Imagini ale paginilor
PDF
ePub

proved advisable to apply the period- | strong in the spectra of the coolest luminosity law, developed by Shapley stars. (see above), to maximum brightness. The small dispersion of the results showed that this procedure was justified. The distances of both nebulæ are found to be 285,000 parsecs, or 930,000 light years! Variables have been found in several other spirals, but not enough plates have yet been collected for their study. The spiral nebulæ, or at least some of them, then, are vast systems far outside the galactic system. Whether they are all very much alike, and those that appear very small are many times as far away as the Andromeda Nebula, we still have no way of knowing. The diverse appearance of the larger ones indicates that they do differ markedly in size and density.

The atom, if sufficiently excited, loses one electron, becoming an ionized atom characterized by new en ergy states and capable of absorbing or emitting entirely different sets of lines. Such lines have long been known as enhanced lines. They are produced at the high temperatures of the spark, some appearing with less excitation, others requiring more. When more than one electron has been removed, the lines are usually far in the ultra-violet and entirely inaccessible in stellar spectra, but certain elements, such as Silicon, give lines, in several successive stages of ionization, which lie in the visible spectrum.

In the atmospheres of stars, ionization is favored both by high tem perature, tending to make the elec trons fly off, and by low pressure, diminishing the opportunity for an ionized atom to pick up an electron.

Atomic Theory and Stellar Constitution. The recent developments in atomic theory have proved of enormous importance to astronomy, in the interpretation of solar and stellar spectra. It now appears that each In the sun's atmosphere, at temspectral line is to be attributed to peratures of 5,000° to 6,000°, the emission (or absorption) taking place elements of easy ionization are alin connection with the transition of most completely ionized. In the exan atom (or molecule) from one defi-treme cases of Lithium and Rhubinite, specifiable state to another, and dium the lines do not show at all in that the number of light-waves per the ordinary solar spectrum, but they second is exactly proportional to the change in energy. A great deal of work has been done on the spectra of many elements, such that the changes of state corresponding to nearly all important lines in stellar spectra are now understood.

do appear in the spot spectrum, where the lower temperature permits some neutral atoms to exist. In general, the arc lines (neutral atom) tend to be strengthened in spot spectra, while the enhanced lines (ionized atom) tend to be weakened. Almost every detail of the difference between the spectrum of sun-spots and that of the rest of the sun can now be explained.

In the atmospheres of the relatively cool stars, the atoms are nearly all at the lowest energy level and are capable of absorbing certain lines, but In the upper atmosphere (the not all. At higher temperatures a chromosphere) of the sun, the pres larger number of atoms are in higher sure is much less than it is in the states, and, in their transitions, are lower atmosphere. There are, therecapable of absorbing a different set fore, fewer neutral and more ionized of lines. The latter are conspicuous atoms there: the enhanced lines are at the temperature of the electric arc, relatively strong in spite of the lower while the former are prominent in temperature. Thus simply is exspectra produced in the electric fur-plained a phenomenon which has been nace at low temperature. Their very puzzling for generations. This characteristic strengthening in sun- explanation receives additional supspots, which was attributed years port from recent studies of the pres ago to the lower temperature of sun-sure in the sun's atmosphere and its spots, now finds complete physical ex- opacity. Where the gas of the sun's planation. The low temperature lines atmosphere has a density as great as of the unexcited atoms are also one one-thousandth that of ordinary

air, various processes render it slight-in stellar atmospheres. Miss Payne ly opaque or hazy and there the sun, finds that the pressures range from although purely gaseous, appears to one ten-thousandth of an atmosphere have a surface (called the photo-in the lower layers to about one-milsphere), revealing definite markings lionth of an atmosphere in the upper corresponding to the hotter and cooler layers. The stars are pretty sharply regions. Stewart and Russell esti- divided into "giants," of low density, mate that a pressure of one one-hun- enormous size and great luminosity, dredth of an atmosphere is sufficient and "dwarfs," which are relatively to produce this effect. Milne, from more dense, smaller and not so bright. additional evidence, makes it one ten- The giant stars, with their comparathousandth. According to the latter tively low density and small force of figure, the whole quantity of gas in gravity at the surface, are believed the sun's atmosphere above the photo-to have much less dense atmospheres sphere appears to be no greater than than the dwarfs possess. This difthat in a layer of air only a few ference may readily account for the inches thick. Laboratory evidence, greater strength, in the giants, of also, is to the effect that this quan- certain enhanced lines and, in the tity of material is abundantly suffi- dwarfs, of certain lines of neutral cient to absorb spectral lines. Since atoms, and thus give physical meanthe sun's atmosphere (the chromo-ing to the method devised by Adams sphere) is several hundred miles in some years ago to determine, from depth, its density must be excessively the relative strength of certain lines small. There is good evidence that in their spectra, the absolute brightradiation pressure nearly neutralizes ness of stars and hence, by comparisolar gravity in the chromosphere son with their apparent brightness, and permits of this considerable the distance. depth.

