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The absorption and re-radiation from successive layers is almost instantaneous, the velocity of heat transference approaching 180,000 miles a second.

In drawing our conclusions the intensity of gravitation on the sun must be borne in mind. It has nearly 28 times the force of gravity on the earth. A man of ordinary size would weigh two tons at the surface of the sun, and would, therefore, be instantly crushed to death by his own weight, were it possible for him otherwise to exist there. Consequently, the pressure to which the vapors of the sun are subject increases with enormous rapidity below the surface.

The average specific gravity of the materials composing the sun can be determined by astronomical theory with great exactness. It is known that the mean specific gravity is about 40 per cent greater than that of water, and one-quarter that of the earth. It is doubtless much smaller than this at the surface, and, therefore, increases toward the centre. A calculation of the resulting pressure shows that near the centre of the sun the pressure produced by the enormous mass and gravitation of the matter composing the solar orb amounts to about 5,000,000 tons per square inch. This pressure is so far beyond any that we can produce at the earth's surface that we are unable to say what effect it would have upon

matter.

Yet another unknown factor is the temperature of the interior. At no great distance toward the centre the temperature exceeds our powers of determination - it may even be 1,000,000°. As the highest temperature which it is possible to produce artificially probably does not amount to 12,000°, it is impossible to say what effect such a temperature would have upon matter. Thus we have two opposing causes, the one an inconceivable degree of heat, such that, were matter exposed to it on the surface of the earth, it would explode with a power to which nothing within our experience can be compared, and a pressure thousands of times any we can produce, tending to condense and solidify this intensely heated matter. One thing which we can say with confidence as to the effect of these causes is that no chemical combinations can take place in matter so circumstanced. The distinction between liquid and gaseous matter is lost under such conditions. Whether the central portions are compressed into a solid, or remain liquid, it is impossible to say.

Modern research shows that the sun, as a whole, is a complex body, the various parts of which are in very different conditions. Beginning at the centre and passing outward, we have first the vast, invisible interior which forms the globe itself, and which our sight can never penetrate. Surrounding this interior is the visible photosphere, or seeming surface, which we see with the naked eye or the telescope, the appearance of which has been fully described. So far as ordinary direct observation could show, this would be the whole of the sun. But the spectroscope, as well as eye observation during total eclipses, has shown most complex surroundings of the sun, which would otherwise have been invisible. The surroundings are formed of two envelopes, the chromosphere and the corona.

The earliest accurate observers of total eclipses with the telescope noticed that during the total phase red cloud-like masses were seen here and there projecting beyond the limb of the dark moon. Moreover, at the beginning or the end of the eclipse, it is found that these projections are connected with a red border extending round the sun. There is, therefore, an envelope which radiates red light and surrounds the sun, and which is invisible except during eclipses. Quite independent of this envelope is a bright effulgence which is seen during a total eclipse. These phenomena are fully described in the article ECLIPSE. What we have now to do is to set forth what they indicate.

The red envelope which rests immediately on the photosphere is called "chromosphere." It is comparatively thin-so thin as to be almost immediately covered when the sun is totally eclipsed. Its nature was first made known by the spectroscope, which showed it to be composed mostly of hydrogen, helium, and calcium vapor. Its principal and lower parts differ in constitution. At the photosphere it comprises nearly all the substances which exist in the latter. This was shown in a very beautiful way by observations of the reversing layer, first made by Young at the total eclipse of 1870. The explanation of the phenomena there described is that the photosphere is hot enough to shine by. its own light, and, being a gas, to give bright spectral lines. But the photosphere is so much hotter than the chromosphere that the latter is, in comparison, a cool gas which absorbs the spectral lines from the light radiated by the photosphere. The question of the density of the chromosphere and reversing layer, as its base is called, has given rise to very varied esti

mates.

The fact that the spectroscope shows bright lines as the last ray of true sunlight disappears at the beginning of a total eclipse shows that the gas from which these lines emanate must be so rare as to be transparent through a distance of thousands of miles. We are, therefore, justified in concluding that the gases of the chromosphere are extremely rare, and the same is probably true of the principal regions of the photosphere.

