SHELTER TO POJTDS. 55 



In the Journal of the Royal Agricultural Society, N.S., vol. iL, 

 p. 1 10, there is a statement, by Mr R. Orlebar, of Willingborough, on 

 the advantage of planting treps around ponds, in which he says, " It 

 is astonishing what efiect a little shade has in checking evaporation. 

 A pond that is well shaded wiU hold water for weeks after one 

 of equal dimensions, but lacking shade, will become dry. The best 

 shade is that given by fir trees, or other evergreens, for they give it 

 aU the year round. The yew, perhaps, would be even better for 

 shade than the fir, but for its poisonous qualities. After the fir, I 

 doubt whether there is, on the whole, any better plant for shade than 

 the hawthorn bush. Its leaves sprout early, and fall late ; and it 

 possesses, besides, the great advantage that it forms its own fence." 

 And the writer tenders the counsel, suitable for Europe and countries 

 in the northern hemisphere, "Alwajs, if possible, have the ffouth 

 of your pond oa the north side, and the shade on the south side.'' 

 A suggestion which commends itself. While the writer speaks only 

 of shade, it may be that the eflFect mentioned by him was to some 

 extent due to shelter afiForded to the pond from wind. 



I find it stated, in reference to the su^estion of Mr Orlebar, by a 

 writer, On the Dew of Heaven and the Influence of Forests, in the 

 Farmer's Magazine, Mr Cuthbert W. Johnstone, that in some parts of 

 England they are careful to have the oak planted around their ponds ; 

 and he mentions, as bearing upon this point, a fact to which I have 

 already referred, that the dew under some large oak trees by the side 

 of his bowling-green, at Croydon, is always considerable of an evening ; 

 but that under a witch-elm, growing by their side, the dew is almost 

 always absent. Observation will show whether this difference is attribu- 

 table to the difference in the shade afforded, or to a difference in the 

 quantity of moisture evaporated through the stomates of the leaves, 

 in accordance with what has previously been stated with regard to 

 the dropping from trees (ante p. 35). . 



Sect. II. — On the Dedecation of Ground hy Drying Winds being 

 Prevented hy the Shelter afforded hy Clumps of Trees. 



In the case of a tree by the side of a stream with the grotmd 

 between the stream and its trunk moist, while all beyond was dry, we 

 had an indication of the effect of a tree in preventing evaporation by 

 protecting the moist ground from the sun's rays, and it nay be, in 

 part, by reflecting or confining vapour rwing from the soU ; but not 

 less important is the influence exerted by a clump of trees ia 



