820 Constituents of the Solar Envelope. solar envelope. There is reason to suppose that this material lies in a more extended atmospheric mass, with but little luminosity. Above the photosphere is a gaseous stratum named (by Lockyer) the chromosphere. It has luminosity, but in a very inferior degree; it is reddish, and it is the base and source of the red flames and prominences seen in eclipses. It is known to rise to a great height above the photosphere (many thousands of miles). The chromosphere shades off into an upper stratum, less luminous, which is the corona, or coronal atmosphere, as already described from its appearance in eclipses. The materials or substances composing the envelope of the sun have been discriminated by means of the spectroscope. Among these are hydrogen gas, and the vapours of the metals, magnesium, caï¿um, sodium, iron, chromium, manganese, nickel, barium, strontium, and titanium. There may be many others, and it is supposed that there are among them some substances that are not found in the earth. The metallic vapours, at a white heat, constitute the photosphere. In the chromosphere, the prevailing substance is hydrogen, at such a temperature as to make it luminous, although not to the degree of the metallic vapours. Above this luminous hydrogen is the same gas in a cooler and less luminous condition, together with another substance not existing in the earth, to which has been in the meantime given the name helium. 1053. The enormous changes, indicated by the spots and prominences, show the intense activity of the solar atmosphere. The hypothesis at present adopted to explain the nature and direction of the activity or movements, is a system of up-and-down-currents, or upheavals and sinkings, accompanied with whirling or cyclonic motions. The upheavals are sometimes so violent as to be compared to volcanic eruptions. A mass of heated matter is forced up from the photosphere below, and leads to an upheaval in the chromosphere above. The rise of photospheric matter makes the bright spots or faculæ, the further upheaval of the chromosphere makes the jets or prominences. These are proved by the spectroscope to be, in many instances, cyclonic; and this is probably the case with them all. Corresponding to the upheavals there must be down-draughts or currents from the (comparatively) colder heights, which would make the dark openings of the photosphere known as the sun's spots. These downward currents may also be presumed to be cyclonic. Estimates have been made of the rapidity of these eruptive up Mercury.-Venus. 821 heavals. Counting the time of throwing up prominences to a measured height, the rapidity of movement of the gases is sometimes not less than 120 miles a second. Considered merely as a hot body parting with its heat and light through incessant radiation, the sun must in course of ages cool down, so as no longer to maintain the present temperature of the planets. For a very long time, no appreciable difference may be felt, but too little is known to enable an exact estimate to be formed of the rate of diminution. The Planets. 1054. Mercury.—A small planet, and the nearest to the sun; it seldom obtains a sufficient elongation to be seen, being hidden in the solar rays. The mean distance from the sun being about onethird of the earth's distance, the intensity of the sun's heat and light must be nine times as great as on the earth, which would bring about a temperature incompatible with life as known to us. Mercury goes through phases like the moon. From the intense brilliancy of the surface, there is an absence of the discriminating marks that give information as to the planet's period of rotation. From such indications as could be had, it has been inferred to rotate on its axis once in about twenty-four hours. The existence of an atmosphere has not been determined. 1055. Venus.-The morning and the evening star. In size and density, Venus nearly resembles the earth. The distance from the sun (sixty-six millions of miles) is rather more than two-thirds the earth's distance, the heating and lighting power of the sun being thus fully double that on the earth. The equatorial heat would be excessive, judging from our standard, but the neighbourhood of the poles would contain climates resembling some of the habitable parts of the earth. The rotation of the planet is very nearly the same as the earth's ; but the inclination of the axis is very much greater, being supposed to be about 50°. This entails an enormous difference of the seasons, which would operate unfavourably upon life. There are appearances that indicate an atmosphere, and mountains, but not very decisively. Recent spectroscopic obversations are in favour of the presence of water, which would imply an atmosphere. The different positions of Venus make her distance from the carth very unequal, so that she varies greatly in apparent size. The accompanying diagram, taken from photographic representations, 822 The Earth's Density. shows both the variation of apparent size, and the phases of the planet. 1056. The Earth.