356 On Fences most eligible for Gardens, fyc. 



when Horticulture began to be practised as a science, 

 down to our own times ; it therefore becomes a great object 

 with the Horticulturist, in this moist, chilly climate, to 

 attend to every circumstance likely to procure an increase 

 of atmospherical warmth. The foliage of living vegetables 

 receives heat during the day from solar influence, but the 

 degree of heat given to a current of air passing through a 

 hedge or plantation of trees is greatly modified by the 

 quantity of water transpired by the leaves. The recently 

 expanded foliage of deciduous and evergreen trees tran- 

 spires a very large quantity of water in the form of aqueous 

 vapour, and this carries off nearly all the caloric received 

 from the action of the sun. The leaf therefore does not 

 become sensibly warm to the touch. On the contrary, in 

 a cloudy day in April and May, the wind seems to be 

 actually refrigerated in passing through a thick Hawthorn 

 hedge, and this may be accounted for on the same prin- 

 ciple, that cool air is obtained in the houses of the wealthy 

 in the East Indies, by sprinkling branches of trees with 

 water in their verandas. Hollies, Laurels, and most ever- 

 greens, exhale but little water from their leaves at any 

 other season of the year, except for about a month in June, 

 when their new leaves are recently expanded ; conse- 

 quently, in April and May, when we most require warmth, 

 and again in September and October, the leaves of these 

 trees, when fully exposed to the sun, become sensibly 

 heated to the touch, I should suppose, if the leaf of a 

 laurel, so circumstanced, were suddenly brought into con- 

 tact with a thermometer having a flat bulb, that the mer- 

 cury would rise to 85 or 90 degrees. 



