Identifier: birdlore141912nati (find matches)
Title: Bird lore
Year: 1899 (1890s)
Authors: National Committee of the Audubon Societies of America National Association of Audubon Societies for the Protection of Wild Birds and Animals National Audubon Society
Subjects: Birds Birds Ornithology
Publisher: New York City : Macmillan Co.
Contributing Library: Smithsonian Institution Libraries
Digitizing Sponsor: Biodiversity Heritage Library

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ed in different ways, for example, by its hotness or cold-ness (temperature); by the rate at which it travels when in motion (velocity);or by the amount of moisture in it (humidity). It is weighed by finding out how much it presses down upon the earth. A thermometer, or heat-measure, is an instrument which tells just how hotor cold the air is. A barometer, or weight-measure, is an instrument whichtells the weight (pressure) of the air. A weather-vane is a very simple instrument, telling in which direction theair is moving, while an anemometer, or wind-measure, tells how fast the airmoves. Other instruments are made to show how wet or how dry the air is, howmuch rain or snow it sheds upon the earth, and other things important forman to know. In order to make pictures of the weather, it is necessary first to weigh andmeasure it in different places, and at different heights. An isotherm is a pictureof the temperature, while an isobar is a picture of the pressure of the air, oratmosphere.
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SPEED OF THE ROBIN MIGRATION(Reproduced by permission of the Biological Survey from W. W. Cookes Some New Facts About theMigration of Birds, Year boolc, Department of Agriculture, 1903, pp. 371-386.) The black band indicates the average position of the isotherm of 35° F. at the several dates men-tioned. The dotted band shows the approximate route of the Robins of northwestern Alaska. The datesin this band represent the average time of arrival of the earliest Robins. The numbers at the side of themigration route indicate in miles per day the average speed of migration between the dated places. (57) 58 Bird-Lore These pictures are simply imaginary lines drawn on a map of the world^.from place to place, wherever the heat (temperature) of the air or its weight(pressure) are the same. They may be called lines of equal temperature and lines of equal pressure,,but the words isotherms and isobars are shorter, and just as easy to rememberwhen once we know what they mean. Now, since the weathe

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