WITH REFERENCE TO HORTICULTURE. 35 



has been established that absorption is greater than evaporation, but 

 when the pressure is diminished, the rate of evaporation exceeds that 

 of absorption. 



When trees are trained upon a wall with a southern aspect, they 

 have the advantage of a greatly increased temperature ; but this tempe- 

 rature, in spring, differs from the warmth of a more advanced period 

 of the year, or of a more southern climate, in not being accompanied 

 by an increase of moisture. 



In this country the sun is frequently sufficiently powerful in summer 

 to raise the thermometer in the free air, at a distance from the wall, to 

 101, whilst the heat in the shade may, perhaps, be only 60, and the 

 dew-point 50. In early spring, with an east wind, a warm sun, and a 

 parched air, it is often useful, and ensures a crop of tender fruit on 

 walls, to check the loss of sap from tender leaves and flowers by a 

 temporary shade. Under such circumstances the degree of dryness 

 would be 20. 



Over the absolute state of vapour in the air we are almost power- 

 less ; and by no system of watering can we affect the dew-point in the 

 free atmosphere. This is determined in the upper regions of the air, 

 and it is only by temporary expedients, such as shelter and shade, that 

 we can modify the effects of excessive exhalations. 



Over rain we may be said to have little influence, though the 

 planting of trees often causes it to fall, and the denudation of forests 

 prevents it from doing so. We can likewise prevent it from falling 

 upon particular plants or objects. By copings we can protect fruit- 

 trees against walls from perpendicular rain, and thus preserve the 

 bloom on the surface of fruit which would otherwise be washed off. 

 The roofs of plant-structures of every kind, and even the surface of 

 the ground, may be protected from rain by thatching or covering with 

 any body that will carry off the rain at particular points, or channels, 

 whence it may be conveyed away in underground drains. By these 

 and other means the soil of a garden in a wet climate may be kept much 

 drier, and consequently warmer, than it otherwise would be. Some 

 situations are more liable to rain than others, such as the vicinity of 

 woods and hills, and places exposed to the Western Ocean generally. 

 Those, on the other hand, which are exposed to the Eastern Ocean 

 have rains less frequently ; but these rains have a better effect on 

 vegetation, because the soil, from the less frequency of rain, being 

 generally drier, is warmer to receive them. 



Though we have little or no power over the moisture of the free 

 atmosphere, we may be said to have the perfect command of the atmo- 

 spheric moisture of hothouses. Till within the last twenty or thirty 

 years the principal points attended to in the atmosphere of hothouses 

 were heat and light ; but meteorological and chemical researches having 

 proved that with every increase of temperature in the open air there 

 is always an increase of aqueous vapour, this condition began to be 

 imitated in hothouses in which tropical plants were cultivated. " Capt. 

 Sabine, in his meteorological researches between the tropics, rarely 

 found, at the hottest period of the day, so great a difference as 10 



D2 



