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or fusiform show the spiral form on plates taken with special care; second, all exposures sufficiently long to photograph one of these objects lead to the discovery of many other similar objects. The number of spirals is much greater than had been supposed, and they may include the majority of the nebulæ. These results were obtained on Mount Hamilton, Cal., where a rich American, James Lick, has founded the observatory which bears his name. No astronomer can visit this model observatory without envy and admiration.

The order in which we take up the objects in the rich collection of Keeler may evidently be open to criticism until an accord is established upon a definite theory. No one, surely, would suppose that the nebulæ have always existed just as they are or that they have acquired a final shape. We must look upon them as still in the process of change. The question we will for the moment consider is whether they are in the process of condensation or expansion; whether the spirals are flowing out from or into the center.

Before forming a too hasty decision, let us first examine the larger, more massive nebula, where the spirals are small and unimportant in comparison with the central nucleus. Afterwards we will consider the more dilated ones, where the greater part of the matter seems dispersed into the spirals. Then we will consider in which direction it is easier to suppose the transition.

In each class we shall place in the first rank those which have the most nearly circular appearance; that is, those whose plane is normal to our line of sight and which will enable us to interpret better the other nebulæ seen at less favorable angles or even edgewise.

Having completed that task, we needs must ask of what are the spiral nebulæ formed; in what way are they changing? Could we answer these two questions, then we would ask two others still more ambitious. How were the spiral nebulæ formed, and what will be their end? But such questions may for a long while yet be premature, and I believe I thus voice the opinion of our master, Poincaré, if I rightly interpret the conclusions stated in his recent book on cosmic hypotheses.

It seems to me that the elements of the spiral nebulæ can be nought else than collections of groups of stars, whence comes the abundance of the luminous points scattered in the outer portions of the spirals where they can be separately seen. A cosmic cloud formed of subtler elements could never show such sharp outlines, nor reveal such clear-cut divisions. The continuous spectra must lead us to suppose that even in the central portions stars predominate, enveloped, if you will, in a common atmosphere which diffuses their light.

Some might argue that if the spirals are formed of stars they would be brighter. I do not see that necessity. The distance of the

spirals is immense, much greater than that of the mean distance of the naked-eye stars, because all the visible stars inclosed in the spirals are telescopic. The light of such stars reaches us weakened by their enormous distances and doubtless by an intersteller absorbing medium.

When we consider the great number of the stars embedded in a nebula like that of Ursa Major (M101), or that of Andromeda, it seems as if we rather minimize their importance either in using these nebulæ to construct a solar system or by regarding them as the result of some very improbable accidental collision. A single nebula is, in my opinion, capable of giving birth to many stars, indeed, to many clusters. By the range in their development, the variety of their structure, the great spirals are comparable without exaggerations to the milky way itself.

I believe that we should not derive from our latest studies the theories of Chamberlain and Moulton or of Prof. Arrhenius, all three of whom interpret the spirals as due to a collision of two stars. Mr. T. J. J. See has raised very strong objections against such theories in his recent work, "The Evolution of Stellar Systems," a book full of erudition and ingenious views, but one whose uncompromising dogmatism must arouse opposition. According to Mr. See, we must not present an explanation to our learned public as possible, but as absolutely necessary. Is the stellar cosmogony of Mr. See, for he has one of his own, truly one of those to which we must subscribe without discussion and hold as definitive? He makes a spiral have its birth in the meeting of two clouds of very elongated form which move through space with different velocities and become deformed before uniting under the influence of their mutual attraction. Each spiral marks the influx toward the common center of one of the original clouds.

I fear that such an explanation would be satisfactory only to readers but little acquainted with the objects themselves. It is not merely two concurrent spirals which we must explain, but often four or five. And when we consider the parsimonious scattering of matter through space, it is truly difficult to admit that upon the path of the deflecting current there will appear first isolated stars, then clusters of stars more and more numerous and more and more dense as we approach the place of conjunction. I do not see whence will be gathered the matter for these suns if there is no central condensation, which, according to See, would not yet have been formed. To me the movement in spirals must be centrifugal and dispersive. The central mass shoots out intermittently groups of stars, giving them a great initial velocity, but the impulsive force acts only for a short distance. The final movement of the liberated stars is governed by the

general attraction, except in the neighborhood of certain points of the spirals which have in turn become centers of disturbances.

