om 
AGRICULTURAL METEOROLOGY. 14) 
a serene night he stretched a handkerchief vertically over a meadow, 
by means of two sticks, and observed that a thermometer placed beneath 
it on the grass, and exposed to the wind, marked from 3° to 5° Cent. 
more than a thermometer placed near by but unprotected by the hand- 
kerehief. This experiment, says Arago,* shows that the walls of espaliers 
afford to the plants, and distribute to them at night, a little more heat 
than they have absorbed during the day, and also mechanically arrest 
the cold winds, acting like screens, and diminishing the great loss of 
caloric which the plants would have experienced by their radiation, if 
a great part of the sky had not been hidden from them by the walls. 
We must ascertain by practical experiment what shape and composi- 
tion of screens are most suitable for protecting wall trees and other 
vegetation from frost. Mr. R. Thompson has shown that different kinds 
of screens will protect, to a certain range of temperature only, some 
species of trees, but not others. For example, straw sereens will protect 
peach trees in blossom from the effects of 12° of frost, while coping- 
boards will be sufficient for about 4°. We must, therefore, combine the 
circulation of air and the conductibility and radiation of heat with the 
form and quality of different screens. 
PHYSICAL PROPERTIES OF DIFFERENT ARABLE SOILS. 
The knowledge of the physical properties of soils is of the highest im- 
portance in agronomy and agriculture. It precedes the study of chemi- 
cal properties, which are secondary and much more complicated. They 
are the only ones that the first agricultural authors have given as 
characteristic signs of the qualities of soil. Virgil, Varro, Columella, 
_and other philosophers of antiquity mention them. The science being 
then in its infancy, these first essays had no appl. ‘ation to agriculture. 
And still, at the present time, agriculturists judge in vain of the quali- 
ties of earths by their color, their relative moisture, their consistence, 
the appearance of their vegetation, &c. These indications, more or less 
vague, not reposing on any scientific basis, have, consequently, very 
little practical value. 
The study of the physical properties of soils did not command the 
attention of savants until 1757, when they commenced the chemical 
analysis of soil. The first experiments were made by a Bernois savant, 
who limited his examinations to the weight of earth, and its facility for 
absorbing water. The celebrated German agriculturist, Thaér, paid 
great attention to the physical properties in his “* Analyses of Earths ;” 
but he did not submit them to a series of comparative experiments. 
Christian L. Schiibler, appointed in 1816 professor of physics and 
chemistry, applied to agriculture, at Hoffwyl, asked himself where was 
the science he was about to teach, and perceived that it did not exist. It 
was then that he studied for the first time agriculture as a physicist, 
and sought for the best means for determining and comparing the 
diverse physical properties of soils.t He discovered that the spe- 
cific weight of earth was always in relation to its facility for retaining 
heat, and of drying promptly; that the facility for retaining water em- 
* Annuaire du Bureau des Longitudes, Paris, 1828. Annales de Agriculture Frangaise, 
Paris, 1827, vol. xi, p. 197. 
+ His researches appeared first in the newspapers of Hoffwyl; by abstract in the 
Bibliotheque Britannique. It forms the second section ef agronomy in a German work en- 
titled “ Principles of Agricultural Chemistry, in more direct reference to the Economy of 
Agriculture and Forestry.” Second edition, revised and improved by Professor Krutzsch. 
‘Tharaud, in Saxony, 1838, and in his German translation of Chaptal’s Agricultural 
Chemistry. 
