unknown. On the mountains of 

 Ferro, one of the Canary Islands, there are trees each of which is 

 constantly surrounded by a cloud: their power of drawing down 

 moisture is well known to the people : the natives call them garol, 

 the Spaniards santo, from their utility. The drops trickle down the 

 stem in one unceasing stream, and are collected in reservoirs con- 

 structed for their reception. Thousands of similar instances might 

 be quoted : Our own revenue surveyors, indeed, could supply an 

 almost unlimited amount of information bearing on the same sub- 

 ject. The whole of this beautiful process depends on the simple 

 laws of temperature, evaporation, and condensation. Trees shade 

 the soil from the sun. They give off vapour during the day, and so 

 mitigate heat, vi^hile they obstruct the direct rays from above, — they 

 radiate out heat during the night, and occasion the precipitation of 

 dew, — many plants being endowed with this faculty to such an extent 

 as to collect water in large quantities from the air. The total 

 quantity of dew believed to fall in England is supposed to amount to 

 five inches annually — and the estimate appears to us to be a vast way 

 under the truth : the average fall of rain is about twenty-five inches. 

 Mr Glaisher states the amount of evaporation at Greenwich to have 

 amounted to five feet annually for the past five years, and supposes 

 three feet about the mean evaporation all over the world : On this 

 assumption the quantity of actual moisture raised in the shape of 

 vapour, from the surface of the sea alone, amounts to no less than 

 60,000 cubic miles annually; or nearly 164 miles a day. Accord- 

 ing to the observations of Mr Laidlay, the evaporation at Calcutta is 

 about fifteen feet annually ; that between the Cape and Calcutta 

 averages, in October and November, nearly three quarters of an inch 

 daily ; betwixt 10° and 20° in the Bay of Bengal, it was found to 



