HORTICULTURAL JOURNAL. 291 



side of a cooled vessel, is the clew point ; and its distance below the ordinary 

 temperature, is in proportion to the degrees of the air. When rain is 

 actually forming, the dew point coincides with the temperature of the weather, 

 whilst in a drier atmosphere, it may require a cup of water to be cooled ten, 

 twenty, or more degrees before clew will be deposited upon the surface of 

 the cup. 



On a clear day when the themometer is ranging from TO to 80 degrees, 

 an experiment may exhibit dew at 60 degrees ; and if the temperature of 

 the nights at the season of trial, be as low as 60 degrees, dew or moisture 

 will be deposited as well inside as outside of the grapery. Should the 

 night temperature not be less than 62 degrees, in this case, tliere will be no 

 excess of moisture. During the season when artificial heat is not required 

 to protect tender plants from frost, spring or well water will be found suffi- 

 ciently cold to indicate the point of the deposition of moisture, in most 

 cases where a small reduction of temperature would cause its presence in 

 the grapery. 



The clew point may be taken with a themometer, a tin or thin metal cup 

 of water, care being taken if the moisture deposits readily, to let the tem- 

 perature of the water rise as near as possible to that of the air, consisently 

 with getting the moisture, and as the temperature rises, the moisture 

 should be wiped off from time to time, that the experimenter may observe 

 the highest temperature at which it will appear : since it is evident that if 

 dew will deposit at 60 degrees, it will do so at 50°, 40° or any lower 

 temperature, although it may not appear at 62 degrees. 



These considerations lead us to the remedy, namely, artificial heat at any 

 period of the year, or clay, when a slight fall in the temperature is accom- 

 panied by moisture enough to cause a deposition of a dew. But as heat 

 both increases, and the capacity of the included air for moisture, the inner 

 air might become more moist than the outer, and on this account there must 

 be ventilation as well as heat, and the latter may be made to furnish the 

 former. 



As in most cases, a very few degrees above the ordinary temperature is 

 sufficient to prevent the deposit of moisture from the air, a small movable 

 stove would answer very well, and this might be placed in the middle or at 

 one end according as the proper circulation of the warmed air would depend 

 upon accidental circumstances, as the course of the wind. It would not be 

 difficult to devise a structure with an arrangement to secure the proper 

 warmth, circulation of air and ventilation. Paving and even flooring the 



