. 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 
temperatures 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 part of the nine- 
teenth century the optical and thermal effects of sunshine were spoken 
of as due to certain imponderable forces called hght 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 we know, measure, and 
study as the phenomena of light, heat, electricity, gravitation, chem- 
ism, and vitality. 
DISTRIBUTION OF CLIMATIC ELEMENTS RELATIVE 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 the leaves, and more especially with the help of 
the yellow part of the solar spectrum, the chlorophy] 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 nutriment 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 
