A Primer of Physical Geography (Paperback)

This historic book may have numerous typos and missing text. Purchasers can usually download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1882 edition. Excerpt: ...them, and are limited in size. When they coalesce, rain occurs; but why they do so, and why they keep separate is not exactly understood. Clouds move over cold districts and upwards into cold regions, and condensation of their remaining invisible vapour occurs, but rain does not invariably follow. Fogs and mists may be so dense that objects moving in them force together the minute spherules and produce wet; but still there may be no rain. Eain is undoubtedly produced by the spherules uniting in little masses and falling and catching up others, and ordinary drops do not increase more than from-.V inch to It inch in diameter. In thunder-storms, the drops are large and the quantity of dense cloud is vast, and the same thing is observed in summer showers, the drops being then produced by the coalescence of spherules high up in the air. In both these instances the falling drops doubtless catch up others and increase in size. The minute spherules of water forming clouds and mists will not unite under Ordinary circumstances, although they must occasionally touch; they rebound. But. under the influence of electricity they will coalesce, that is, if the electrification is feeble. If (according to Lord Eayleigh) the electrical condition of the particles is more decided, actual contact will be prevented. Bain may thus be due to sudden and rapid changes in temperature in clouds, and also to the presence of minute amounts of electricity. The Rainfall.--The quantity of rain that falls on the earth is enormous, and it restores a large quantity of the heat and water which disappeared during the process of evaporation. Different parts of the earth receive different amounts of rain during the year, and some are rainless; and the annual amount of rain is...