Lectures, reports, letters, and papers on sanitary questions / by Robert Rawlinson.

  • Rawlinson, Robert, 1810-1898.
Date:
1876
Catalogue details

Licence: Public Domain Mark

Credit: Lectures, reports, letters, and papers on sanitary questions / by Robert Rawlinson. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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    siderable variations. In tlie equatorial regions, for example, where the sun's heat is greatest and most powerful, evaporation is neces- sarily most copious and the rainfall is heaviest; and, as we approach either pole, with decreasing solar heat evaporation decreases and the rainfall is in a proportionately diminished volume. But in no region of the earth's surface does evaporation cease. Vary as it may, evaporation is general and constant. It takes place inces- santly, wherever the ocean extends. Even icebergs and the broad floes of the polar oceans evaporate. In tropical regions, as well as in the colder zones of the earth's surface, vapoiu* is constantly passing into the atmosphere, again to be condensed and become fresh-water; and at all tem2:)eratures (boiling or freezing) this act of evaporating water implies the absorption of latent heat, which heat is again given out, sometimes accompanied by lightning and thunder, when invisible vapour is condensed into water, as tornado, water-spouts, deluging sheets of water, gentle rain, snow or hail. The fall of rain is determined by the amount of invisible vapour which is carried up into the atmosphere, as also by terrestrial causes, such as latitude, the relative position of land and water, plains, valleys, mountains, and air-currents (winds) which all act and re-act upon the process of precipitating vapour into rain. This may be illustrated simply by what takes place year-by-year in our own country. In England the annual average fall of rain is from 30 to 36 inches. The distribution of this rainfall is, however, singularly arbitrary, as the average fall of rain in one portion of the Lake districts is not less than 150 inches in each year; and even 300 inches of rainfall has been experienced at Styehead. In contrast with this, on a portion of the eastern coast of England the average yearly fall of rain does not exceed 20 inches ; and, in a dry season, it sinks down to 14 inches. In the Thames valley, the average yearly fall is 27 inches. On the western coast of England, as also on the southern, the rain which regularly falls year by year con- siderably exceeds in volume the rainfall on the eastern coast. This result is produced by a greater local prevalence of the south-west wind, which brings in the vapour-laden atmosphere from the Atlantic Ocean, and much of this vapour, as it sweeps towards and over the backbone ridge of our island, becomes condensed, and precipitates heavy rains in the districts that lie towards the south and the west. During the actual descent of the heaviest rain over any area of fall, the process of evaporation is maintained. Recent researches have led to the discovery that, in England, the amount of constant evaporation is greater than previously had been supposed. [There do not appear to exist any certain data which may be accepted as de- termining accurately the comparative amount of evaporation in the tropics.] But, if we take the valley of the Thames, which contains an area of about 5,000 square miles, and where the yearly average rainfall amounts to 27 inches, we find something nearly approach- ing to two-thirds of the entire fall of rain re-evaporates and passes again, in the condition of invisible vapour, into the atmosphere.