APPENDIX V.

REPORT ON THE ASTROPHYSICAL OBSERVATORY.

SIR: I have the honor to present the following report on the operations of the Smithsonian Astrophysical Observatory for the year ending June 30, 1910:

EQUIPMENT.

The equipment of the observatory is as follows:

(a) At Washington, in an inclosure of about 16,000 square feet, are contained five small frame buildings used for observing and computing purposes, three movable frame shelters covering several out-of-door pieces of apparatus, and also one small brick building containing a storage battery and electrical distribution apparatus.

(b) At Mount Wilson, California, upon a leased plot of ground 100 feet square in horizontal projection, are located a one-story cement observing structure, designed especially for solar constant measurements, and also a little frame cottage, 21 feet by 25 feet, built and furnished last September for observer's quarters. It is highly satisfactory to note from the decrease in probable error of the observations secured in 1909 on Mount Wilson, compared with those of previous years, that the new cement observatory there, located as it is far from the dust, smoke, and disturbances of the other parts of the mountain, is excellently adapted for securing the most exact results.

WORK OF THE YEAR.

The present year's results are of uncommon interest, for they appear to fix within narrow limits the value of the solar constant of radiation. When in 1902 the first attempts were made here to measure it, that first-rank constant of nature, the intensity of the solar radiation at the earth's mean distance from the sun, was unknown within the wide range between 1.75 and 4 calories per square centimeter per minute. This range of values is given, with a preference for Langley's value (3 calories), by Hann in his standard work on meteorology, published in 1905.

It is improbable that this observatory would have continued since 1902 in solar-constant work had it not been that the results of 1903 gave strong indications of considerable variability of the sun in short intervals and that later work also strongly supported this presumption. The late director, Secretary Langley, shared, with many others of the most competent judges on the subject, the impression that to determine the solar constant of radiation with any considerable degree of accuracy or certainty was, if not impossible, yet a thing which would probably be long deferred and would involve spectro-bolometric measurements at the highest possible altitudes at which men may exist. He did not at all believe that our results of 1903 approximated to the true value of the solar constant, but only that they might be so far independent of ordinary atmospheric changes as to be used in determining the probability of solar variability. Hence, in 1905, he instructed the present writer to bear in mind, in going to Mount Wilson for the first time, that it was not the solar constant but

the possibility of solar variability which was the result to be determined by the expedition. This inquiry has, indeed, been the primary one in all the subsequent work, but not to the exclusion of attempts to fix the value of the solar constant itself.

There were at that time two principal and seemingly formidable difficulties hindering the determination of the solar constant of radiation. First, there was no instrument capable of absorbing fully and adapted for measuring completely the energy received at the earth's surface, excepting, perhaps, the littleknown and rarely used instrument invented by W. A. Michelson, of Russia, about 1894. Second, there was grave doubt if a true estimate of the loss of radiation in traversing the air could be made. Langley has somewhere described the first obstacle as "formidable," the second as “perhaps insurmountable.”

As stated in previous reports, much attention was given from 1903 onward to devising a standard pyrheliometer, and thus establishing the absolute scale of radiation measurements. A considerable degree of success seemed to be attained in 1906, but the results obtained in that year were found, by comparison with instruments of the United States Weather Bureau, to differ so much from the generally adopted scale of Ångström that further work, involving finally the construction of two additional water-flow pyrheliometers, was done. The last of these instruments, and by far the most perfect of them all, was completed and tried at Mount Wilson in October, 1909. A fairly close agreement seemed to hold between it and its immediate predecessor, but when the electrical constants of both instruments were determined with extreme care in February, at Washington, by Mr. Aldrich, the gap widened. A source of error, till then little regarded, was reconsidered, and painstaking comparisons of pyrheliometers were carried through at Washington by Messrs. Aldrich, Abbot, and Fowle. These were finished in June, 1910, and the two standard pyrheliometers were found to agree together well within the probable error of the highly accurate experiments. Not only so, but each instrument was found to take up and measure between 99 and 100 per cent of such various quantities of electrically introduced heat as were used as tests. Finally these definite measurements indicated that while the results published at page 46 in volume 2 of the Annals, made with standard pyrheliometer No. 1, are 4 or 5 per cent above the true scale, yet when all the experiments made with that instrument, at Washington as well as Mount Wilson, are collected their mean result is almost in exact agreement with the results obtained in 1910 with standard pyrheliometers Nos. 2 and 3.

