tory was devoted to it. The establishment of a standard system of photographic magnitudes proved a difficult and intricate problem, but again the results are of primary importance, for the color of a star, which is best measured by the difference between its visual and photographic magnitudes, proves to be almost as important as its spectral type, to which it is very intimately related. Here again the principal work of observation was done by othersMiss Leavitt, Professor Bailey and Professor King-but the unifying guidance was Pickering's. Closely related to this is the discovery of variable stars, which, previously largely a matter of chance, was reduced to a system, whether by the comparison of plates of the same field taken at different times, or by means of certain spectral peculiarities. The new methods were so successful that the number of variable stars discovered at Harvard within a few years was three times as great as that of all those detected by all the astronomers of the world during the previous history of the science.

Finally, and by no means least, should be recorded his deep interest in, and support of, cooperation between the whole fraternity of astronomers, whether in this country or abroad. There was hardly an organization for the furtherance of any specific astronomical aim, such as the Committee on the "Carte du Ciel" or the Solar Union, in which he did not take an active part, and his counsel and advice were always of weight. But equally influential, though less conspicuous, was his ever generous aid to individual investigators, to whom he was continually transmitting invaluable material from the treasures under his charge, sometimes observations already made, but unpublished, and again data concerning stars which had been put upon his observing lists for that especial purpose.

His abiding willingness to use his powerful influence to aid other astronomers in obtaining instruments for the expansion of their researches, or funds to provide assistance in the reduction and publication of their observations, is known to all.

It may be pardonable to speak of one or two instances. In conversation, referring to the Metcalf Telescope, for which he had found the funds to purchase the glass disks for the lens, and provide the mounting, while the figuring of the lens was done, as a labor of love and in his spare time, by the distinguished amateur whose name it bears, "I felt as if a great artist had said to me 'If you will buy the canvas, and the brushes and paint, I will paint you a picture.

If a more personal allusion may be excused, it may be recorded that, shortly after the writer's first interview with Professor Pickering (during which he had described his first serious astronomical work, on stellar parallax) a letter arrived from Harvard, saying in substance "I think that it would be useful to determine the magnitudes and spectra of all your stars. If you will send me a list of them, we will have them observed, and send you the results." This involved the photometric and spectroscopic observation of some three hundred stars (the photometric settings being made by Professor Pickering himself) and was offered as an unsolicited contribution to the work of a young and unknown instructor! The Harvard Observatory never admitted graduate students of the ordinary sort; and doctoral theses are absent from the long list of its publications. But, under Pickering, it was an educational center of the first rank, and its pupils were not the immature students, but the working astronomers of the country. Who among us has not gone to Harvard, enjoyed the delightful hospitality and finished courtesy of the director, and returned, loaded down with

data for investigations new or old, and inspired by his experience with new enthusiasm alike for the magnificent researches of the great observatory, and for his own humbler work?

Such a career deserved unusual recognition, and received it in a merited degree. Almost all the honors of the scientific world fell to his lot, and the list of these distinctions is too long to detail here. But those who knew him will mourn less the disappearance of the distinguished leader of science than the loss of a warm and loyal friend, one of the kindliest and most generous of men.

HENRY NORRIS RUSSELL

PRINCETON UNIVERSITY OBSERVATORY,
February 6, 1919

SOME RECENT CONTRIBUTIONS TO THE PHYSICS OF THE AIR1 THERE has come to us from ancient times the story of a foolish man who sold his birthright for a mess of pottage, and that story to-day is right applicable to us physicists, except in one important particular-we haven't even got the pottage. No department of learning has a richer birthright than has the department of physics in meteorology-the physics of the air. And yet the few institutions that even profess to teach this subject in any form offer it through the department of geology, or, more frequently still, that omnivorous department which, for want of a better name, is called the department of geography. Statistical meteorology, if such expression will be permitted, or climatology, is of course of great interest alike to the geologist and the geographer and this they should teach and in great measure do teach, but climatology is no more meteorology than de

1 Address of the vice-president and chairman of Section B-Physics, American Association for the Advancement of Science, Baltimore, December,

1918.

scriptive geography, for instance, is geology. Its value is great and unquestioned, but its function, like the function of geography, is merely to describe and not to explain.

