76 ATMOSPHERIC MOISTURE, CONSIDERED 



SECT. II. Atmospheric Moisture, considered with reference to Horticulture. 



242. The existence of water in air, even when the latter is in its driest, 

 coldest, and purest state, is easily proved ; and the quantity of aqueous 

 vapour which it holds hi suspension has been ascertained by experiment. It 

 varies with the temperature, increasing as the heat is greater, in something 

 like a geometrical ratio. " At 50 Fahr. air contains about 1-50 of its 

 volume of vapour ; and as the specific gravity of vapour is to that of air 

 nearly as 10 to 15, this is about 1-75 of its weight. At 100, and supposing 

 that there is a free communication with water, it contains about 1-14 part 

 in volume, or 1-21 in weight." (Davy's Ag. Chem. Oth ed. p. 198.) Water 

 is also held in the atmosphere hi a grosser form than that of elastic vapour ; 

 for example, as mist, fog, or clouds, which three forms only differ in their 

 appearances, and not in their nature. Mists are those clouds near the surface 

 of the ground ; and fogs are only more dense mists, or, perhaps, mists diffused 

 to a greater height in the atmosphere. Mists are of a floating nature, and 

 the air is often seen clear above and below them ; but fogs are generally more 

 dense, and they pervade the atmosphere to a greater extent. It will be found 

 afterwards that it is of some importance to bear in mind the distinction between 

 water held in suspension in the atmosphere in the state of invisible elastic 

 vapour, and held in suspension in the state of steam, mist, or fog : in these 

 latter states it is frequently found in greenhouses in the winter season, and 

 in frames and pits, where the heat is communicated through the moist soil by 

 a bed of fermenting dung laid below it. 



243. To measure the quantity of elastic vapour in the atmosphere, Hy- 

 grometers have been invented, and the degree of moisture is indicated in 

 these instruments by what is called the dew-point. The best hygrometer 

 is that of Daniell ; but as some nicety is required in its use, a substi- 

 tute has been found in two common thermometers. The mode of ren- 

 dering these a substitute for a hygrometer is thus explained by Mr. 

 Wailes: "The dew-point is that degree of temperature, in any place, 

 at which moisture is deposited from the surrounding atmosphere upon 

 any object of that particular temperature ; and it depends, of course, upon 

 the humidity of the air. If, therefore, the air is very moist, the slightest 

 depression of the heat of the body in it will cause dew to form ; and, on the 

 contrary, if very dry, it will require a considerable fall of temperature to 

 produce that result. Hence it is that the cold produced by evaporation of a 

 liquid will be proportioned to the hygrometric state of the surrounding 

 medium ; and by measuring that degree of cold, we readily ascertain the 

 degree of humidity. The common thermometer is the best instrument for 

 the purpose of showing the temperature ; and we have only to keep its bulb 

 wet with some evaporating liquid of the same temperature as the medium 

 it is suspended in, to measure the degree of cold produced by such evapo- 

 ration, and thereby to find the dew-point." (Gard. Mag. vol. xv. p. 506.) 

 Two thermometers being obtained and placed together, one must have the 

 bulb dry to mark the temperature, and the other the bulb wet to indicate 

 the cold produced. The bulbs of both thermometers should be covered with 

 a fold of white silk or muslin, in order that both may be on a par, with 

 respect to the reception of heat from the atmosphere in which they are 

 placed, and pure water must be supplied to one of them from a phial or 

 other vessel placed near it, by a thread of floss silk acting as a siphon. 



