WITH REFERENCE TO HORTICULTURE. 



77 



The cover of the moistened bulb and the silk thread must be renewed 

 occasionally. The greater the difference between the heat indicated by the 

 moistened thermometer and the dry one, the greater will be the want of 

 atmospheric moisture. A table, with explanatory observations, will be found 

 in our Appendix. Mason's hygrometer, which Mr. Newman informs us was 

 in use upwards of thirty years ago by Sir H. Davy and others, (tliougli 

 recently brought into notice by Mason,) contains two common thermometers 

 mounted side by side, with a glass fountain for water fixed between them. 

 It is a very neat instrument ; but the mode of using two thermometer? above 

 described is sufficient for all ordinary purposes. Still though the liygrometric 

 state of the air may be known by a dry and a moistened thermometer, such 

 as that bearing the name of Mason, the latter showing a depression corre- 

 sponding with the rapidity of evaporation at the time, yet it is allowed by all 

 who have studied the subject maturely, that the results are not so much to 

 be depended on as when obtained by means of Daniell's hygrometer. 



244. Having described the means which may be resorted to in order to 

 ascertain the liygrometric state of the atmosphere, v/e shall now give an 

 example of the utility of that knowledge for horticultural purposes. W e 

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

 imagine the plant growing under the most favourable circumstances in its 

 native country, at the time of its flowering; enjoying 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 refreshing moisture, or, in other words, it is in a state of 

 saturation. The leaves expand, and the shoots become rapidly extended. 

 The conditions 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 diminished 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 accu- 

 rately ascertained by means of the hygrometer ; and it will sometimes, under 

 such circumstances as are stated, indicate as much as 20° of dryness, or the 

 double of what the vine naturally had in the day, instead of being in the 

 natural state of saturation at night. 



245. The amount of evaporation from the soil, and of exhalation from the 

 foliage of the vegetable kingdom," Mr. Daniell observes, " depends upon two 

 circumstances, — the saturation of the air with moisture, and the velocity 

 of its motion. They are in inverse proportion to the former, and in direct 

 proportion to the latter. When the air is dry, vapour ascends in it with 

 great rapidity from every surface capable of affbrding it ; and the energy of 



