PHYSICAL GEOGRAPHY. 



EOGRAPHY from^-/, the earth, 

 V_J I write in its simple and literal significa- 

 tion, is that science which describes the superficial 

 appearance and conditions of our globe. It 

 naturally divides itself into two great branches 

 I. Physical Geography, which treats of the earth 

 as a superficies composed of land and water ; 

 considers the position, extent, altitude, and general 

 character of the former ; and the position, extent, 

 depth, currents, and other motions of the latter : 

 in short, all that relates to the distribution of land 

 and water, variations of surface, temperature and 

 climate, and distribution of plants and animals 

 as dependent thereon, are the legitimate objects 

 of this species of geography. 2. Political Geo- 

 graphy, which treats merely of the division of 

 the earth's surface by man into territories and 

 states, describing their boundaries, the history 

 of their occupation, their produce, commerce, 

 population, laws, religion, and other topics which 

 constitute the fundamental features of human 

 polity. The latter of these branches will form the 

 subject of several subsequent treatises ; to an 

 exposition of the former dwelling more on prin- 

 ciples than on mere descriptive details we intend 

 to devote the present number. Before doing so, 

 however, it will be necessary to advert briefly to 

 the cosmical relations and constitution of our 

 planet, as determined by astronomy, geology, 

 chemistry, and meteorology. 



GENERAL CONSTITUTION OF THE GLOBE. 



Astronomy informs us that the earth we inhabit 

 is one of a number of planets which revolve round 

 the sun as a common centre, constituting what is 

 usually denominated the SOLAR SYSTEM. These 

 planets are situated at different distances from 

 the central orb, and differ also in their magnitudes, 

 their densities, and in their periods of revolution. 

 They are nearly spherical in form, are opaque, 

 have no light of their own, but merely reflect that 

 of the sun ; and all move from west to east in 

 nearly circular orbits. Several of them serve in 

 turn as centres for other bodies of revolution, 

 which are known by the name of satellites as 

 the moon, for example, which is the satellite or 

 attendant of the earth. Besides the planets and 

 their satellites, there is a third and numerous class 

 of bodies belonging to the solar system namely, 

 comets, which revolve round the sun in regular 

 periods, but in orbits so elliptical, that in parts of 

 their course they approach nearer to the great 

 orb than any of the planets, and in others recede 

 so far into the regions of space, as to be entirely 

 beyond the reach of our most powerful telescopes. 

 The stars belong to other systems of revolution, 

 and have, so far as has yet been determined, no 

 perceptible effect upon the conditions of our globe, 

 though undoubtedly bearing, like everything in 

 nature, a universal harmonious relationship. 



The earth, as an individual planet, is situated 

 at the mean distance of 91,350,000 miles from the 

 sun ; has a mean diameter of 7912 miles ; per- 

 forms a revolution round the sun in 365 days 5 

 4 



hours 48 minutes and 49 seconds, which consti- 

 tutes the space of time called a year ; rotates on 

 its own axis once in 24 hours that is, in one day ; 

 and in these movements is attended by the moon, 

 which is distant 237,000 miles, is 2160 miles in 

 diameter, and which completes her synodic revo- 

 lution in 29 days 12 hours and 44 minutes, or in 

 one lunar month. We have spoken here of the 

 mean diameter of the earth, because, upon accurate 

 measurement, it has been found to be not a perfect 

 sphere, but an oblate spheroid, whose greater 

 diameter is 7925, and whose lesser is only 7899 

 miles. This gives a difference of 26 miles between 

 the two diameters, or a flattening at each pole of 

 about thirteen miles a result that may be arti- 

 ficially illustrated by twirling with rapidity a ball 

 of any yielding material, such as putty, round a 

 spit thrust through it as an axis, when a bulging 

 at the outer circumference will take place, causing 

 the ball to lose its original spherical form. This 

 bulging takes place through what is called the 

 law of centrifugal force ; and from what we know 

 of this law, it is concluded that the earth was in a 

 soft or yielding state at the time when it assumed 

 its present form. Besides the bulk, revolutions, 

 and configuration of our globe, science has also 

 determined its density with considerable accuracy. 

 By weighing the most prevalent rocks, it has been 

 found that the solid crust composed of them is 

 about two and a half times heavier than water ; 

 but from experiments made on the attraction of 

 mountains of known bulk, compared with the 

 attraction and bulk of the globe, and by other 

 means, it has been inferred that the density of 

 the whole mass is five or six times that of water : 

 in other words, the earth, as at present consti- 

 tuted, is five or six times heavier than a globe of 

 water of similar dimensions, and more than twice 

 the weight of one composed of such rocky sub- 

 stances as those with which we are acquainted. 

 In addition to what may be called its own proper 

 material, the earth is surrounded by a gaseous 

 envelope or atmosphere. This atmosphere or air 

 is peculiar to, and inseparable from, our globe 

 it rotates with the solid mass upon its axis, and 

 does not, as may at first be supposed, occupy the 

 space in which the rest of the heavenly bodies 

 revolve. Like all aeriform and liquid masses 

 whose particles press upon each other equally in 

 every direction, the portions or strata next the 

 earth are more pressed upon than those in higher 

 regions ; and continuing this conception, a height 

 must be arrived at where the air becomes so 

 attenuated as to be inappreciable. That limit 

 used to be assumed at 45 to 50 miles ; but there 

 is evidence to shew that there is an atmosphere 

 of some kind even at the height of 400 or 500 

 miles. 



From its planetary relations, as a part of the 

 solar system, the earth derives its figure and 

 motions, its light and heat, and consequently the 

 changes of season, and the alternation of day and 

 night j the phases of the moon, and the rising and 

 falling of the tides ; the vicissitudes of wind and 

 weather, and all the varied results and phenomena 



