140 AGRICULTURAL REPORT. 
During the night the influence of the coloring of walls differs from 
that exerted during the day; then, especially after midnight, an equal 
temperature prevails before walls of opposite colors, and also in the 
stratum of air which circulates upon their surfaces. 
In sixty-one observations made ketween 9 a. m. and 3 p. m., Vintry * 
found the thermometer higher before a white than before a black sereen. 
The more brilliant the sun the greater was this difference. The range 
was from 2° to 3°, and reached even as high as 5° Cent. He made use of 
bitumened paper, one washed over with lime and the other with gas tar. 
An exception to this rule is found when a cold wind paralyzes the ae- 
tion of the reflected rays and causes an accidental coolness; in this case 
the excess of caloric accumulated in a black wall might exercise some 
effect upon the air near its surface. Following this hypothesis, it would 
seem desirable to color and even to blacken walls for trees bearing 
seeded fruits, which shun a temperature too elevated. 
If the facts here presented are correct, it is possible that there still 
exists a mass of circumstances which should be taken into consideration. 
Heat and light do nct produce the same effect. In cloudy weather a 
black wall may heat a great deal; is it the same with a white one? 
What action attends a current produced along garden walls struck by 
the sun, which arises only from air traversing the surface of the ground ? 
The nature of this surface, the special plants which cover it, and its 
dampness or dryness, may also exert some influence. The black wall 
being warmer at sunset than the white, communicates during a part of 
the night more heat to plants trained against it. It retards, perhaps, 
the moment when those plants become colder than the air, and when 
the deposit of dew affects the surface of the leaves. Is this result fay- 
orable or unfavorable? Is it of the same nature during spring, sum- + 
mer, and autumn, and equally favorable in every season ? 
Able horticulturists assert that terrace walls which closely press the 
ground on one side, are less favorable support for espatiers than those 
which are exposed to the air on both sides. A French writer thinks that, 
if this be true, it would be consistent that a terrace wall presents 
one surface of only 10° or 12° below zero, and that in the other, receiving 
the solar rays, the heat which they tend to produce is constantly de- 
stroyed, or at least greatly diminished by the relative cold of the first- 
named surface. We may not compare the effect of a good wall, permeated 
on the one side with warmth from the sun’s rays, and on the other by 
an air with a temperature of 25° or 30°, and which yields this heat but 
slowly during the night, to a sheet of pasteboard, as Vintry has done in 
his observations, for the latter, as soon as the sun clouds or sets, continues 
to receive the temperature of the surrounding air. 
By covering the ground with charcoal-dust, dark-colored straw, the 
remains of heath or faded leaves, the maturity of certain plants is 
accelerated fifteen or twenty days, a result which is surprising. We 
observe an analogous fact when we place on the snow two pieces of 
cloth, one white and the other black; the former does not produce, 
properly speaking, any effect on the suow, even in a fine sunlight, while 
the latter quickly occasions melting, and sinks rapidly below the surface. 
is not this effect somewhat similar to the opposite action of black and 
white walls?+ 
Wells} made experiments sufficiently decisive in this regard. During 
* Journal de la Société d’Horticuiture, Paris, 1857, vol. iii, pp. 480-483. - 
t Journal de la Société d’ Horticulture. Paris, 1857, vol. ili, pp. 600-608. | 
tAn Essay on Dew. London, 1818 and 1821; new edition, with annotations by L. P, 
Casella, and an appendix by R. Strachan, London, 1866, Sve. 
