(8) Mild, and at the same time dry, winters are associated with a tendency of the earth temperature to rise above the average.

(9) The earth temperatures exhibit a tendency to fall, if not already too low, during winters in which, with alternate freezing and thawing, the mean temperature is below the normal.

(10) In the same way even a covering of snow can only to a limited extent prevent the cooling of the earth when severe cold follows the mild and rainy weather of the first part of winter.

(11) In continuous severe winters, on the contrary, when even December generally brings a permanent covering of snow, the negative departure of the earth temperature is either limited to the higher strata or is unimportant.

(12) A warm spring, which, as a rule, brings only a moderate quantity of rain, causes a relatively decided rise of the earth temperature. (13) When a cold and rainy late winter is directly succeeded by warm spring months, the temperatures of only the upper strata of the ground rise, while those of the lower strata may fall still further below their normal values.

(14) In certain warm and at the same time rainy springs the earth temperatures remain on an average unchanged with respect to the normal [or the cold rain counterbalances the warm weather. C. A.]

(15) An exceptionally cold spring, which is generally distinguished by heavy snows, is, with few exceptions, accompanied, and to a considerable depth, by a notable lowering of the temperature of the ground in comparison with its normal temperature.

(16) In cold and at the same time dry spring weather the relative lowering of the temperature of the ground will generally be inconsiderable if it has not been preceded by an immediate very rainy

season.

(17) A warm summer is always accompanied by a high temperature of the ground or by a rise of its temperature. The increase is the more decided the more the excess in the temperature of the air is accompanied by a large quantity of rain, or has been immediately preceded by it. In warm and comparatively dry summers the rise of the earth's temperature does not perceptibly exceed the normal.

(18) The relative lowness of the temperature of the soil which follows without exception a cool summer generally extends down only to a comparatively moderate depth, scarcely to 4 meters. Those months in which we find it extending to 6 meters will be found to have been at the same time rainy months.

(19) A warm autumn, with very few exceptions, causes a corresponding small rise in the temperature of the soil, but this may even, on the contrary, become a fall when the late autumn, by reason of much rain, resembles a mild type of winter.

(20) Low air temperature is generally accompanied in autumn by an excess of rain, the consequence of which, as regularly and frequently observed, is a falling in the temperature of the earth.

(21) In the rarer cases of cool and dry autumns there is observed only a very inconsiderable influence on the temperature of the earth. (22) The dampness of the soil is (under the climatic influences prevailing in Munich) sufficient to allow the variations in the temperature of the air in winter and spring to exercise a decided influence upon those of the soil, whereas in summer an excess of rain would be

necessary to accomplish this, and that, too, to a greater degree if the soil be covered with vegetation. The phenomena of autumn generally resemble closely those of summer.

(23) In general the fluctuations in the temperature of the earth are not less dependent on the precipitation than on the variations in the temperature of the air.

SOIL TEMPERATURES AS AFFECTED BY SURFACE SLOPE AND COVERING (WOLLNY).

In reference to the effect of the slope of the earth's surface on the temperature of the soil, Wollny (1888, p. 364) has made an extensive series of measurements at Munich from which he draws the following conclusions in continuation of those published by him in 1883. His temperatures were measured bihourly at a depth of 15 centimeters under both fallow soil and grass sod; the differences referred to amounted to 3° and 4° F. in individual cases, but on the average to scarcely 1° F.

(1) That soil whose exposure is toward the south is the warmest, then comes the east, then the west, and finally the north exposure.

(2) The southern exposure is warmer in proportion as the inclination to the horizon is greater.

(3) The difference of temperature between the north and south exposure is much greater than between east and west.

(4) The difference in the warming of the soil for north and south exposures is greater in proportion as the surfaces have a greater inclination.

Wollny (1888, p. 415) has also investigated the influence of the covering of straw and chaff on the temperature and moisture of the soil. He finds the following conclusions:

(1) That at a depth of 10 centimeters the naked soil is warmed. more with rising air temperatures and is cooled more with falling air temperatures than under any one of the different forms of straw covering.

(2) That the variations in the temperature within the straw litter are very much less than in the earth.

(3) That the earth is in general somewhat colder than the material of which the litter is made, except when the latter is moss.

(4) That among the various materials forming a litter the pine needles are warmed the most, the oak leaves and the fir-tree needles are less warm, while the litter of moss is the coldest.

The different temperatures observed were as follows, on the average of the months April to September: Pine needles, 16.93° C.; oak leaves. · 16.62° C.; fir needles, 16.34° C.; the naked soil at a depth of 10 centimeters, 16.18° C.; moss, 15.95° C.

The difference between the morning and evening temperatures shows:

(1) That the cooling during the night and the warming during the day is appreciably larger for the naked earth than for the various kinds of litter.

(2) That the pine needles warm up most during the day and the moss warms up least; that the fir needles cool most during the night and the pine needles least.

The power of retaining moisture varies with the different kinds of litter as follows:

(1) Any litter of forest leaves or needles is moister than the earth, but the moss is less moist than the earth; the gradation is from oak leaves, the highest, through fir needles to moss, the lowest.

With regard to evaporation Wollny shows that the naked earth loses a greater quantity of moisture by evaporation than do the various kinds of litter.

(2) That the moss litter evaporates the most, but the litter of forest leaves the least.