Among the most extraordinary phenomena exhibited by the sun are the mountainous elevations of the chromosphere, which we see as the red protuberances already described. These are of two kinds, the eruptive and the cloud-like. The latter present to us the appearance of vast clouds floating in an atmosphere of the sun. It seems certain, however, that they cannot be what they seem, because there can be no atmosphere there to support them. They are probably held up by an impulsion of the solar rays, which will be described presently. The eruptive prominences seem to be due to outbursts of intensely hot gases, mostly hydrogen, from the sun. These are thrown up with a velocity of several hundred miles per second, like immense mountains of fire. They sometimes rise to a height of many thousand miles, their ascent being doubtless aided by the impulsion of the solar rays; then they fall back again to the sun. The chromosphere and prominences can now be photographed in projection against the sun's disc with the spectroheliograph. When such photographs are made with the light of the red hy

drogen line, they show great vortex phenomena, centering in sun spots and closely related to the vortices in the photosphere which constitute the spots themselves. See Plate III.

The violent forces seen in action in the chromosphere are in singular contrast to the soft white light of the corona. Much mystery still surrounds the constitution of the latter. It was supposed to be an atmosphere of the sun; but this view is rendered untenable by the fact that an atmosphere supported by its own weight would more than double in density for every mile that it was nearer its base. It probably consists of exceedingly minute molecules of gaseous matter, similar to those which make up the tail of a comet, and possibly having some resemblance to the latter. The newly-discov

an electrical thermometer. The unit usually employed is the calory, which is the amount of heat required to raise one gram of water 1° C. The solar constant is then the number of calories which would be received on each square centimeter of the earth's surface, exposed perpendicularly to the solar rays, if there were no loss in transmission through the atmosphere.

The following small table gives the mean values of the solar constant obtained from observations made at Washington, Mount Whitney and Mount Wilson. Observations were made at the second station, whose elevation is nearly three miles, in order to test the accuracy of the laws assumed for the absorption of solar rays by the atmosphere.

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ered fact of the impulsion of the solar rays probably affords a clew for an explanation of these and similar phenomena. More than 40 years ago it was announced by Maxwell, as a result of his electro-magnetic theory of light, that light and heat emitted by the sun should exercise a very minute pressure on any object which they struck. Conclusion showed that this pressure was so slight that no apparatus then known was so delicate as to make it sensible. Within the last few years, however, E. F. Nichols and others have succeeded in showing experimentally that on very finely divided matter this action of light can be observed and measured. It follows that particles below a certain size will be repelled by the .sun's light with greater force than they are attracted toward it, and will thus be driven from the sun when in its neighborhood, or supported temporarily at a certain height above the sun. Hale has proved, by spectroscopic methods similar to those employed in his discovery of magnetic fields in sun spots, that the entire sun is a magnet, with a field about 80 times as intense as the magnetic field of the earth and with its magnetic axis inclined about six degrees to the sun's axis of rotation.

The Sun's Rotation.-As the light and heat which we receive from the sun are the source of all life on the earth, the important work is at once suggested to measure exactly how much radiant energy we receive from the sun in a given time, and especially, if possible, to find whether this is growing greater or less, or if it varies from time to time. Until so late as 1905 the measurements were comparatively very crude, but the sensitiveness of the instruments employed has recently been so increased and observations with them have been so carefully and continuously carried on that this quantity has now been well determined.

For measuring the amount of heat received on a square unit of the earth's surface, the so-called pyrheliometer is employed, an instrument which presents a surface of known area to the solar rays, the rise in temperature due to the heating being communicated to a stream of water (in the best form), and measured by

Prior to 1905 the true value of the constant was in much doubt; numbers ranging from 1.76 to 4.10 were stated for it, and the average value 3.0 was frequently accepted. There can be no doubt that the value 1.95 is very near the truth, and this may be regarded as the best value now obtainable. It has been well established, however, especially by the recent work of Abbot, that this fundamental constant varies slightly and irregularly. In 1919 a solar station was established at Calama, Chile, at which it is planned to make constant measures of the sun's radiation for several years, in conjunction with northern observations, with a special view of ascertaining, if possible, the law of this variation, and its effects upon terrestrial climates.

