HOW THE SUN WARMS THE EARTH 



By C. G. Abdot 

 Secretary, Snilthsonian Institution 



[With 6 phites] 



James Arthur in 1931 bequeathed a considerable sum to the Smith- 

 sonian Institution for the study of the sun. A condition of the gift 

 directed that each year a lecture should be given on some phase of 

 our knowledge of the sun either by a member of the Institution's 

 staff or by some other competent person. The first Arthur lecture 

 was given 2 years ago by Dr. Henry Norris Kussell, who discussed 

 the sun's chemical and physical composition. Last year, Dr. Ernest 

 William Brown, in the second Arthur lecture, treated of the sun's 

 gravitational control of the solar system. In this third Arthur lec- 

 ture I propose to give some account of the sun's radiation, whereby 

 the earth is kept at a habitable temperature. 



HEAT EXCHANGE BY RADIATION 



Experiment: Here I have a lamp which I can heat more and 

 more by electric current until, as you see, it glows brightly. At the 

 other side of the stage, too far away for direct heating by the lamp, 

 is a concave mirror which forms an image of the lamp upon a sen- 



FlODRB 1. — Heat exchange by radiation. 



The lamp, a, heated by electricity, sends rays to the concave mirror, b, which focuses 

 them on the thermopile, c, whose warming Is Indicated by the galvanometer, d. 



sitive electrical thermometer called a thermopile (fig 1). That you 

 may all observe its behavior, I record the indications of the elec- 

 trical thermometer on the screen. Please observe this little spot of 



iThe third Arthur lecture, delivered at tlie Smithsonian Institution on Feb. 2G, l"j:>4. 



149 