Meteorology, on the other hand, is concerned with causes, it is the physics of the air, a vast subject of rapidly growing importance upon which peace and war alike are becoming more and more dependent. Only yesterday we

Heard the heavens fill with shouting, and there rained a ghastly dew

From the nations' airy navies grappling in the central blue;

and to-day

Saw the heavens fill with commerce, argosies of magic sails,

Pilots of the purple twilight, dropping down with costly bales.

It is, therefore, no longer an opportunity, a shamefully neglected opportunity, that invites, but an imperative duty that commands our leading institutions to add to the various subjects taught, studied and investigated in their departments of physics that eminently valuable and fascinatingly difficult branch of geophysics-the physics of the air.

No doubt the great majority of colleges and universities would find it highly impracticable to add a proper course in meteorology to their present long list of electives. Neither is it practicable nor desirable for all of them to teach anthropology, say, despite its fascination, nor even any whatever of the a-to-z kinds of engineering. But it is insisted with all possible emphasis that if taught at all it be taught right-taught as a branch of physics. It is also insisted that there is a growing need, especially in connection with both the science and the art of aviation, for young men who understand the phenomena of the atmosphere. Nor should it be forgotten that when our army called for men trained in meteorological physics it called in vain-they did not exist. Furthermore, it would be a godsend to our national Weather Bureau if in the future it could secure a larger portion of its personnel from among university gradu

ates highly trained in the subjects with which they have to deal. And, finally, it is insisted that the physics of the air offers many opportunities to the creative scholar, and every university must realize that its paramount duty is the fostering of research and the training of investigators, for in no other way can it meet the growing and compelling demands of a progressive civilization.

It must be admitted, however, that it is not now easy to give a connected course on atmospheric physics, for there is no suitable text and the isolated articles upon which such a course must needs be based are scattered through the journals from Dan to Beersheeba and buried under a babel of tongues. But this is only a difficulty, and not, in the face of imperative needs, an excuse. A far greater and very real difficulty has, it is true, confronted most of us, for, until the last decade, or less, several important lectures in such a course would of necessity have been restricted to the same brevity as characterizes Horrebow's famous chapter on snakes in his "Natural History of Iceland"-there aren't any. Some of these lectures are still unwrittentantalizing challenges to the skill of the experimentalist and acumen of the analystwhile others have been at least partially supplied, a few of which it will be interesting to review in what follows.

TEMPERATURE OF THE FREE AIR

Although efforts to determine the temperature of the free air by means of thermometers carried by kites were made as early as 1749, the experiments being conducted at Glasgow by Alexander Wilson and his pupil Thomas Melville; and although, beginning with Jeffries in his ascent from London in 1784, balloonists have often carried thermometers on their flights, it was only after the development of self-recording instruments and the sounding balloon-both at the very end of the last century-that the vertical distribution of temperature up to even 7 or 8 kilometers became at all accurately known. As is now known, and as shown in Fig. 1, the average

temperature decreases slowly with elevation near the surface, then more and more rapidly to a maximum at some such considerable altitude as 7 to 9 kilometers, where it roughly approaches the adiabatic rate for dry air of approximately 1° C. per 103 meters.

These are the observed facts; but here too, as in the investigations of other physical phenomena, a knowledge of what happens is only so much raw material out of which some one happily may fashion the finished productwhy it happens. In this case the why is found in the presence of water vapor in the air, its condensation and the latent heat thus rendered sensible. As air is carried to higher levels by vertical convection it progressively expands against the continuously decreasing pressure, and thereby does work at the expense of its own heat. During the dry stage of this convection, that is, until saturation is attained, the cooling is roughly at the rate of 1° C. per 103 meters increase of elevation. Immediately condensation begins, however, latent heat is set free and the rate of cooling with elevation correspondingly decreased. But as the amount of vapor condensed per degree drop in temperature decreases with the temperature, it follows that the latent heat set free and the corresponding check in the rate of cooling with elevation also decreases. Hence the continuous temperature-elevation coordinates of a rising mass of saturated air form a curved line. Furthermore, the curve thus formed approximately coincides with the average temperature-altitude curve of the free air throughout all cloud levels, or from 0.5 kilometer, say, to 9 kilometers, or thereabouts, above sea level. This agreement necessarily occurs more or less closely during every rain and in all deep clouds and, therefore, very frequently. Nor can there often be much departure from it between such occasions for during these intervals the whole of this portion of the atmosphere is, as a rule, simultaneously warmed or cooled, and thus the curve in question usually shifted essentially parallel to itself.