It may now be accepted that the absolute scale of radiation is established within three parts in 1,000, and that we may express all our measurements of solar radiation made since 1902 with this degree of accuracy in absolute calories per square centimeter per minute.

Three secondary pyrheliometers, the cost of whose construction after my designs has been defrayed from the Hodgkins Fund, have been standardized and sent to Russia, France, and Italy. Two others have been sold by the Institution to the United States Agricultural Department. Thus steps are being taken to diffuse the standard scale of pyrheliometry. The new scale is about 5.2 per cent above that of new Ångström pyrheliometers.

The second obstacle mentioned above seems now less serious than the first. It was found in 1905 and 1906 that practically identical values of the solar constant resulted from good series of spectro-bolometric observations of the same day taken at Washington (sea level) and Mount Wilson (6,000 feet elevation). But in August, 1909, Mr. Abbot ascended Mount Whitney (14,500 feet) with a complete spectro-bolometric outfit, and, notwithstanding many days of

unpromising weather, succeeded on September 3, under the most perfect sky and in exceptionally dry air, in making a complete and satisfactory series of solar constant measurements. A prism of quartz and two mirrors of magnalium were the only optical parts to affect the rays, so that it was possible to observe from wave length 0.29 μ to wave length 3.0 μ. This extended region includes not only all the visible but the ultra-violet and infra-red spectra, with sufficient completeness to include in the discussion apparently within 1 per cent of all the rays which the sun sends the earth and to make the allowance for rays not observed practically sure. During the same day Mr. Ingersoll observed with the usual complete spectro-bolometric outfit on Mount Wilson, and his results were in accord with what would be expected from his preceding and following day's work there and agreed within 1 per cent with those obtained simultaneously on Mount Whitney.

In view of the agreement of results on the solar constant of radiation obtained at sea level, 1 mile, and 2 miles elevation, it now seems highly probable that we can really by Langley's method of homogeneous rays allow for losses in the air and get the same values that we would observe directly if we could take our instruments above the air altogether.

The reduction of spectro-bolographic work to the absolute scale of pyrheliometry enables us to give as the average value of the solar constant of radiation for the epoch 1905 to 1909, 1.924 calories per square centimeter per minute. It is probable that observations at sun-spot minimum will tend to raise this value by rather more than 1 per cent, so that we may suppose the mean value of the solar constant for a complete sun-spot cycle will be about 1.95 calories.

Experiments made in 1909 at Mount Wilson with various optical systems agree within their probable error with one another, and with the results obtained on Mount Whitney in fixing the distribution of energy in the spectrum of the sun outside the atmosphere. In the Mount Whitney work the curve of energy distribution was followed to a wave-length estimated (not very accurately) as 0.29 and it there practically reached zero intensity, although the quartz and magnalium apparatus would have been capable of transmitting the rays, had they existed, of much shorter wave-lengths. In the spectrum of the "perfect radiator," corresponding to the apparent temperature of the sun, the intensity of the ultra-violet rays would be of some importance for a considerably farther stretch of wave-lengths beyond this. It therefore appears that either the earth's atmosphere, even above Mount Whitney, or else the sun's envelope, effectually hinders the solar rays. If it is the former, then it may be that the above-mentioned value of the solar constant should still be raised a few per cent. But the known powerful selective absorption of vapors in the sun's envelope seems quite reasonably competent alone to produce the observed weakness of the solar spectrum in the ultra-violet. This view is confirmed by experiments of Miethe and Lehmann, who found no extension of the solar spectrum with increasing elevation, although they shifted their observing station from Berlin (50 meters) to Monte Rosa (3,500 meters), thus greatly diminishing the layer of air traversed. Their shortest wave-length was 0.2911μ, closely agreeing with ours. From our experiments of 1909 the apparent average solar temperature is 6430°, 5840°, or 6200 ̊of the absolute, according as we follow Wien's displacement law, Stefan's law, or Planck's law as the method of computation. But the temperature of the sun, apart from the uncertainty of terms when dealing with such high values, is probably a quantity which has very various values, from the center to the limb of the sun's disk, depending on the depth within the sun at which the radiation originates.