We have also advanced far toward Spectra of Stars.-The same prin- an understanding of the conditions ciples applied to the spectra of stars which prevail in the deep interior of and with equally suggestive results. a star. Here ionization must be Take, for example, the spectrum of much more nearly complete, many of Silicon, the atom of which in its the atoms being stripped down to bare neutral state, has four electrons nuclei, which, together with the free outside the "complete shell"; the electrons, form a mixture of so low lines due to neutral Silicon appear a mean molecular weight, that, in in the spectra of the sun and spite of the great pressure, it behaves of the stars which are known to as a gas. Equilibrium in the stars have a low temperature; those due is thought to be of the radiation type to Si+ (Silicon atoms which have the change in temperature from lost one electron) are observed in stars like Sirius; still hotter stars show the lines of atoms which have lost two electrons; and in the very hottest stars is found evidence of Silicon atoms which have lost three. In general, the hotter stars show the lines of ionized atoms. The lines of metals appear at lower temperatures Eddington and Jeans, while disthan do those of the permanent gases, agreeing on certain technical matters, such as Oxygen and Nitrogen, which agree in concluding that the brightare harder to ionize. As the tem-ness should depend mainly on the peratures can be found from the total mass and relatively little on color of the stars and from the dis-the surface temperature of the startribution of their radiation, it is pos- a conclusion which is in excellent sible, since the degree of ionization agreement with observation. The also depends upon pressure, to use theory predicts the relative brightthe observational data concerning line ness of stars with satisfactory acintensities to determine the pressure curacy, but gives too high an abso

point to point is determined from the rate at which heat flows out to the surface. It has now become possible to calculate the opacity (roughly, the resistance to the flow of heat) of the material, and so to develop a theory of the internal condition of the stars.

lute brightness. This disagreement |ture of about thirty million degrees between theory and fact may be re- processes come into play which start moved by further work. According the gradual transformation into heat to Eddington, the density at the cen- of the main mass of the star. Durter is about twenty times the meaning this part of its evolution the star density throughout the star, and the maintains a nearly constant central central temperature ranges from temperature, steadily decreases in about one million degrees centigrade mass, increases in density and opaquefor the huge red stars, such as An-ness and passes down the main setares, to about thirty million for a quence represented by Sirius, the Sun large proportion of the stars, of very and the small faint red stars. Finaldifferent characteristics, such as the ly, the white dwarfs, such as the com Orion stars, Sirius, the Sun and the panion of Sirius, may be brought into faint red stars typified by 61 Cygni. the orderly sequence of evolution by The white dwarfs, such as the com- the assumption that, after the transpanion of Sirius (see later) prob- formation of the main mass, there ably have central temperatures of remains only a highly refractory mamore than one hundred million de- terial which is only transformable at grees. still higher temperatures. If, after Evolution of the Stars.-These re- the transformation of the main mass, sults cause modification of the older the star is still of considerable size, theory (Year Book, 1914, p. 590) it continues to contract to greater of stellar evolution; in particular density, grows hotter and brighter they necessitate the abandonment of until this final transformation sets the assumption of a fall in the in- in. In such discussions the use of ternal temperature in the denser dwarf stars in consequence of the close-packing of the atoms. There is now reason to ascribe the low surface temperature of these stars, not to a low central temperature, but to the greater opacity with greater density, which prevents the heat from leaking out to the surface. There is good geological evidence that the radiation of the sun has changed very little in the past billion years. The life of a star must be many times as long. The secret of the maintenance of radiation throughout this great The following list includes books length of time appears to lie in the that were published previous to the equivalence of matter and energy. present year, but which are still the Present investigators are agreed that most recent in their fields. The most a consistent theory of stellar evolu- recent material of all is to be found, tion can only be built up by assuming of course, in the journals: such as that a very considerable part of the the Astrophysical Journal; the Pub material of a star is gradually trans-lications of the Astron. Soc. of the formed into energy and radiated away Pacific; Monthly Notices of the Royal into space. Astron. Soc., and Astronomische Nachrichten.

Russell finds it necessary to conclude that rate of transformation of matter depends upon the temperature. He suggests that at some critical temperature, in the neighborhood of a million degrees, the degradation of some form of matter begins, thus supplying the radiation of the giant stars of large diameter and low central temperature; that at a tempera

familiar measures of time involves numbers which are inconveniently large. Smart suggests the use of the "Cosmogonic Unit" of 7.7 x 1013 years, which he finds to be the interval required for a star to pass from an infinitely large mass to the pres ent-day mass of the sun. The life history of a star probably extends over a large number of these units.

SELECT REFERENCES ON
ASTRONOMY

JONES, H. Spencer, General Astron omy. (N. Y., Longmans, Green, 1922.) The most recent book cov ering the general field of astronomy. A satisfactory textbook for the college student and a book that, in its exclusion of mathematics, is pleasant reading for the amateur.

PHILLIPS, Rev. T. E. R., and STEAVENSON, Dr. W. H., assisted by others, The Splendour of the Heavens. (2 Vols., London, Hutchinson, 1923.) These two volumes were brought out in twenty-four fortnightly parts written by various authorities. A popular book of descriptive astronomy. Over 1,000 illustrations, averaging more than a picture to a page. Of particular value to those who give popular lectures.

