EARTH 



2764 



EARTH-HOUSE 



form the moon. This process re- 

 peated on a small scale the method 

 by which the earth itself was 

 formed from the sun. The earth's 

 diameter is 7,900 m., less than a 

 hundredth part of that of the sun. 

 The eartb is the third planet in 

 order from the sun, Mercury, the 

 nearest and smallest planet, being 

 36 million miles while the earth is 

 92 J million miles from the sun, and 

 Neptune, the most distant, is 2,775 

 million miles from it. The earth 

 rotates once in 23 hrs. 56 mins., the 

 sun in 25 days, and Jupiter in 9 

 hrs. 55 mins. While the revolution 

 of the earth round the sun occupies 

 a year, Mercury only takes 88 days 

 and Neptune takes 165 years. 

 The Moon and the Earth 



The whole solar system is held 

 together by the influence of gravity. 

 While the sun is very hot and the 

 moon quite cold, the earth still 

 retains within it a certain amount 

 of heat which tends to be dissi- 

 pated through space. The moon 

 is without an atmosphere, but the 

 earth and Mars have a gaseous 

 envelope surrounding them, the 

 troposphere. The moon causes 

 eclipses of the sun, the wave move- 

 ments of terrestrial waters called 

 the tides, and reflects a small 

 quantity of solar light and heat to 

 the earth, but the main external 

 source of supply of energy to the 

 earth is the sun. The earth, how- 

 ever, only receives a very small 

 portion of the total radiant energy, 

 light, heat, electricity, etc., which 

 streams continuously into space 

 from the sun. 



The earth is a rigid body, ap- 

 proximately twice as rigid as steel, 

 in that it resists compression, yet 

 although the interior is not fluid, it 

 is fluidl ble and will flow when op- 

 portunity offers, as in lava streams. 

 Hydrosphere and Lithosphere 



The earth is a cooling body, and 

 therefore is very slowly becoming 

 smaller. The geometrical solid 

 which combines a maximum of 

 surface area with a minimum of 

 volume is a tetrahedron, which has 

 four corners, four faces, and six 

 edges. Because the earth's crust 

 is attached to a shrinking interior 

 the crust crinkles into folded moun- 

 tains which have adopted the 

 tetrahedral outline. One corner is 

 Antarctica, the opposite face is the 

 Arctic Ocean. Three edges, the 

 Andes, the African mts., and the 

 ridges of Malaysia and Austra- 

 lasia, point towards the Antarctic 

 Corner. The remaining three edges 

 are formed by the mountain ridges 

 which run east and west in the 

 northern hemisphere. 



The hydrosphere, or water 

 covering of the earth, consequently 

 tends to form four basins ; the best 



marked of these is the Arctic Ocean. 

 Of the total surface of 196|- mil- 

 lion sq. m., 141 J million sq. m. are 

 water, most of which is found in the 

 Water Hemisphere, which has its 

 pole close to Antipodes Island and 

 includes Australasia, Antarctica, 

 and small portions of South Amer- 

 ica and S.E. Asia. The maximum 

 depth of the hydrosphere does not 

 reach 6 m. The hydrosphere is the 

 source of the water which in the 

 form of rain, ice, and flowing 

 streams tends to wear down the 

 surface of the lithosphere or solid 

 earth to a uniform slope which 

 would leave only the ' tetrahedral 

 edges as mountain ridges ; this 

 process is most marked all round 

 the Arctic Ocean in the great 

 plains of Canada and Russia. Al- 

 though it is probable that the bulk 

 of the lithosphere is of the same 

 composition, the known diversity 

 in the composition of the outside 

 crust is responsible for the minor 

 variations of ridge and valley which 

 are infinitesimal in relation to the 

 earth's bulk (1,000 ft. compared 

 with 8,000 miles, i.e. roughly 

 1 : 40,000). 



Flux of Atmospheric Conditions 



The earth is the home of man. 

