106 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



This peculiar quality of vapour may be illustrated by a diagram in 

 which the length of the vertical lines shows the relative weight of vapour 

 required to saturate a space of one cubic foot at temperatures from 0 ° to 

 100 ° F. (Fig. 20.) 



0*F 20° 46)' 60' 80" /OO'P 



TEMPERATURE 

 Fig. 20. 



The botanical physicist requires, however, to know something about 

 the temperature of the soil as well as that of the air, and the observations 

 of this element which are usually made refer to the lowest temperature at 

 the ground's surface, and the temperature at certain depths below it — 

 generally one foot, two feet, and four feet, and occasionally also at six 

 inches, below the surface. 



For obtaining the lowest surface temperature a minimum thermometer 

 is laid horizontally upon the ground with its bulb in contact with closely 

 cut grass, and the reading, which is generally taken in the morning 

 indicates the lowest temperature which plants similarly exposed have 

 experienced since the last observation was made. It is obvious that this 

 thermometer requires a perfectly open and unsheltered exposure, so that 

 it may experience the full cooling efi'ect of the radiation of heat from the 

 earth's surface into space. 



On still nights, when the sky is perfectly clear of cloud, this radiation 

 of heat will proceed very rapidly, and the temperature of the earth's 

 surface will fall to a point much below that which may be indicated at 

 the same time in the closed screen four feet above the ground. One effect, 

 familiar to everyone, of this chilling of the earth's surface is the formation 

 of dew, which is caused by the layer of air next the ground becoming so 

 chilled by contact with the cooled earth or grass that it is no longer able 

 to retain in a gaseous form its present amount of vapour, and the excess 

 becomes deposited on the grass in the form of w^ater. A cloudy sky will 

 check the radiation of heat, and a cloudy night is therefore not usually 

 a dewy one ; and the deposit of dew will be very slight also if the night 

 be windy, so that the air is kept in motion and the stratum next the ground 

 is not left in contact with it long enough for its temperature to fall very 

 much. 



