NO. 1865 OUR KNOWLEDGE OF THE SUN — HALE 333 



In this presence it is quite unnecessary to dilate upon the im- 

 portance of the general question of evolution, or to discuss the re- 

 lationship of the problems of the astronomer to the more complex 

 ones encountered by the student of evolution in biolog}^ It is evi- 

 dent that if we are to acquire a correct understanding of evolution 

 in all of its phases, w^e should start from a knowledge of those pro- 

 cesses which result in the formation of stars and the development 

 of planetary systems. The generalizations of thinkers like Kant, 

 Laplace, and more recent writers who have furnished hypotheses 

 to explain the origin of suns and planets must be put to the test of 

 observation. But these hypotheses leave untouched scores of ques- 

 tions relating to the physical state of stars in various stages of 

 growth ; their relation to one another and to their environment ; their 

 connection in systems, and the part they play in the universe as a 

 whole. All of these questions lie within the province of the student 

 of stellar evolution and call for the exertion of his best efforts to 

 contribute toward their solution. 



We thus see that solar research may be divided into two classes: 

 (i) measurement of the intensity of the Sun's radiation, to deter- 

 mine whether the heat received by the Earth is constant or under- 

 goes fluctuations; and (2) observation of the various phenomena of 

 the Sun's disk, to determine the laws by which they are governed. 

 The first of these subjects has been investigated with great success 

 by the Smithsonian Astrophysical Observatory, established by the 

 late Secretary Langley and directed by Mr. Abbot. The work of 

 the Mount Wilson Solar Observatory lies in the second field. The 

 two departments are closely related, and I am glad to say that 

 through a plan arranged with Doctor Langley and extended by 

 Doctor Walcott, the work inaugurated here in Washington is being 

 continued by Mr. Abbot on the summit of Mount Wilson, in close 

 cooperation with our own investigations. 



It has been conclusively shown by Koppen, and confirmed by New- 

 comb, that the average temperature of the Earth, as determined by 

 the combination of a great number of thermometer observations 

 made at several stations, indicates a fluctuation of from o°.3 to 0° ."/ 

 C. during the eleven-year Sun-spot period. The mean temperature is 

 greatest at the time of minimum Sun-spots, and least at the time of 

 maximum Sun-spots. This relationship having been proved to exist, 

 it remains to inquire whether there is any direct connection between 

 the mean temperature of the Earth at a given time and the total 

 heat radiation of the Sun as measured at a point outside of the 

 Earth's atmosphere. Since all observations must be made within 



