82 REPORT OF THE SECRETARY. 



the presence of the oceans, would prevent so great a change of temperature as this 

 resulting from a temporary diminution of solar radiation of only a few months' dura- 

 tion. The observed fall of about 2°. 5 C. in the mean temperature of the land areas 

 of the North Temperature Zone during April, 1903, seems to be therefore in good 

 accord with the observations of solar radiation. 



(Jwing to the uncommon cloudiness of the tirst six months of 1904 few measures of 

 the "solar constant" worthy to be compared Avith the series of 1903 have been 

 obtained, but taking the best of the measures it appears that high values of solar 

 radiation in February, 1904, and lower ones in the subsequent months are indicated, 

 as shown in Table 2, given below. This appears to be in general accord with the 

 mean temperatures observed over the North Temperate Zone, except that it seems 

 probable that the solar radiation was high in January as well as February, but the 

 lack of good observing weather prevented our recognition of it. 



Forecasts of temperature. — If subsequent research shall confirm these indications 

 of a general parallelism between measures of solar radiation and terrestrial temper- 

 atures, we are now entering upon a new field of climate forecasting. But if such fore- 

 casts had to depend on measures of the solar radiation outside our atmosphere the 

 observing station should be removed from Washington to a more favorable situation, 

 for the experience of the last two years has shown that hardly a score of days in a 

 year are uniformly clear enough to allow even relatively good "solar constant" 

 values to be obtained; and these good days are very unevenly distributed. Fortu- 

 nately, the distinct work which you have planned on the absorption of the solar 

 envelope, already mentioned, seems to promise a far easier method of forecasting, 

 which requires much less of constancy in the atmospheric conditions. Owing to the 

 preliminary nature of the installation of the great horizontal telescope as thus far 

 used, and to the fact that the great coelostat has been sent to the exposition at St. 

 Louis, only the single instance above mentioned of a comparison of the radiation 

 outside our atmosphere with solar absorption has yet been made; but if future work 

 shall confirm the general agreement between the indications afforded by the study 

 of the absorption of the solar envelope and those afforded by the measures of total 

 solar radiation, the significance of the result will be very great, for the bolometric 

 investigation of the solar image can be made at any time when the sun shines clear 

 for five minutes, by a method practically independent of the disturbances of our 

 own atmosphere, whereas the measures of total radiation require three hours of 

 unvarying transparency of the air. Accordingly the former measurements may be 

 made almost daily, and will, it is hoped, prove of great service in temperature 

 forecasts. 



The effects of changes of the transparency of our own atmosphere are, perhaps, of 

 equal importance in temperature forecasts, and the recognition of these of course 

 depends on such spectro-l)olometric measures as are involved in determinations of 

 the "solar constant." Referring to my last year's report, it will be remembered 

 that the earlier months of 1903 were found distinguished by more than the average 

 absorption of light in our atmosphere. In September, 1903, there was a marked 

 increase in transparency, so that in the latter months of the year the clearness was 

 almost as great as that of 1901-2. Of course as these studies are made only at Wash- 

 ington no general conclusions as to the transparency of the air at other stations can 

 safely be drawn. But if the clearing above mentioned was general it might perhaps 

 explain the ui)ward tenden(;y during the last months of 1903 of the curve of average 

 temperatures shown in Plate VII. 



I venture to think the importance of studies of atmospheric absorption at other 

 stations would warrant observers elsew'here in taking up the spectro-bolometric work 

 involved in measures of the "solar constant." Measures of the absorption of the 

 solar envelope, on the other hand, which require the provision of a great solar image, 

 would not necessarily be duplicated elsewhere. 



