Elements of Mathematical and Physical Geography (Paperback)

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1868 Excerpt: ...earth. Clouds are the masses of haze, which are seen floating in the higher regions of the sky, and which do not descend to the ground. They moderate the rays of the sun by day, and the radiation, or passing off, of heat from the earth's surface by night. Dew.--After sunset the solid objects on the earth's surface begin to throw off their heat by radiation into space, and soon become colder than the air, which loses but little heat in this way. When the air has remained for some time in contact with these objects, a portion of its vapour becomes condensed into specks of water, which adhere to the objects, and are D called dew-drops. There is a minimum temperature at which aqueous vapour can remain, suspended in the atmosphere. This minimum temperature is called the dew-point, or the Eoint of temperature at which dew begins to be precipitated. -ew falls most copiously on a calm, clear night, when there are no dense clouds to prevent the radiation of the heat. When the temperature of the surface bedewed becomes reduced below the point of freezing by the amount of radiation of heat, the precipitated vapour is frozen, and forms what is termed hoar-frost. Rain, Hail, Snow.--When the temperature of a cloud becomes reduced by the mixture of different currents of air, the vapoury particles of which it is composed increase in number, and begin to descend by their own weight. During their descent they increase in size by attaching themselves to other particles, and ultimately fall to the ground as rain-drops. When rain-drops fall through a stratum of air below 32, they become frozen, and are then called hailstones; and when the watery particles are formed in air under 32, snow is the result. The Distribution of Rain.--The quantity of rain which falls in differen...