476 SUN has been that the sun is now giving off the heat imparted to it at its creation, and that it is gradually cooling down ; another ascribed it to combustion, and a third to currents of elec- tricity. Newton and Buffon conjectured that comets might be the aliment of the sun, and of late years a somewhat similar theory (first broached by Mr. Waterston in 1853) has been in vogue, viz., that a stream of meteoric matter constantly pouring into the sun from the re- gions of space supplies its heat, by the con- version into it of the arrested motion. As the sun may indeed derive a small amount of heat from this cause, it deserves more attention than previous conjectures. But conjecture and hy- pothesis may be said to have given place to views which claim a higher title, as it is now becoming generally recognized, in accordance with modern physical theories of heat, that in the gravitation of the sun's mass toward its centre, and in its consequent condensation, sufficient heat must be evolved to supply the present radiation, enormous as this undoubtedly is. It appears to be susceptible of full demon- stration that a contraction of the sun's volume of a given definite amount, which is yet so slight as to be invisible to the most powerful telescope, is competent to furnish a heat sup- ply equal to all that can have been emitted du- ring historical periods. According to this the- ory then (which is due largely to the develop- ment by Helmholtz of Mayer's great generali- zation), the sun's mass remains unaltered, and its temperature nearly constant, while its size is slowly diminishing as it contracts ; so slowly, however, that the supply may be reckoned on through periods almost infinite as measured by the known past of our race, and which are in any case to be counted by millions of years. It would appear from early measurements of Secchi that the different portions of the solar disk do not radiate heat in uniform degrees, and his tables show that the equatorial regions are slightly hotter than the polar. It has been explained that the rapid decrease of brightness toward the edge of the sun obliges us to admit the existence of a shallow atmosphere around it. Prof. Langley has recently published tables from more extended measurements, showing the rate of absorption both of heat and light, the latter being greater than the former. As he does not now find the difference between the equatorial and polar heat observed by Sec- chi in 1852, the latter concludes from a com- parison of his own observations with Lang- ley's, that great changes occur in the distribu- tion of the heat on the sun's surface. Prof. Langley has further shown that this atmos- phere absorbs one half of the sun's total ra- diation, and he considers that its function in the solar emission is of great importance to us. A slight alteration in the thickness of this obscuring envelope would induce changes on the earth greater than those known to have occurred in its 'climate in past geologic epochs, which may themselves not impossibly SUN BIRD have been due to this hitherto unrecognized cause. M. Fizeau has found that the chemical rays are similarly reduced in amount toward the edge of the solar disk, a fact which is also abundantly shown by the darkening near the edge of photographic sun pictures, like those by Rutherfurd and De la Rue. To sum up briefly the received hypotheses of the physical constitution of the sun : Of its internal struc- ture we know nothing, but we can infer from the low density of the solar globe as a whole that no considerable portion is solid or liquid. The regions we examine appear to consist of cloud layers at several levels floating in a com- plex atmosphere, in which probably most of the elements are known to us, and certainly many of them exist in the form of vapor. Out- side this complex atmosphere extend envelopes of simpler constitution, though into them oc- casionally arise the vapors which ordinarily lie lower down. The sierra, for instance, consists in the main of glowing hydrogen gas, and that gas, whatever it may be, which produces the line near the orange-yellow sodium lines. The prominence region may be regarded as simply the extension of the sierra. The inner corona is still simpler than the sierra so far as its gaseous constitution is concerned ; but here meteoric and cometic matter appears, extend- ing to the outer corona and to great distances beyond even the visible limits of the zodiacal. Returning to the photosphere, we find it sub- ject to continual fluctuations, both from local causes of agitation and from the subjacent vapor acting by its elasticity to burst through it ; the f aculae, which are found to be above the general level of the photosphere, are taken to be heapings up of the luminous matter like the crested surges of the sea. All the strata are subject to great movements, which sometimes have the character of uniform progression analogous to our trade winds, and sometimes are violent and resemble in their effects our tornadoes and whirlwinds. Eruptive action appears to operate from time to time with exceeding violence, but whether the enormous velocities of outrush are due to true explosive action (which would compel us to believe that the sun is enclosed by a liquid shell, so as to resemble a gigantic bubble), or to the uprising of lighter vapors from enormous depths, as heated currents rise in our own atmosphere, is not as yet certainly known. SUN BIRD, the name commonly given to the promeropidce, a family of tenuirostral birds, with a long, slender, and usually curved bill, the nostrils placed at the base and covered with a scale, wings of moderate size, and short tarsi covered with broad scales. They inhabit the tropical regions of both hemispheres ; the subfamily promeropince, including by far the most species, is confined to the old world, and the ccerebina to the new. The true sun birds belong to the former, and have a long, slender, curled, and sharp bill, sometimes finely serrated on the margins; the tail is long, the central
