116 Mr. Daniell on Climate. [Feb. 



be necessary to have a full knowledge of the peculiarities of the 

 climate of every region of the earth, a knowledge which we are 

 very far from yet possessing^ ; but to which, rapid advances are 

 daily making. But above all, it seems necessary that we should 

 understand the atmospheric variations of our own situation. 

 These, though not constituting the greatest range with which we 

 are acquainted, are great, and oftentimes sudden. The range 

 of the thermometer m the shade is from 0*^ to 90° of Fahrenheit's 

 scale ; but under favourable circumstances the heat of the sun's 

 rays reaches 135°, the changes of moisture extend from 1-000, 

 or saturation, to '389.* Now the great object of the Horticul- 

 turist is to stretch, as it were, his climate to the south, where 

 these extremes of drought and cold never occur ; and not only 

 to guard against the injurious effects of the ultimate severity of 

 weather, but to ward oiff the sudden changes which are liable to 

 recur in the different seasons of the year. To enable us to 

 understand the methods of effecting this end, it will be neces- 

 sary to consider the means by which these changes are brought 

 about in the general course of nature. The principal of these 

 >vill be found to be, wind and radiation. 



The amount of evaporation from the soil, and of exhalation 

 from the fohage of the vegetable kingdom, 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 affording it, and the energy of this 

 action is greatly promoted by wind, which removes it from the 

 exhaling body as fast as it is formed, and prevents that accumu- 

 lation which would otherwise arrest the process. 



Over the state of saturation, the Horticulturist has httle or no 

 control in the open air, but over its velocity he has some com- 

 mand. He can break the force of the blast by artificial means, 

 such as walls, palings, hedges, or other screens ; or he may find 

 natural shelter in situations upon the acchvities of hills. Exces- 

 sive exhalation is very injurious to many of the processes of 



• The Dew-point (a term which will often occur in the course of this paper) is the 

 degree of temperature at which the moisture of the atmosphere would begin to precipi- 

 tate, and may readily be ascertained at all seasons by means of the hygrometer. The 

 natural scale of the hygrometer is included between the points of perfect dryness and 

 perfect moisture : the latter, of course, being that state of the atmosphere at which the 

 deiL'-point coincides with the temperature of the air. The intermediate degrees may be 

 ascertained by dividing the elasticity of vapour at the temperature of the dew-point by 

 the elasticity at the temperature of the air : the quotient will express the proportion of 

 moisture actually existing, to the quantity which would be required for saturation ; for, 

 calling the term of saturation 1-000, as the elasticity of vapour at the temperature of the 

 air is to the elasticity of vapour at the temperature of the dew-point, so is the term of 

 saturation to the actual degree of moisture. The necessary tables for facilitating this 

 calculation, and more detailed explanations than it is possible to comprise in a note, may 

 be had, with the hygrometer, at Mr. Newman's in Lisle-street, or may be found in the 

 author's Meteorological Essays. 



