METEOROLOGY IX .ITS RELATION TO HORTICULTURE. 105 



This is clone by placing the thermometers in a box or " screen " whose 

 four sides are formed of double louvres, whilst its top is double, and its 

 bottom is so arranged that a free passage is given to air, in or out, at the 

 same time that the bulbs are quite protected from terrestrial radiation. 

 This is generally known as the " Stevenson screen," after its designer, the 

 well-known lighthouse engineer, and the father of the more famous Robert 

 Louis Stevenson of literary renown. 



In this screen are usually placed, at the height of 4 feet from the 

 ground, a maximum and a minimum thermometer and two ordinary ther- 

 mometers ; the bulb of one of the ordinary thermometers is closely covered 

 with thin muslin, which is kept constantly wet by water led to it by 

 threads from a small reservoir at its side. The effect of the evaporation of 

 this water from the muslin is to lower the temperature of the bulb, owing to 

 the abstraction from it of the heat required to turn the water into vapour. 



The extent to which the temperature is lowered depends, therefore, 

 directly upon the rapidity with which evaporation goes on ; and that in 

 its turn depends of course upon the temperature and relative dryness of 

 the air, and its consequent readiness to absorb moisture. If the air already 

 contains as much vapour as it can hold without precipitating it as water, 

 or if, in other words, it is " saturated," there will be no evaporation from 

 the wet bulb and consequently no lowering of its temperature, and both 

 it and the dry bulb will read alike. 



If, on the other hand, the air happens to be very dry, and especially if at 

 the same time it should be also warm and be moving quickly, evaporation 

 will proceed rapidly, and so much heat will be extracted from the bulb that 

 its temperature will fall considerably — possibly fifteen or twenty degrees 

 — below that of the dry-bulb thermometer. Knowing the temperature of 

 the air and the amount by which the temperature of the wet bulb is 

 lowered by this evaporation, the actual amount of vapour in the air can be 

 readily found by means of tables prepared for the purpose ; but the usual 

 way of describing the hygrometrical condition of the air is to give the 

 percentage of the amount of vapour present to the amount which the air 

 could contain if it were saturated, or, as it is termed, the "relative 

 humidity." 



The capacity of air for holding water in the form of invisible vapour 

 depends upon its temperature ; or, put in another way, the weight of 

 vapour which a given space can contain depends directly upon the tem- 

 perature of the vapour. 



Into a closed glass vessel of two cubic feet area one might put about 

 4J grains of vapour if it were at the temperature of 32° without any of it 

 becoming precipitated as water. If more than that amount were forced 

 into the vessel, the excess would cease to exist as vapour, and would be 

 deposited on the sides of the glass as minute particles of water. 



If, however, the temperature were raised from 32° to that of an 

 ordinary summer day — say 60° — not only would the precipitated water 

 disappear again, but we should have to add more vapour up to a total 

 weight of 11^ grains before precipitation would again occur ; and if the 

 temperature were then to be raised still more to that of a hot summer's 

 afternoon, say 80°, as much as 22 grains of vapour might be squeezed 

 into the vessel without any deposit of water taking place. 