LEWIS, Isabel M., A Hand-Book of Solar Eclipses. (N. Y., Duffield, 1924.) A book of 118 pages dealing with the phenomena of total eclipses of the sun and evidently timed to meet the popular interest excited by the eclipse of January 24, 1925.

DINGLE, Herbert, Modern Astrophysics. (London, Collins Sons, 1924.) -An up-to-date account of modern astrophysical discoveries and theories. In the preface the author says: "While the popular appeal has been given first place, difficulties have not been evaded. It has been assumed here that the reader is prepared to think. No specialized knowledge is required." MITCHELL, S. A., Eclipses of the Sun. (2d ed., N. Y., Columbia University Press, 1924.)-An account of the observation of eclipses; what they have contributed to our knowledge of the constitution of the sun; the eclipse test of relativity; and what remains to be done in the future. It is very well illustrated, and many of the experiences of the author on eclipse expeditions in which he took part are interestingly recounted.

OLIVIER, Charles P., Meteors. (Baltimore, Williams and Wilkins Co., 1925.) A general work on meteoric |

astronomy written by the leading authority on meteors in this country. Detailed methods of observation and computation are given and the results discussed. The literature of the subject is thoroughly reviewed. The author's own opinions are frankly stated.

RIGGE, William F., The Graphic Con-
struction of Eclipses and Occulta-
tions. (Chicago, Loyola University
Press, 1924.)-The author has long
been interested in the representa-
tion, by graphs, of the circum-
stances of eclipses. Many of his
diagrams have appeared in Popular
Astronomy. There are also chap-
ters on graphic methods of deter-
mining the times of rising of the
sun, moon and stars, and hints on
drawing and computing.

PAYNE, Čecilia H., Stellar Atmos-
pheres. (Cambridge, Harvard Col-
lege Observatory, 1925.)--The first
of a series of research monographs
to be issued from the Harvard Ob-
servatory. An analysis of stellar
atmospheres by means of modern
theories of atomic structure and
ionization. Necessarily technical.
JEFFREYS, Harold, The Earth: Its
Origin, History, and Physical Con-
stitution. (Cambridge University
Press, 1924.)-The content of this
scholarly work is perhaps sufficient-
ly indicated by the title. One re-
viewer says:
"The Earth is em-
phatically not a book to be read
through from cover to cover as one
would read an essay. Nor is this,
even principally, due to the mathe-
matics which enters into it, for the
text is full of close reasoning. We
take it to be a compliment to a
book to say that the digestion of
it should only be undertaken in
doses, and these when the mental
state is pretty vigorous."

DIVISION XXIX
ENGINEERING AND CONSTRUCTION

CIVIL ENGINEERING

BY JOHN M. GOODELL

CIVIL ENGINEER, NEW YORK

reached from a consideration of the experience with existing bridges was that an increase in working unit stresses is justified, provided the steel is of standard quality, the designing is properly done and the workmanship is good. This conclusion has, naturally, drawn attention to the determination of the actual strength of old bridges, and in December the public discussion of the subject was started with a highly technical paper presented to the American Society of Civil Engineers.

Steel Bridges. The outstanding unit stresses in steel for buildings feature of the steel-bridge field in became so important that the Ameri1925 has been the highly technical can Society of Civil Engineers ap matter of the loads which it is safe pointed a committee to investigate it. to place on existing bridges. Almost The committee reported in January, as interesting although not so im- 1925, and this report is still, at the portant from an engineering view-close of the year, a subject of anipoint has been the unexpected revival mated discussion. So far as it reof the determination of the better lates to bridges, the conclusion public policy, tolls or bonds, in financing very expensive bridge projects, a subject which was believed to have been settled in favor of bonds, until very recently, for all public bridges. Stresses on Railway Bridges.-The safe loading of railway bridges seems to be easily settled. They have all been designed by experts using approved unit stresses for their tension members, columns and other parts. Nominally, their safe load is that for which they were designed. Railway engineers are human, however, with a strong desire to have a substantial anchor to windward in matters where trouble may arise. They knew by experience that getting appropriations for heavier bridges from their directors is far from easy. They have also known for a long time that the standard unit stresses for structural steel, adopted when the material was less uniform and reliable than it is to day, were very low, but they have said nothing. These low unit stresses used in their designs were their safeguard when operating officials insisted on loading bridges beyond their nominal capacity and directors could not see how to spare money for stronger bridges. In 1922, however, the subject of a private

Toll Bridges.-Turning to public toll bridges, the other major subject in the field of steel bridges, the public seems now to be where it was about forty years ago, floundering in uncertainty. Then, as now, there was a feeling that the only way in which money could be raised for very costly bridges was to meet at least a part of their cost by tolls, much as such charges were disliked. During later years tolls became so unpopular for public bridges that they were repealed quite generally. Since the World War, with all the demands for public works, tolls have again become fairly common. The suspension bridge across the Hudson River was built by company and tolls are

« ÎnapoiContinuă »