WITH REFERENCE TO HORTICULTURE. 79 



in the shade may, perhaps, be only 6G, and the dew-point 60. \Ve should 

 in this case say, that the degree of dryness was 10, and not 51, as would 

 be the result of subtracting 50 from 101, as Mr. Daniell has done 30 from 

 ] 50. Supposing a screen were put so as to intercept the sun's rays from the 

 thermometer, the latter would soon fall, and it would be found that the tem- 

 perature of the air was really not 101, and therefore that the latter number 

 should not have been taken for the purposes of giving the difference or degree 

 of dryness. 



248. 4i Some of the present practices of gardening," Mr. Dauiell continues, 

 " are founded upon experience of similar effects ; and it is well known that 

 cuttings of plants succeed best in a border with a northern aspect protected 

 from the wind ; or if otherwise situated, they require to be screened from 

 the force of the noon-day sun. If these precautions be unattended to, 

 they speedily droop and die. For the same reason, the autumn is selected 

 for placing them in the ground, as well as for transplanting trees; the 

 atmosphere at that season being saturated with moisture, is not found to 

 exhaust the plant before it has become rooted in the soil. 



249. Over the absolute state of vapour in the air we are wholly 

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

 free atmosphere. This is determined in the upper regions ; it is only, 

 therefore, by these indirect methods, and by the selection of proper seasons, 

 that we can preserve the more tender shoots of the vegetable kingdom from 

 the injurious effects of excessive exhalation." (Hort. Trans. , vol. vi. p. 7.) 



250. Over rain, we may be said to have little influence ; but though we 

 cannot prevent rain falling from the clouds, we can 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 by it. 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 gene- 

 rally drier, is warmer to receive them. 



251. 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 atmospheric 

 moisture of hothouses. Till within the last twenty or thirty years the prin- 

 cipal points attended to in the atmosphere of hothouses were heat and light ; 

 but meteorological and chemical researches having proved, as we have seen 

 (242 and 253), 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. u Capt. 

 Sabine, in his meteorological researches between the tropics, rarely found, 

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

 temperature of the air and the dew-point ; making the degree of saturation 

 about 730, but most frequently 5 degrees, or 850; and the mean satura- 



