TABLE 1.—('oefficients of atmospheric transmission for radiation from zenith The mean results will be found to lie extremely near the corresponding ones for the year 1903-4, so that no further discussion of them seems necessary. In Table 2 are given the values computed for the solar constant of radiation outside our atmosphere, in continuation of the series published in former years. The method of computation and the assumptions on which it is based have been given in preceding reports and are substantially those employed by you many years ago at Allegheny. TABLE 2.-Values of the solar constant of radiation outside the earth's atmosphere from Washington observations. The rating of the character of the observation is made chiefly by inspection of the logarithmic plots used in determining the atmospheric transmission, similar to those published facing page 80 of your report for the year ending June 30, 1903. On an excellent day the observations thus plotted, like those just referred to, lie close to representative straight lines, but such days are unfortunately very rare in Washington, and especially within the last two years, owing in part to building and other operations producing smoky and nonuniform atmospheric conditions. The bearing of the results given in Table 2 on the question of the variability of the sun has already been given. Before leaving the subject of solar constant observations it may well be remarked that Mr. Fowle has published within the year a valuable paper giving a comparison between the solar constant values deduced by the method of homogeneous rays and those deduced for the same days by the old method of high and low sun observations of the total radiation of the sun by the actinometer alone. This old method, the method of Pouillet, gives necessarily too low results, as you long ago demonstrated. What is valuable in Mr. Fowle's paper is that he shows that the defect is almost constant at Washington, no matter what the time of the year, the transparency of the air, or the humidity, provided the extrapolation is made with moderate solar zenith distances. Thus if observations be made in Washington with the actinometer alone and extrapolated by the aid of a logarithmic plot to the limits of the atmosphere, and a correction of 14 per cent is then added, the result will be practically the same as if the spectrobolometric method had been used. If additional measurements should confirm this result (that 2 constant difference between the two methods holds here and at other localities), then the process of detecting variations in solar radiation outside our atmosphere would be much easier, for it could be made to depend on actinometer measures alone, and, indeed, old series of observations made years ago could be utilized. I give in illustration the following table, taken in part from Mr. Fowle's paper above cited, showing how closely solar constant values deduced from pyrheliometer measures alone, by the application of the 14 per cent correction, agree with values deduced when possible from spectrobolometrie work of the same days. TABLE 2a.-Solar constant values from pyrheliometry. a F. E. Fowle, Smithsonian Miscellaneous Collections, vol. 47, p. 399, 1905. S. P. Langley, American Journal of Science (3), XXVIII, p. 163, 1884. TABLE 2a.-Solar constant values from pyrheliometry—Continued. (b) Transmission of the solar envelope.--As stated above the apparatus for examining the solar image has been provided with appropriate shelters, and is now much improved over its condition last year. The great coelostat was exhibited at St. Louis in 1904, and as a substitute a smaller coelostat was arranged at the Observatory shop. The larger coelostat was not returned from St. Louis until January, 1905, and, in the press of preparation for the Mount Wilson expedition, time could not well be spared to set it up, so that the smaller instrument has served throughout the year. Many measurements, both of the distribution of radiation along the diameter of the solar disk and of its distribution in sun spots, have been made after the manner explained last year.a A revision has been made of all the data obtained relating to the distribution along a diameter, on the assumptions (1) that we study a phenomenon of absorption only; (2) occurring in a homogeneous medium situated outside the photosphere, and (3) extending to 21 per cent of the solar radius. With these assumptions it appears that the form of the distribution curves as shown in Plate VIII of last year's report agree within the experimental error with that deduced from the ordinary simple exponential absorption formula. As this formula in a logarithmic form is peculiarly adapted to graphical illustrations and comparison of results, all the measurements have been reduced on the above basis and the results to be given below in Table 3 depend on it. It goes without saying that the assumptions made are not harmonious with our conceptions of the sun's absorbing envelope, and they are only made for want of better and for the sole purpose of more readily comparing the results of dif • Smithsonian Report for 1904, p. 85. ferent days. This arbitrary method of reduction is necessary, for differences of atmospheric transparency and differences of solar distance make a direct comparison of one observational curve with another complicated and unsatisfactory. Plate VI shows two of the solar curves plotted with abscisse as air-masses derived on the above assumptions, and ordinates as the logarithmic deflections. The general absorption of the solar envelope appears to be like that of the earth's atmosphere, greatest at the violet end of the spectrum. This is shown, and also some of the changes which have been noted in the apparent solar transmission, in Table 3. In this table the numbers purport to represent the percentage transmission of the solar envelope for vertical rays, on different days, and for various rays between wave lengths 0.4u in the violet, and 2.0u in the infra-red. The results are based on the assumptions stated above, and a considerable difference will be noted from the corresponding table of last year, owing to a change in the assumed thickness of the absorbing envelope. TABLE 3.-Transmission of solar envelope. September 25 1904. February 20. 0.445 0.495 0.585 0.635 0.675 0.700 0.715 0.740 0.790 0.830 November 17 November 28 (.440) .495 .575 625 .510 560 .640 690 730 750 .770 .800 .830 (.850) (.660) .525 .605 March 16. .480 .530 .605.660 .690 .710 730 760 .480 .530 .605 .660 .690 710 .730 .760 795 830 .530 .605 660 .690 .730 760 795 .830 530 .605 .660 .690 730 760 795 830 |