-Next in order from the sun is our earth. What we do not know about the other planets, or know by precarious inference, we know with ease and certainty about the earth. Yet, in its planetary character, there are some things that we do not discover with the same directness as in the case of the planets commonly so called. Placed as we are at a distance, in their case, we can discern at once their round shape, and their motions through space. The first property of the earth, considered as a planet, is its Figure. It is nearly, but not exactly, a globe. It is flattened at the poles, and it bulges at the equator; the difference of the two diameters, as already stated, is twenty-six miles, or about one threehundredth part. The equator itself is not a perfect circle, there being a difference of about two miles between its longest and its shortest diameter. The form and size of the earth being ascertained, the next important fact is its Density considered as a whole. We know the density of the materials composing the crust, so far as we are able to penetrate it. The rocks that are accessible to us have a specific gravity of between two and three, water being one. But these superficial rocks may not represent the interior. In the depths there may be a great quantity of the heavy metals, as iron, lead, copper, tin, silver, gold, and their prevalence in any considerable proportion would raise the average specific gravity much above the specific gravity of the ordinary rocks. Moreover, we do not know the limits to the condensation of bodies under hundreds of miles of a superincumbent mass, although probably the utmost amount of compressibility of the ordinary minerals is not great. The difficulties of ascertaining the mean density of the whole earth, so as to estimate the quantity of matter contained in it, are very considerable. Differe: t methods have been resorted to, Methods of finding the Density. 823 The most direct method is to measure the deviation of a plumbline from the perpendicular, when in the neighbourhood of a large mountain. If this deviation (which is a very small quantity) could be accurately measured, and if the mass of the mountain (combining its bulk and its specific gravity) could be measured, we could deduce the mass of the earth. This operation was performed in the last century, in Perthshire, by Dr. Maskelyne, and is called the "Schehallien" determination. By it the mean density was given at between 4'56 and 4.87 Another method is the Cavendish experiment by the torsion balance, a far more delicate apparatus for testing attraction in small amounts. Instead of a mountain, which, although from its size it is able to exert a considerable influence, is very difficult to measure, Cavendish substituted two heavy spheres of lead, which were brought into the neighbourhood of the little balls at the end of the torsion lever. The joint attraction of the spheres deflected the balance, and the amount could be measured and compared with the downward gravity of the earth. The result was to make the density of the earth as a whole 5'48. The same experiment, repeated since with still more care, has since given 5·66 as the figure. The third method, called the pendulum method, is carried out in two forms. In the one, a pendulum is taken to the top of a mountain, and the swing compared with what it would be (known by calculation) at the same height above the unelevated surface of the earth. On the other method, the pendulum is taken to the bottom of a deep mine. In such a spot, attraction is diminished, in so far as there is less matter to attract (the portion overhead counts for nothing), and increased in so far as the attracted body is nearer the centre of the earth. The last effect is the greater of the two, and the gravity is actually increased, as shown by the increased rate of vibration of the pendulum. By this method, which was carried out under the direction of the Astronomer Royal, Sir George Airy, in the Harton Colliery, South Shields, at a depth of 1260 feet, the calculation showed a density of six and a half times water, or 6·565. The previous estimate is still preferred, and it is usually assumed that the average density of the whole earth is 5'6. This determination is the key to the estimate of the densities of all the other bodies of the system. The comparative masses of the sun, planets, and satellites, are found by their relative gravitating energy. But we cannot tell the absolute masses till some one is 324 Distance of the Sun from the Earth. estimated, and this one must be the earth. The earth's density multiplied by its bulk gives its mass or quantity of matter. 1057. The distance of the earth from the sun is ascertained on the principles already described in Art. 1020. It is measured by means of the angle of horizontal parallax; which, however, is so small that errors greatly affecting the result may easily be made. The earth's semi-diameter as seen from the sun, amounts to less than nine seconds of an arc. It is for this determination that so much importance is attached to the TRANSITS OF VENUS across the sun's disc, which occur at alternate intervals of 8, 122, 8, 105, 8, 122, years. Two occurred near the middle of last century (1761, 1769); from them the conclusion was drawn that the parallax of the sun is between 8"-5 and 8"7. From this was obtained the estimate of the sun's distance that long prevailed, namely, 95 millions of miles. The next transit occurred in 1874; but in the meantime other methods have led to the adoption of an increased angle of parallax, and consequently a diminished estimate of the sun's distance. The method of proceeding for the transit of Venus consists in choosing two stations in the earth as widely as possible apart in latitude, or north and south, and as nearly as possible in the same longitude. The difference of the position of the observer will make a difference of position in the projection of Venus in the sun. The design is to measure exactly the interval of the two apparent tracks, which is done in a very efficacious manner by the difference of time of the apparent transits. The transit that is nearest the sun's centre will be longest from the nature of a circle; and from the difference of time, the difference of the two projections can be known. In the diagram let s be the sun, E, the earth, and V, the planet Venus, supposed to be in line with the earth and the sun, and A B Fig. 323. a G D thus casting its shadow on the sun's disc. Let A and B be two stations in the earth as far asunder as possible in the north and south directions and as ncar as possible in the same meri. |