Hygiene and public health / by B. Arthur Whitelegge.

  • Arthur Whitelegge
Date:
1894
Catalogue details

Licence: Public Domain Mark

Credit: Hygiene and public health / by B. Arthur Whitelegge. Source: Wellcome Collection.

    157/610 (page 145)
    absorb and evaporate twelve times the rainfall.^ Herbage lessens absorption of heat by the soil, and also true radiation, but increases evaporation and convec- tion owing to the vast surface of the blades of grass. Grass, therefore, like trees, renders the soil cooler and drier, and more equable in temperature. Earth-temper- atures are usually determined by thermometers pro- tected by open wooden covers, suspended by a chain at the required depth in the soil, and withdrawn at daily or other intervals for reading. An iron tube driven vertically into the' ground affords a passage for the thermometer. Systematic records are usually kept of the daily readings of two earth-thermometers, at depths of one foot and four feet. At greater depths the internal heat of the earth produces an appreciable effect, causing a rise of about 1^ Fahr. for every 55 feet. Moisttire.—-At a certain point below the surface in permeable soil, all the interstices are full of water, to the complete exclusion of air. At and below this level there is therefore a continuous sheet of water, . known as the subsoil water or ground-water, which n3 extends downwards to the next impermeable stratum. In marshy ground the surface of the subsoil water may coincide with that of the ground itself, and under ordinary conditions it may be from two or three feet to a hundred feet or more below the ground level. The surface of the ground-water is not necessarily horizontal, nor parallel with that of the ground. It flows (usually towards the nearest watercourse or the sea) with a vf^lopity that varies with the permeability ^ of thp, snil^ the «tpppTiP«« nf f.l^P gi-nrliPnf anrl ihf^ ^ T£\ absence of obstruntiop by roots of trpps f The level is * As regards the air, forests render both the daily and annual temperatures more equable, and increase the humidity [Galtoii). t In Munich Pettenkofer estimated the velocity of the ground- water to be 15 feet j)er day. K—28 ^