CHAMBERS'S INFORMATION FOR THE PEOPLE. 



ments and changes, of almost incredible velocity 

 and magnitude, constantly going on in the spots 

 and bright patches lead irresistibly to the conclu- 

 sion that the luminous surface of the sun his 

 fihotosfihere, as it is termed is of a cloudy nature. 

 But during a total eclipse of the sun, when his 

 shining disk is covered by the dark body of the 

 moon, there is readily seen a white halo called 

 the corona, surrounding the moon ; within this 

 there were first observed fantastically shaped 

 masses of a red colour, projecting considerably 

 here and there beyond the moons edge, and 

 variously called red flames and red prominences ; 

 and closer observation has shewn that these larger 

 prominences are connected by a continuous belt 

 of similar colour at a lower level. This less 

 brilliant envelope is known as the chromosphere. 



By bringing that new and marvellous instru- 

 ment of investigation, the spectroscope, to bear on 

 those appearances, a good many points have 

 late been established regarding the physical con- 

 stitution of the sun that is, what substances it 

 composed of, and the condition in which those 

 substances exist. To explain the action of the 

 spectroscope belongs to Optics ; it is sufficient 

 here to say, that it tells whether a distant luminous 

 body is in a solid or liquid state, or whether it is 

 in the state of vapour; and can discriminate 

 between the light proceeding from incandescent 

 hydrogen, for instance, and that from the vapour 

 of sodium or of iron. The chief results arrived at 

 may be thus summed up : The sun is composed 

 of substances identical, in part at least, with those 

 composing our earth; hydrogen, sodium, iron, 

 magnesium, and several other metals have already 

 been fairly ascertained. The matter of the sun is 

 so intensely hot as to be to a great extent in the 

 state of vapour ; at all events, it is so to a consid- 

 erable depth at the surface. The outer envelope, 

 or cromosphere of the sun, consists mainly of 

 hydrogen. Below this, we have the photosphere, 

 or region containing metallic vapours, along, prob- 

 ably, with numerous deposited cloud-particles of 

 these vapours, which particles are the chief source 

 of the light and heat of the sun. The photosphere 

 is in a state of constant agitation, like that of 

 boiling, caused, apparently, by the portions on the 

 surface cooling by radiation and rushing down at 

 one place, while hotter matter from the interior 

 is heaved up at another. When these 'convection- 

 currents' are exceptionally violent, they become 

 visible as faculae and spots, the blackness of the 

 latter being caused by a great down-rush of the 

 comparatively cold matter from above into a 

 hollow of the photosphere. This agitation of the 

 photosphere causes a corresponding commotion 

 in the chromosphere; where the photosphere 

 is upheaved, masses of the red-hot hydrogen 

 envelope are projected far above the general 

 level, sometimes to the height of tens of thou- 

 sands of miles, and form prominences. The velo- 

 city of these uprushes, and still more of the 

 whirling motions going on in the hydrogen, is 

 astonishing, being sometimes not less than 120 

 miles a second. The nature of the corona is yet 

 not fully ascertained. 



As to the sun's heat, it has been calculated 

 that the amount given out by one yard of his 

 surface is as great as that which would be pro- 

 duced by burning six tons of coals on it each 

 hour. The amount received by the earth in one 



year would be sufficient to melt a layer of ice 100 

 feet thick all over the earth's surface. But the 

 sun's heat is given out equally all round, so that 

 the portion intercepted by the earth at its distance 

 of 92,500,000 miles must be an inconceivably smal) 

 fraction of the whole that is lost by the sun. 



Is the sun becoming cooler with all this loss > 

 It must be so, unless the loss is in some way made 

 up. The most probable theory of the origin of 

 the existing store of solar heat is, that the matter 

 composing the sun was originally diffused in a 

 nebulous form throughout space, and that this 

 matter, falling together by gravity, had its motion 

 converted into heat, just as two stones are heated 

 by clashing together. But we know of no source 

 from which the continual dissipation is replen- 

 ished ; and without fresh fuel, the fire, however 

 big, must in the end go out. ' There will come a 

 time when the sun, with all its planets weldedf 

 into one mass, will roll, a cold black ball, through 

 infinite space.' Lockyer's Astronomy. 



Mercury () and Venus ($), the two members of 

 the system next to the central body, are called 

 inferior planets, from their orbits being within 

 ! that of the earth. Owing to their position, they 

 can never appear at any great distance or elonga- 

 tion from the sun. Mercury, in fact, even at its 

 greatest elongation, sets long before the end of 

 i twilight, or if west of the sun, does not rise till 

 the dawn has begun ; so that in our latitude and 

 cloudy climate, it is rarely seen with the naked 

 eye. Venus attains more than twice the elonga- 

 tion of Mercury, and is seen long after night has 

 fairly set in, or before the morning twilight. Both 

 inferior planets go through phases like the moon,, 

 and when seen through a telescope, have a cres- 

 cent or a gibbous shape. They are never seen 

 when full, or when new, being then near the sun. 

 Venus is the most conspicuous of the heavenly 

 bodies, next to the sun and moon ; when it rises- 

 before the sun, it is called the Morning Star- 

 the Lucifer of the ancients ; and when it sets after 

 the sun, it is the Evening Star or Hesperus. 



Telescopic observations of Mercury and Venus- 

 are very difficult, owing to the intense brilliancy 

 of the surface ; hence nothing has been ascer- 

 '< tained as to their physical constitution, 

 the times of their rotation on their axes cannot be 

 held as accurately determined. 



The Earth () is the third planet in order from 

 ! the sun, and closely resembles Venus. The 

 ! earth is not a perfect sphere ; it does not measure 

 the same in all directions, but has its axis, or 

 the diameter on which it rotates, shorter than 

 its diameter at the equator. This is a general 

 law in all planets, the cause of which will 

 explained afterwards. The earth, then, is flat- 

 1 tened at the poles, and its shape is called by 

 astronomers an oblate spheroid. If it were cut 

 in two, by a plane passing through the two poles., 

 the section would be, not a circle, but an oval, or 

 ellipse; and the excess of the equatorial diamete 

 over the polar diameter is called the ellipticity of 

 the spheroid. The ellipticity of the earth is found 

 to be irk, or the equatorial diameter exceeds the 

 polar by ^th of its length. This was determined 

 in two ways : by measuring degrees. of the men 

 dian at different latitudes ; and by measuring the 

 variation in the force of gravity, between the 

 equator and the poles. The polar diameter, omit- 

 ting fractions, is 7899 miles, the equatorial 79 2 5 ; 