It appears, then, that the average tempera

ture gradient (rate of decrease of temperature with elevation) of the free air is approximately that of a rising mass of saturated air; and for the reasons (a) that frequently the air is rising and saturated, and (b) that departures from the thus established saturation curve develop but slowly, as explained, and are soon eliminated by its reestablishment.

THE ISOTHERMAL STATE OF THE UPPER AIR

In April, 1898, Teisserenc de Bort began at Trappes, near Paris, a long series of frequent atmospheric soundings with small balloons carrying automatic registering apparatus.

Among other things, he soon obtained temperature records that indicated the existence either of surprising errors in his apparatus, or of wholly unsuspected conditions in the upper atmosprere. The records generally were tolerably satisfactory up to some 10 to 12 kilometers satisfactory, because through at least the upper half of this region they showed the temperature to decrease with elevation at, very roughly, the adiabatic rate for dry air. But somewhere in the neighborhood of 11 kilometers elevation everything seemed to go wrong, for from here on the records no

longer indicated a decrease of temperature with increase of elevation, but often even a slight increase! There were but two possible conclusions. Either the apparatus had developed, in actual use, faults that the cross questioning of the laboratory had failed to reveal, or else the upper atmosphere really was in a most unorthodox thermal state. However, numerous records obtained with sounding balloons at different places, by different people and with different apparatus all showed the same thing, namely, that the temperature of the upper atmosphere, though varying slightly from day to day, is, at any given time, substantially the same at all levels, as illustrated by Fig. 1.

Here, then, was a conflict between observational evidence and tradition. Actual measurements had declared the upper atmosphere to be essentially isothermal-declared it in the face of a tradition to the effect that the temperature of the atmosphere must steadily decrease to, or very nearly to, the absolute zero. The name of the joker who first perpetrated this scientific hoax may be lost to fame, but the worst of it is we physicists thoughtlessly perpetuated it. The qualification, thoughtlessly, is used advisedly, for it seems impossible than any process of reasoning could have led to such an erroneous conclusion. If the surface temperature of the earth is maintained, as we know it is, by the absorption of solar radiation, it is equally certain that in turn the temperatures of objects in the full flood of the necessarily equivalent terrestrial radiation can not drop to zero; nor, therefore, can the air, generally, cool by convection to a lower temperature than that which this radiation can maintain. These ideas, so simple that they seem hardly worth expressing, embody the fundamental explanation of why the upper atmosphere is essentially isothermal.

In addition to being exposed all the time to earth radiation the upper air is also exposed much of the time to solar radiation, but there is abundant evidence that the atmosphere at all levels is far more absorptive of

the relatively long wave-length terrestrial radiation than of the much shorter wavelength solar radiation. Hence in computing from á priori considerations the probable temperature of the isothermal region, or stratosphere, as it generally is called, it is sufficient, as a first approximation, to consider the effect of only the outgoing radiation, which, according to the work of Abbot and Fowle, of the Smithsonian Institution, is approximately equal in quantity and kind to that which would be emitted by a black surface coincident with the surface of the earth and at the temperature of 259° A. As a further simplification the surface in question may be regarded as horizontal and of infinite length and breadth in comparison to any elevation attained by sounding balloons, and, therefore, as giving radiation of equal intensity at all available altitudes.

Now consider two such surfaces, parallel and directly facing each other at a distance apart small in comparison to their width, and having the absolute temperature T2, and let an object of any kind whatever be placed at the center of the practically enclosed space. Obviously, according to the laws of radiation, the final temperature of the object in question will also be approximately T. If, now, one of the parallel planes should be removed the uncovered object would be in substantially the same situation, so far as exposure to radiation is concerned, as is the atmosphere of the isothermal region in its exposure to radiation from the lower atmosphere. Of course each particle of the upper air receives some radiation from the adjacent atmosphere, but this is small in comparison to that from lower levels and may, therefore, provisionally be neglected. Hence the problem, as an approximation, is to find the temperature to which an object, assumed infinitesimally small, to fit the case of a gas, will come when exposed to the radiation of a single black plane at a given temperature, and of infinite extent.

But whether an object lies between two planes of equal temperature, as above assumed, or, like the upper air, faces but one,