The Sun's Magnetism.- In the year 1908, Prof. G. E. Hale of the Mount Wilson Observatory, from an examination of the spectral lines in sun spots, discovered that around each spot there is a more or less powerful magnetic field. A powerful field will double many of the lines of the spectrum; a less powerful one will merely widen them. A few lines were found triple in sun spots, and afterward these same lines were found to become triple in the laboratory when viewed along the magnetic lines of force. Thus the lines of sun spots near the sun's limb tend to become triple, while those from spots near the centre of the solar disc are doubled merely. In many cases a pair of sun spots quite near together are found to have opposite polarity, and while in general the polarity of spots in the southern hemisphere is different from that in the northern, many cases of exception occur. It has, however, been well established that the sun, like the earth, has a north and south magnetic pole; the inclination of the sun's magnetic axis has been determined, and the fact has been established that the magnetic pole is in slow rotation about the pole of the sun.

A curious relation is found by the study of magnetic storms on the earth. The latter consist in occasional perturbations of the magnetic needle, which are very irregular in their character and are felt over the whole globe. They

generally occur when there is an unusually bright aurora. Now, investigation shows that the number of these disturbances follows exactly the period of the solar spots. During those years when the spots are most numerous magnetic storms are most frequent, and viceversa. The conclusion is that the sun spots and magnetic storms are due to the same cause. The sun's spots can be due only to something going on in the sun, and it follows that there must be some emanations from the sun which produce magnetic storms. Modern investigation has not been able to detect or define these emanations, though they are supposed to be electrically charged particles, shot out from the sun, and drawn in around the magnetic poles of the earth, where they give rise to the aurora. We have the strongest reasons to believe that neither the magnetic field of the sun as a whole nor the much more intense local fields in sun spots, produce any measurable magnetic effect at the distance of the earth.

Age and Duration of the Sun.- The greatest problem connected with the sun is suggested by modern science. Up to 100 years ago students and philosophers saw no reason why the sun should not continue to shed heat and light on the earth for an indefinite period without undergoing any change whatever. But toward the middle of the 19th century the laws of energy were developed and understood. These laws set forth that the radiation of heat always involves the expenditure of something called energy; and that the latter is necessarily limited in supply. It was also seen that the sun must be a hot body, and must lose all the heat it radiated. To make this subject clear, we must remark that what the sun really radiates is not properly called heat in scientific nomenclature; the more exact term is radiant energy. But this differs in no respect from what is radiated into a room from a hot fire. Radiant energy goes out from the fire and strikes the walls of the room, where it creates heat, and thus warms the walls. All the heat thus transmitted to the walls comes from the coal, although in its passage from the fire to the walls it passes through an intermediate stage called radiant energy. That the latter does not necessarily warm a medium through which it passes is shown by the striking experiment of making a large lens out of ice instead of glass. When the sun's rays are concentrated on a point by passing through this mass of ice they will burn the substance on which they fall, as if they had passed through glass. We see, then, that however cold the space between us and the sun may be, all the radiant energy reaching us from the sun must come from a source in the sun limited in supply.

If the sun were merely losing energy like an ordinary hot body cooling off, a very simple calculation will show that it would be so cooled off in the course of 3,000 or 4,000 years as no longer to radiate much heat. It is clear that such has not been the case. Yet the most careful study shows no possibility that it can be receiving any considerable part of its energy from any outside source. Moreover, the geologists assure us that the stratification of the rocks, as well as many other other phenomena associated with them, proves that the sun has been radiating heat to the earth at not much

less than its present rate for hundreds of millions of years.

The only solution of the puzzle that was suggested until 1903 was based on the mutual convertibility of heat and motion. From the time that the theory of energy was developed it was known that when the motion of a material substance is arrested without any other effect being produced, heat is generated. For example, the waters of Niagara are warmed by about onequarter of a degree Fahrenheit in striking the bottom of the falls. The blacksmith by hammering a piece of cold iron can make it hot, because the energy which he puts into the motion of the hammer is converted into heat when the latter strikes the iron. It follows that if bodies of any sort are falling into the sun, heat will be generated by the fall. Moreover, owing to the power of the sun's attraction, such bodies may fall with great velocity; and the heat thus generated increases as the square of the velocity. Thus arose the first theory as to how the sun's heat could be kept up. It was supposed that meteors were continually falling into that luminary. But further study showed it to be impossible that meteors could fall in such quantity as to have this effect.