(3) That the quantity of evaporation is greater the thinner the layer of the litter.

In general, then, the litters of leaves and of pine needles give up the rain water that falls upon them to the ground beneath in larger proportion, but still continue to be very moist because they lose, relatively, little water by evaporation; furthermore, that the moss litter is distinguished by large variations in its contained water because it has on the one hand a large capacity for water and on the other hand a very considerable evaporating power.

SOIL TEMPERATURES OBSERVED AT GREENWICH, ENGLAND.

Among the limited number of long-continued series of observations of temperatures of soil near the surface is that maintained at Greenwich Observatory, England, since June, 1846. This series embraces observations at considerable depths that will not interest the student of agriculture, but we reproduce in the following table the results of observations at 1 inch in depth, as given in the annual volumes of the Greenwich Observatory for 1878, and as given in J. D. Everett's memoir of 1860. These soil temperatures can be used in any subsequent study of English crops throughout the southern half of England or in analogous climates.

Monthly and annual means of noonday readings of a Fahrenheit thermometer whose bulb is 1 inch below the surface of the soil at Greenwich Observatory.

Annual

Year.

Jan. Feb. Mar. Apr. May. June. July. Aug. Sept. Oct. Nov. Dec. mean.

1847.

1848.

1849.

41.5 43.6

1850.

36.7 44.4

1851.

44.2 42.7

1852.

42.8 42.0

1853.

44.3 37.0

37.8 38.0 42.9 47.2 58.0 61.1
64.7 56.9 54.5 48.7 44.5
37.4 44.1 44.6 49.5 61.6 61.7 65.0 60.8 58.8 53.5 45.5 45.5
44.3 46.3 56.5 63.3 65.0 65.2 61.8 53.0 46.5 41.1
41.9 50.4 53.0 64.1 65.2 63.0 58.7 49.5 48.7 42.7
44.0
48.5 54.8 62.2 63.8 65.5
60.0 54.7 41.2 42.2
43.0 49.9 55.1 59.4 71.0 65.2
41.8 47.4 55.7 62.3

67.4

45.3 52.7 54.2 59.8 64.4
41.0 48.9 52.9 61.3 65.6
41.5 43.3 41.7 50.4 52.6 63.0 64.8 66.7 59.0 54.5 43.9 41.9
38.9 40.7 43.7 48.3 57.6 65.6 67.0 67.9 62.5 55.2
39.6 37.8 42.2 49.6 54.3 68.6 64.5 66.0 62.7 54.2
42.4 43.5 47.3 49.3 55.8 64.4 70.7 66.7 59.7 54.4
37.4 42.1 45.2 56.7 58.2 61.3 60.4 57.0 52.4
42.6 44.6 47.7 54.9 63.2 64.6 66.2 60.3 57.5
40.7 42.9 45.6 50.6 57.8 60.0 62.6 63.5 60.6 54.3 42.7
43.2 45.0 51.1 54.3 60.5 64.0 63.9 57.6 54.3 48.0
39.6 39.0 43.2 50.0 56.3

1865.

38.8 38.9 39.3 53.3

57.8

43.1 42.0 50.3
38.3 46.1 40.7 49.9 55.9
60.5 52.6 45.2 39.9
44.5 45.9 50.5 59.8
63.0 52.2 45.0 47.0
46.8 40.8 51.5 54.3 58.6 66.2 63.2 60.9 52.4 45.8 40.5

40.38 41.40 43.25 49.50 55.54 62.08 65.44 64.46 60.21 53.45 45. 43 42.50

51.97

SOIL TEMPERATURES OBSERVED AT BROOKINGS, S. DAK.

Among the agricultural experiment stations in the United States whose work will be used in this preliminary report are some whose observations of the temperature of the soil will be needed for comparison with the observations on the growth of plants and resulting crops or for denfonstrations of the relations between the temperature of the air and of the soil. The following table gives for Brookings, S. Dak., the daily maximum readings of the thermometer in the air and shade, the daily rainfall, the maximum temperatures of the soil at depths of 2 inches and 12 inches as far as published in Experiment Station Bulletin No. 6 for a portion of the summer of 1888. These figures show that in summer and for the growing season generally the temperature of the soil near the surface is higher than that of the air in the shade only when the sun shines on it, and that it is lower than the temperature of the air in the shade only when the radiation cools it at nighttime or when the rain falls in the daytime. and is for a short time followed by rapid evaporation. The average

of the maximum temperatures of the air, less the temperatures of the soil at 2 p. m. at a depth of 2 inches was 2.3° F. in July, 1888, and 3° F. in August, 1888. On the other hand, the average value of the maximum temperature of the air, less the temperature of the soil at 2 p. m. at a depth of 12 inches was 12° F. for the observations here given, scattered through July and August, 1888.

It would appear that the reading of the soil temperature is frequently omitted when rain falls; this is a bad practice, but the records suffice to show us that in this dry country and during the summer time the maximum surface temperatures of the soil will not differ much from the maximum temperatures of the air, while the soil temperatures at 12 inches will closely follow the mean temperature of the air. The latter mean, viz, one-half the sum of the maximum and minimum record for any day is greater than the mean temperature of the layers of soil at 2 and 12 inches depth, as observed at 2 p. m., by about 6° F.