A treatise on materia medica, pharmacology, and therapeutics / by John V. Shoemaker ... and John Aulde.

  • Shoemaker, John V. (John Veitch), 1852-1910.
Date:
1889-1891
Catalogue details

Licence: Public Domain Mark

Credit: A treatise on materia medica, pharmacology, and therapeutics / by John V. Shoemaker ... and John Aulde. Source: Wellcome Collection.

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    sunbeam forms with the surface, the greater is the area submitted to its iiittueuce, and the less is the quantity of heat actually received. If, then, the surface of the earth were plane, and if the atmosphere were motion- less, the temperature at a given latitude might be determined b}' mathe- matical calculation. Temperature, however, steadil}^ declines with the elevation above the sea level. The rate of this diminution is quite variable, l)ut has been estimated at one degree Fahrenheit for eveiy three hundred feet of per- ])endicular ascent. Upon lofty mountains a line is reached at which the moisture of the air is permanently frozen, and the lowest point at which this congelation occurs is termed the snow-line, or line of perpetual snow. The height of the snow-line is greatest at the equator, but is a variable point even upon the same parallel. Its position is not determined solelv by altitude, but is governed, to some extent, by other circumstances, such as habitual dryness or moisture of the atmosphere, form of the mountain, etc. The sun’s rays strike upon the earth much more obliqueh' in winter than in summer at latitudes distant from the equator. Temperature is likewise modified by winds. Currents of motion are excited in the atmosphere by the expansion of portions of air in contact with the heated surface of the earth. The heated air is borne upward to higher strata, Avhile its place is taken by colder air which rushes in from above. Thus the temperature of the earth’s crust, of the atmos- phere, and of the aAirial currents act and react upon each other. The direction in which winds bloAv depends upon the distribution of bent upon the surface of the globe,—the rarefaction and displacement of the heated air and the rotation of the earth upon its axis. It has been shown b}' Buchan,* that prevailing winds are due to difterence of atmospheric pressure, and that they How from a region of high to one of low pressure. Air always contains some aqueous vapor. When capable of holding no more it is said to have reached its point of saturation. The higher the temperature of the air, the more moisture it is capable of absorlfmg. This capacity is doubled for every twenty-seven degrees Fahrenheit of temperature above the freezing-point, the temperature increasing in an arithmetical and its absorbent poAver for moisture in a geometricnl ratio. The quantity of Avater in a given volume of air is called the absolute humidity. The degree of dampness is termed relative humidity, and represents the mutual influence of absolute humidity and tenq)erature. 0\'er large bodies of water, as the ocean and great lakes of North America, the atmosphere is constantly charged with a large quantity of moisture; hence ocean winds are laden AA-ith moisture. If the}’ * “On the Mean Pressure of the Atmosphere and the Prevailing AA’inds over the Globe.” Trans. Roy. So. Ediub., 1868. 22