Then it was suggested by Helmholtz and Thomson that if the sun were a gaseous body, as it is now supposed to be, radiating energy, the loss of the latter would continually be made up by the fall of its outer portions involved in the continual contraction of the sun through loss of heat. All bodies, and gaseous ones in a higher degree than any others, diminish in volume when they cool off. Accordingly, when the photosphere of the sun cools off, it diminishes in volume, grows smaller and falls down upon the mass of the sun below it. Careful calculation shows that if the sun contracted about 250 feet per annum, the energy thus generated would keep up all the heat which the sun radiates. An important addition to this theory was made by J. Homer Lane, who showed that if the sun contracted like a mass of pure gas it would continually grow hotter as it contracted. This is now known as Lane's law. But there is a necessary limit to the quantity of heat which can thus be generated. If the sun has been thus growing smaller through long ages, there must have been a time when it filled the whole space now occupied by the solar system. What is more, the contraction must have been far more rapid the larger the sun was; because the force of attraction at the sun's surface diminishes as the inverse square of the diameter of the sun. For example, when the sun was twice as large as it is now, this force was only one-quarter as great; consequently it would have to contract four times as much to generate a given amount of energy as it does now. Finally, exact computation showed that even on this theory there was still a limit to the existence of the sun too narrow to satisfy the demands of geology. It could not have been radiating heat for more than 50,000,000 or 100,000,000 years. Before that time it must, according to the theory, have been a gaseous mass filling the whole space now occupied by the solar system, which contracted and formed sun and planets, in accordance with the theory known as the "nebular hypothesis."

It also seemed very improbable that the sun's

heat could have been at all constant for even 20,000,000 years; on the other hand, geologists went hundreds of millions of years. Thus apparently an irreconcilable contradiction was presented to scientific investigators when in 1900 the discovery of radium began to put a new face upon the fundamental theories of physical science. We now know that there is an immense amount of energy stored in the atom, which is a very complex thing. With the socalled "radioactive" substances, the atoms may be broken up, the result of the process being an element of lower atomic weight than the original substance. And in this breaking up of the atom a great amount of energy is liberated. Though it has been known for many years that Helmholtz' theory was inadequate, whether a large part of the sun's energy is of this sub-atomic origin, we do not know, but it is reasonable to suppose that it is. And it is only necessary to suppose that a part of the energy of the atom is in this way changed into heat energy to almost indefinitely prolong the life of the sun.

The most recent semi-popular, but authentic work is "The Sun' by C. G. Abbot (New York 1911); this contains many references to more extended works or detailed publications. A larger and very important work is 'Physik der Sonne,' by E. Pringsheim (Leipzig 1910). Numerous papers will be found in the Proceedings of the Royal Society, London, and in the Astrophysical Journal, Chicago. SIMON NEWCOMB. Revised by ERIC DOOLITTLE.

SUN, Eclipses of the. See ECLIPSE. SUN, Order of the. See ORDERS (ROYAL) AND DECORATIONS.

SUN-BIRDS, a large family (Nectariniida) of small insect-eating birds of the tropics of the Old World, having elongated, slender and curved bills, wings of moderate size and the central tail-feathers usually prolonged beyond the others. These birds occur in the Eastern Archipelago, India and Africa. They take the place of the humming-birds of the New World, and in brilliant coloration and habits much resemble these, but are far removed from them in classification, the honey-eaters (Meliphagida) being their nearest relatives. They are constantly hovering about flowers secking the minute insects found within the petals and sipping the flower-juices, so that they have been named sucriers or sugar-eaters, by French authors. Some certainly eat fruits. The song is sweet, but without any special characteristics, and in habits they are exceedingly lively, quarrelsome and even pugnacious. The gaudy plumage is chiefly confined to the male sunbirds and depends for effect upon intensity of color and not upon metallic or prismatic lustre. The nests are built in the hollows of trees or are placed in thick bushes. Some species (such as Nectarinia lotenia and N. asiatica) make dome-like nests, which are suspended from the extremities of twigs of bushes, and are covered with cobwebs for the purpose of concealment. A magnificent treatise upon the sun-birds, with colored plates, has been written by Shelley, entitled A Monograph of the Nectariniida' (London 1876-80). The name is sometimes given to various other birds. Thus the sugarbirds or banana-birds (qq.v.) of the West

Indies are often so called; and a large South American bird, also called finfoot, for which see HELIORNITHIDÆ.

