22 



guished from the deep-earth temperatures that have for a century 

 past interested the student of terrestrial physics but do not affect 

 agriculture. I shall hereafter give a synopsis of such records so far 

 as they are available to me ; but so much agricultural data has been 

 collected, both in Europe and America, without corresponding soil 

 temi3eratures that we also need the data and methods that may be 

 used for estimating soil temperatures from ordinary meteorological 

 observations. 



SUNSHINE. 



Climatology usually considers the temperature of the air as given 

 by thermometers that are shaded from the effect of sunshine; this is 

 the temperature of the air very nearly as given by the whirled or 

 ventilated or sling thermometers and is that which is needed in 

 dynamic meteorology. But the sunshine produces important chem- 

 ical effects besides its thermal effects, and these have no simple rela- 

 tion to each other. It is therefore very important that we have some 

 method of recording the duration, intensity, and quality of the total 

 or general radiation that the plant receives from the sun and from 

 the sun and the sky combined. Up to the early i^art of the nine- 

 teenth century the optical and thermal effects of sunshine were spoken 

 of as due to certain imponderable forces called light and heat that 

 were supposed to be combined in the complex solar rays, but which 

 can be separated from each other. But we now believe it to be cor- 

 rect to speak of the sunshine as a complex influence, a radiation of 

 energy, whose exact nature is problematical, but whose mechanical 

 effects when it acts upon terrestrial matter w^e know, measure, and 

 study as the phenomena of light, heat, electricity, gravitation, chem- 

 ism, and vitality. 



DISTRIBUTION OF CLIMATIC ELEMENTS RELATR-E TO THE LIFE OF THE 

 PLANT. 



As before stated, plants respire during both day and night. The 

 pores of the leaves are always absorbing and emitting gases, but 

 when the sun shines on thedeaves, and more especially with the help of 

 the yellow part of the solar spectrum, the chlorophyl in the leaf cells 

 is able to decompose the carbonic acid absorbed by the plant, retain- 

 ing carbon and rejecting the oxygen. 



So long as the plant absorbs more carbon from the air and more 

 nitrogen from the soil than it loses by any process it is continually 

 increasing its leaf surface and the nutrition in its sap, laying up a 

 store of nutrnnent for future use. This process ceases in the case of 

 annual plants when the seed or grain or fruit begins to ripen; from 

 this time forward the seed makes a steady draft upon the nutriment 

 already stored up in the plant which goes to perfect the seed. In 



