34 ATMOSPHERIC MOISTURE, CONSIDERED 



Having denoted the means which may be resorted to in order to 

 ascertain the hygrometric state of the atmosphere, we shall now give 

 an example of the utility of that knowledge for horticultural purposes. 

 We shall suppose that the grape is to be forced in a vinery ; and we 

 shall first imagine the plant growing under the most favourable cir- 

 cumstances in its native country, at the time of its floAvering, and enjoy- 

 ing a temperature of 70 or 80 through the day, with 8 or 10 of 

 dryness, according to the hygrometer of Mason or Daniell. At night, 

 whilst the air has still a genial warmth, it is also charged with a re- 

 freshing moisture, or, in other words, it is in a state of saturation. The 

 leaves expand, and the shoots become rapidly extended. The con-, 

 ditions under which this takes place, in the native country of the grape, 

 we would wish to imitate in its artificial culture in our vineries. In a 

 vinery we can, even in cold weather, command heat, and the degree of 

 dryness through the day will not be much in excess ; but when night 

 comes, although we can still keep up the heat, the moisture is dimi- 

 nished instead of being increased. More fire-heat being required, the 

 air in contact with the hot flues, or hot-water pipes, ascends upwards 

 in consequence of its increasing elasticity, till it reaches the cold glass; 

 the latter condenses the vapour which the air contains, just as the 

 refrigerator of a still condenses, by its coldness, the spirituous or other 

 vapour contained in the worm ; and the condensed vapour may be seen 

 trickling down the glass roof. The portions of air thus successively 

 drained of moisture being also cooled by contact with the glass, become 

 specifically heavier, sink and give place to a fresh supply of warmer 

 air, which in its turn descends, likewise deprived of its moisture. 

 Herein we have discovered the source of an evil, the amount of which 

 may be accurately ascertained by means of the hygrometer ; and it 

 will sometimes, under such circumstances as are stated, indicate as 

 much as 20 of dryness, or double what the vine naturally had in the 

 day, instead of being in the natural state of saturation at night. 



Over the state of saturation the horticulturist has little or no control 

 in the open air ; but over its velocity he has some command. He can 

 break the force of the blast by artificial means, such as walls, palings, 

 hedges, or other screens ; or he may find natural shelter in situations 

 upon the acclivities of hills. Excessive exhalation is very injurious to 

 many of the processes of vegetation, and no small proportion of what 

 is commonly called blight may be attributed to this cause. Dr. McNab, 

 of Cirencester, has proved by numerous experiments that "in the sun 

 plants transpire most in a saturated atmosphere ; in the shade transpi- 

 ration ceases when the atmosphere is loaded with watery vapour," so 

 that " plants in the warm moist air of a hothouse will give off very 

 large quantities of fluid by transpiration, unless this is modified and 

 regulated by proper shading." Evaporation increases in a prodigi- 

 ously rapid ratio with the velocity of the wind, and anything which 

 retards the motion of the latter is very efficacious in diminishing the 

 amount of the former : the same surface which, in a calm state of the 

 air, would exhale 100 parts of moisture, would yield 125 in a mode- 

 rate breeze, and 150 in a high wind. Under pressure, moreover, it 