SUN-BITTERN, an extraordinary somewhat rail-like bird of Brazil and Guiana (Eurypgahelias). It is about 16 inches long, body small and thin, neck long and slender, head like that of a heron, with a long, powerful beak compressed at the sides and slightly arched at the culmen; the plumage is minutely variegated with bars and spots of many colors, and it has the habit of spreading wings and tail in courtship or on other occasions of excitement, forming a rosette about its head and neck fancifully compared to a "sunburst." It is often made a pet by the Brazilians, who call it peacock. A larger species (E. major) inhabits Venezuela and Colombia. Their nearest relative is the kagu (q.v.). Consult Newton, A., 'Dictionary of Birds' (New York 1893–96). SUN CRACKS. See MUD CRACKS.

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SUN-DANCE, a ceremony performed, with local variations, by most of the prairie Indians, including the Mandan, Omaha, Pawnee Loup, Cheyenne, Arikara, Hidatsa, Blackfeet, Nez Percé, Winnebago, Yankton, Santee and Kiowa. It is held apparently at the full moon occurring at or next after the summer solstice, and lasts from three to six days. The budding of the wild sage also indicates the times for holding the ceremony, and all neighboring tribes, whether friendly or not, are usually invited. The dance begins at sunrise and ceases the following sunrise. As may be inferred from the length of the festival, including the fasting and purification of the devotees and other preparatory acts, the actual sun-dance is but the chief episode in a ritual comprising a congeries of ceremonies. The motive or purpose of the dance is to promote welfare through the gratification of desires and wants and to avoid ill-fare through the dispelling of hostile agents. The devotee or sun-dancer indulges in the ceremony to fulfil a vow, made by him during the previous winter or season from various motives, that he would make a prayer to the disposer of what he needs through an appeal to the sun, to "Wakanda" (among the Sioux). The Tetons call the ceremony by a name which means "They dance looking at the sun." In it the moon is regarded as the representative of the sun, hence the dancers gaze at it just as they do at the sun. Among the principal objects in the festival of the sundance is the sun-pole or "mystery tree" (symbolic of the centre of the four quarters of the heavens), the sacred tent of preparation erected within the so-called camping-circle of the tribe, wild sage, a sweet-smelling grass called wachanga by the Teton and the dancing-lodge. Each devotee persists in his part until he has received a vision from the sun; but if at the close of the ceremony no such vision has been vouchsafed to him, resort is had to self-sacrifice, which is called "vision-hunting." One of the characteristic forms of self-sacrifice is that of having two wooden skewers inserted underneath strips of skin raised by slashing the breast or back, to which stout thongs are made fast, by which the devotee is drawn up and fastened to the sun-pole, to which he remains suspended until his weight, sometimes made

greater by having a buffalo-skull hung to his person by similar skewers, causes the latter to rend the skin, thus letting the devotee fall to the ground, usually in a faint; another may have a buffalo-skull suspended from thongs passing through raised strips of the skin on the back or breast, which is allowed to hang thus until the skin is parted by violence and the thongs are freed. Some men who do not intend to dance seat themselves near the sun-pole, and small pieces of flesh are cut out in a row from the shoulders of each; these are offered to the being represented by the sun-pole. Women do not scarify themselves in the sun-dance, and self-torture and the shedding of blood are not practised in the Kiowa ceremony. Consult Catlin, G., North American Indians' (new ed., 2 vols., Philadelphia 1913); Lowrie, R. H., "The Sun Dance of the Crow Indians' (New York 1915).