The spirals, essentially irregular in their sections and projections, are neither currents nor trajectories. The axis of each one is a synchronous curve of the places which at any given instant are occupied by the products of a prolonged and intermittent eruption. The latter are continually evolved in the same central mass which slowly turns upon itself. The spirals therefore tend to become, with increasing distances, normal to the radius. The general motions of the matter in this class of nebulæ thus conform to the stellar currents of our own milky way if we adopt the views expressed by Schwarzschild.

THE RADIATION OF THE SUN.

By C. G. ABBOT,1

Director of the Astrophysical Observatory of the Smithsonian Institution.

[With 4 plates.]

The sun presents many interesting aspects. Although controller of the solar system, an object rich with beautiful and curious features, the nearest of the fixed stars, and typical of a large class among them, the sun also has a still greater claim on human interest as the fountain of heat, light, and life upon the earth. It is this latter aspect which we shall consider mainly, still further confining our attention almost wholly to work done under the auspices of the Smithsonian Institution.

When James Smithson died in Genoa in 1829 he left his estate, subject to certain conditions, "to the United States of America, to found at Washington, under the name of the Smithsonian Institution, an Establishment for the increase & diffusion of knowledge among men." On May 9, 1838, by decree of the English Court of Chancery, the Smithson bequest, amounting to about $500,000, was adjudged to the United States. By the act of establishment in 1846 the control of the Smithsonian Institution is vested by Congress in a Board of Regents, comprising the Vice President and the Chief Justice of the United States, three Senators, three Representatives, and six private citizens. In the years that have elapsed the Smithsonian private funds have increased by gifts and economy to nearly $1,000,000. For many years the institution has administered the annual congressional appropriations for the support of the National Museum, National Zoological Park, Bureau of American Ethnology, Astrophysical Observatory, Bureau of International Exchanges, and International Catalogue of Scientific Literature. The immediate administration is in the hands of the secretary of the Board of Regents, at present Dr. C. D. Walcott, the fourth of the secretaries. Dr. S. P. Langley, the third secretary, a distinguished American astronomer, founded in 1890 the Astrophysical Observatory of the

1 Reprinted with revision and addition from Science Conspectus, Boston, vol. 2, No. 5, April, 1912. Illustrations in part from "The Sun," by permission of D. Appleton & Co.

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Smithsonian Institution, and was its director until his death in 1906. His own principal investigations, and those of the Astrophysical Observatory begun under his direction and still continued, have lain in the field of measuring the quantity and quality of the sun's radiation, the effect of the earth's atmosphere thereon, and the dependence of terrestrial temperatures and plant life on solar radiation. This is a utilitarian branch of astronomy, whose applications to terrestrial concerns may be expected to increase in future years and result in the promotion of the arts of meteorology and agriculture. But the interest of such studies for the promotion of pure knowledge is also very high. Let us imagine that the Greek philosophers, the Arabians, and the astronomers of Galileo's time, had all possessed the means to measure accurately the quantity and quality of solar radiation. How interesting it would be now to compare their measurements with our own, and determine thereby what, if any, appreciable changes have occurred in 2,500 years in that energy which supports heat and life upon the earth! The astronomer of the future will have, we hope, trustworthy measurements of our own time to compare with his own. Referring to another branch of the measurements which I am to bring before you, our knowledge of the approximate temperatures prevailing in the sun, and our conclusions as to the sun's nature rest on such work as is being done at the Smithsonian Astrophysical Observatory.

By the term solar radiation, I propose to your minds not only the solar rays which affect our eyes as light, but the extensions of the spectrum beyond the violet and beyond the red, where the eye is not sensitive. All these rays, whether visible or not, may be absorbed by blackened surfaces and will thus produce their just and proportional effects as heat. For the measurement of solar radiation, Langley, about 1880, invented the delicate electrical thermometer shown in plate 2, which he called the bolometer; figure 8 of plate 2 shows its most important part. This is a pair of tapes of platinum, each about 1 centimeter long, 0.01 centimeter broad, and 0.002 centimeter thick. These tapes are blackened with camphor smoke or by a deposit of platinum black. One is exposed in the path of the rays to be measured, and the other is hidden. Hence one tape is warmed with respect to the other. Thereby a minute electrical current is caused to flow through the delicate galvanometer connected with the Wheatstone's bridge, of which the tapes form two arms. In this way a change of temperature, which may be as small as one-millionth degree Centigrade, may be detected in ordinary practice. By special devices the sensitiveness may be increased beyond this one-hundred fold. But though so sensitive the bolometer is far behind the eye in its capacity to detect faint yellow light. It is used in preference to the eye because it can detect and

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