At Washington Messrs. Fowle and Aldrich have continued experiments on the transmission by moist columns of air for long-wave radiation, though with

many interruptions due to the difficulty of the research. The work has been carried to wave-length beyond 15μ in the infra-red, and for columns of air 800 feet long. It is not yet possible to summarize the results.

Messrs. Fowle and Aldrich and Miss Graves have made rapid progress with the reduction of solar-constant work of 1909.

Experiments have been begun for the purpose of devising economical means of utilizing solar energy for domestic purposes.

PERSONNEL.

Dr. L. R. Ingersoll served as temporary bolometric assistant on Mount Wilson to September 6, 1909.

Mr. L. B. Aldrich was given a temporary appointment as bolometric assistant at Washington beginning September 1, 1909. He passed a competitive examination and was reappointed provisionally on January 10, 1910. His appointment was made permanent, to begin July 1, 1910.

SUMMARY.

The work of the year is notable for the determination of the absolute scale of pyrheliometry and for the success of spectrobolometric observations of the solar constant of radiation on Mount Whitney. These agree with simultaneous observations of the same kind on Mount Wilson. Reducing these and other results to the absolute scale of pyrheliometry, we may fix the average value of the solar constant of radiation at 1.925 calories per square centimeter per minute for the epoch 1905-1909. Making allowance for the higher values which must prevail at sun-spot minimum, the solar constant may be estimated at 1.95 calories as an average value for a sun-spot cycle. No reason has been found for departing from the view heretofore held that short-interval variations of 5 per cent or more from this value occur. The energy distribution in the solar spectrum outside the atmosphere has been determined with the bolometer on Mount Whitney between wave lengths 0.29μ in the ultra violet and 3.0μ in the infra red. This region appears to contain full 99 per cent of all the solar energy outside the atmosphere. The apparent temperature of the sun as computed by three different methods comes out 6430°, 5840°, and 6200° of the absolute scale. Researches on the transmission of moist columns of air for long-wave rays, such as the earth emits, have been continued to wave lengths beyond 15μ, and for columns of air 800 feet in length: Secondary pyrheliometers, standardized to the absolute scale, have been sent to Russia, France, and Italy, and also furnished to the United States Weather Bureau and Department of Agriculture.

Respectfully submitted.

Dr. CHARLES D. WALCOTT,

C. G. ABBOT, Director.

Secretary of the Smithsonian Institution.

APPENDIX VI.

REPORT ON THE LIBRARY.

SIR: I have the honor to present the following report on the operations of the library of the Smithsonian Institution for the fiscal year ending June 30, 1910: The accessions recorded for the Smithsonian deposit, Library of Congress, numbered 2,653 volumes, 2,879 parts of volumes, 1,396 pamphlets, and 623 charts, making a total of 7,551 publications. The accession numbers run from 495,195 to 500,000. These publications were forwarded to the Library of Congress immediately upon their receipt and entry. In their transmission 270 boxes were required, containing approximately the equivalent of 10,800 volumes. The actual number of pieces sent, including parts of periodicals, pamphlets, and volumes, numbered 36,526. This statement does not, however, include about 2,948 parts of serial publications secured in exchange to complete sets and transmitted separately.

The Institution has continued the policy of sending public documents presented to it to the Library of Congress without stamping or entering. The number of publications given above does not include these, nor does it include other publications for the Library of Congress received through the International Exchange Service.

The libraries of the Smithsonian office, of the Astrophysical Observatory, and the National Zoological Park have received 473 volumes and pamphlets and 253 parts of volumes and charts, making a total of 626 publications, and a grand total, including the publications for the Smithsonian deposit, of 8,177. The actual decrease in the number of publications entered for the Smithsonian library is not as great as would at first appear, owing to the fact that in the present report a statement has been made of the number of completed volumes accessioned, rather than, as was formerly the custom, of the number of parts constituting a volume. Special attention has been given to the checking up and completing of the Smithsonian deposit sets of publications of scientific societies and learned institutions of the world, together with the series of scientific periodicals contained in the library.

The parts of serial publications entered on the card catalogue numbered 26,772, and 1,605 slips for completed volumes were made; 277 cards for new periodicals and annuals, together with 418 donor cards and 1,114 catalogue cards for separate publications were made and filed.

Inaugural dissertations and academic publications were received and accessioned from universities at the following places:

The establishing of new exchanges and the securing of missing parts to complete sets of publications in the Smithsonian library required the writing of