SUN-DEW, any herb of the genus Drosera, which is classified near the pitcher-plants and the roses. Several species are found in America. The flowers are very pretty, like that of the saxifrage, five-petaled, and borne at the top of a leafless scape, in a raceme, the buds in which are bent downward, the blossom of each day surmounting the arch and facing the sky. They are white or purplish in color. Sundews grow in bogs or wet ground, the roots are poorly developed and yet the small plants thrive even in sphagnum; this is because they are flesh-eaters, and live on the nutriment obtained from such insects as they can catch on their foliage; the roots, therefore, serving principally to anchor the plants and to supply the large amount of water needed. The leaves, varying in shape in the several species, from round to filiform, are covered thickly on the upper face with wine-red filaments having a glistening drop like dew at the tips. These are stalked glands, destined for a purpose as deadly as that of the tentacles of the octopus. The leaf blades of the Drosera rotundifolia, a common sun dew, are round, and are arranged in a rosette around the base of the flower-scape, the smooth green under surfaces resting on the ground. In times of inaction the tentacles radiate in concentric circles and are tipped by their globular translucent glands, which sparkle with a viscid secretion exuded by them. But let a fly light on one of the glands and remain there, glued fast by the viscous fluid, and there is immediately a change in the state of things. In its efforts to release itself, the struggling insect is only besmeared more completely, chokes the organs of respiration and is ultimately smothered. In the meantime, the tentacles, disturbed by the fly, have become excited and have transmitted the stimulus to the other glands so that they all bend toward the tiny body, converging over it, and striving to touch it. They even shift the inert object toward the centre of the leaf-blade, so that the greatest number of tentacles may reach it. Such glands as succeed in touching the meat secrete an acid juice, with the addition of a ferment which is entirely similar to pepsin, and apply this secretion to the fly, digesting it, as it were. The glands then absorb the flesh and blood of the meat, and also their own secretions. The tentacles straighten up, the undigested portions of the insect resting on the dry

glands are blown away, and the glands soon begin to exude their viscid secretion again, and make themselves ready for a fresh victim. When a large insect is entangled, the leaf-blade itself folds inward slightly, so that a maximum number of tentacles may concentrate upon the food. D. filiformis has erect, very narrow leaves, and when an insect is caught by the glands, the leaves themselves bend toward it. In D. longifolia the leaf-blade enfolds the fly. Sun-dew glands respond by bending to repeated touches, although no object rests upon them. It is only nitrogenous food which is obtained by this digestive process; carbonic acid is assimilated from the air as by other plants. Consult Darwin, C., 'Insectivorous Plants' (1875; new ed., 1900).

SUN-DIAL, an hour-measuring instrument known from the earliest times to the Egyptians, the Chaldæans and the Hebrews. It is worthy of remark, however, that no ancient Egyptian sun-dials have been found. Those connected with Egyptian remains have been recognized as all of Greek origin. The Greeks adopted it from their Eastern neighbors, and it was introduced into Rome during the First Punic War. One of the earliest types of sundial found in Egypt, and still in use there, consists of a palm rod set upright in the ground, with a circular arc around it set out with stones to mark the hours as the shadow of the rod traverses the circle. Another more primitive form still in existence in Egypt has a rod laid horizontally in a north-and-south direction on two forked uprights, a short distance above the ground. At equal distances east and west of the rod are placed two stones or pegs. When the shadow of the rod lies across the westerly peg the day's work begins; when it reaches the easterly peg the day's work is ended.

It was discovered in very early times that a vertical rod could not throw a shadow that would accurately denote time, and the correct inclination of such a rod or the stile of a sun-dial was evidently a matter of experiment and approximation before the ancient_astronomers fixed the angle by calculation. This surmise is borne out by the various inclinations found in ancient dials. Some of these were constructed arbitrarily at 45°, an angle having no relation whatever to the latitude of its location.

The first historic sun-dial dates from about 1000 B.C. It was found in Rhodesia, and is believed to be of Semitic origin. Sun-dials are referred to in Grecian literature in 560 B.C., and a certain sun-dial is specifically spoken of as having been set up at Athens by the astronomer Meton in 433 B.C. It is said of the Turks that wherever they build a mosque they place a sun-dial. In China they are everywhere, and small ones which may be carried in the pocket are very common. The correct use of these portable dials depends, of course, upon their accurate orientation when reading them.

Sun-dials have been classified under three headings, according to their superficial form: spherical, cylindrical and plane. The spherical form is the most ancient. It consists of a hemispherical hollow cut into a rock or built up in that form, the flat of the hemisphere being horizontal. An upright rod was set in

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