 In common with the forms of life 

 which provide his sustenance man 

 exists on the earth because of the 

 atmosphere. His yearly and daily 

 round is regulated by the motion 

 of the earth. The annual revo- 

 lution, with the consequent varia- 

 tion in the earth's tilt relative to the 

 sun, causes the seasons and the well- 

 marked differences between the 

 tropical and polar belts. In rela- 

 tion to these differences the tetra- 

 hedral conformation produces varia- 

 tions in atmospheric circulation 

 which govern the several climatic 

 differences which cause the weather 

 to vary from place to place. 



The net result of these differen- 

 tial conditions is the production in 

 the neighbourhood of the tetra- 

 hedral ridges of the northern hemi- 

 sphere of areas where the atmo- 

 spheric conditions are in a state 

 of continuous flux. It has been 

 demonstrated that man reaches 

 and retains his highest develop- 

 ment in response to such a kaleido- 

 scopic environment. It is not an 

 accident that modern civilization 

 in its highest expression is found on 

 the edge of the Alpine ridge, where 

 the northern slope abuts on the 

 junction of the Arctic and Atlantic 

 basins. See Planet. B. c. wants 



Earth. Term used in several 

 senses besides that of the name of 

 a member of the solar system. Two 

 only need be noted here, one in con- 

 nexion with electrical and magnetic 

 phenomena, and the other with the 

 so-called rare earths. 



The chief rare earths are cerium, 

 terbium, and ytterbium, occurring 

 in small quantities in Scandinavia, 

 the Urals, America, and Australia, 

 and yielding radium, the discovery 

 of which has revolutionised the 

 theories of the constitution of 

 matter. 



In 1600 Gilbert showed that the 

 earth was a magnet ; the magnetic 

 poles corresponding approximately 

 to the geographical poles. The 

 cause of this magnetisation of the 

 earth is unknown, but it has been 

 observed to vary in strength with 

 the appearance of spots on the sun. 

 That there is an intimate connexion 

 between solar phenomena and mag- 

 netic storms on the earth has long 

 been known. These magnetic 

 storms are of such strength as to 

 interfere with and often prevent 

 the transmission of telegraphic 

 messages. They are notably severe 

 at times of brilliant displays of the 

 aurora borealis. See Magnetism ; 

 Telegraph and Telegraphy. 



Earth Colours. Pigments uni- 

 versally employed before the intro- 

 duction of oils. The cave men used 

 red, yellow, black, and white for 

 their crude but spirited drawings 

 of animals, and the ancient Egyp- 

 tians and Greeks resorted to similar 

 though somewhat more numerous 

 pigments. Honey or one of the 

 gums was probably the binding 

 medium. The earth colours in- 

 clude the ochres, umbers, sienna 

 brown, and terre verte. 



Earth Density. This has been 

 determined experimentally by 

 several physicists, beginning with 

 Cavendish. The mean density as 

 calculated most recently by C. V. 

 Boys is 5 '5268 ; or rather more than 

 five and a half times that of water. 

 See Cavendish Experiment. 



Earthenware. Name given to 

 all pottery that is not translucent. 

 It includes Faience, Delft, Stone- 

 ware, and such modern makes as 

 Granite ware, Silicon, Semi-porce- 

 lain, and so forth. It may be 

 glazed or unglazed, the terra- 

 cotta of the Greeks, flooring tiles 

 and building blocks, or the Majolica 

 and Enamelled wares all coming 

 under the general term. Its origin 

 is lost in antiquity, dating back 

 before the age of metal working to 

 a primitive culture when plaited 

 baskets were encased in clay for 

 cooking food. See Pottery; also 

 illus. pp. 139 and 2083. 



Earth-House. Primitive under- 

 ground structure of the early metal- 

 lic age, especially in Scotland. Nor- 

 mally it is a round or rectangular 

 chamber of unhewn masonry, with 

 a beehive roof, beneath an artificial 

 mound. Frequently one or more 

 chambers are approached by stone- 



