Earth Thermometer

The Earth Thermometer is for ascertaining the temperature of the soil at various depths. It is protected by a brass frame, pointed and strengthened at the end to facilitate insertion into the ground, as in fig. 51.

Utility of a Knowledge of the Temperature of the Soil.—The temperature of the soil is an important element in the consideration of climate, as it concern

the vegetable kingdom.

Dr. Daubeny, in his Lectures on Climate, gives the following statement with respect to some temperatures which have been observed just beneath the earth’s surface, in different parts of the globe:—

Country.	Temperature.	Authority.
Tropics, often	162-184°	Humboldt.
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Egypt	133-144	Edwards & Colin.
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Orinoco	In white sand, 140	Humboldt.
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Chili	113-118, among dry grass	Boussingault.
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Cape of Good Hope	150, under the soil of a bulb garden	Herschell.
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Bermuda	142, thermometer barely covered in earth	Emmet.
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China	Water of the fields, 113; adjacent sand, much higher; blackened sides of the boat at midday, 142-150	Meyer.
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France	118-122, and in one instance 127	Arago.

“The importance of this to vegetation may be estimated by the following considerations:—

“It is known that every plant requires a certain amount of heat, varying in the case of each species, for the renewal of its growth, at the commencement of the season.

“Now when this degree of heat has spurred into activity those parts that are above ground, and caused them to elaborate the sap, it is necessary that the subterranean portions should at the same time be excited by the heat of the ground to absorb the materials which are to supply the plant with nourishment. Unless the latter function is provided for, the aerial portions of the plant will languish from want of food to assimilate. Indeed, it is even advisable that the roots should take the start of the leaves, in order to have in readiness a store of food for the latter to draw upon.” In another place the professor remarks:—“It has been calculated by Mr. Raikes, from experiments made at Chat Moss, that the temperature of the soil when drained averages 10° more than it does when undrained; and this is not surprising, when we find that 1 lb. of water evaporated from 1,000 lbs. of soil will depress the whole by 10°, owing to the latent heat which it absorbs in its conversion into vapour.”