182 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 8 



investigations have shown that there is very little variation indeed. 

 There is a small effect, which will be discussed later, which is of great 

 importance and which has only recently been detected, but the varia- 

 tion with sidereal time is very much less than would be expected if the 

 stars of the Milky Way were the origin of the rays. 2 



Now all the stars that one sees with the naked eye, or can be seen 

 even with a large telescope, are members of a huge group of stars 

 called the galactic system. This system contains some 30,000 

 million stars. These stars form a kind of huge flattened disk, of 

 which the size can be roughly estimated as being 00,000 light years in 

 diameter and 5,000 light years thick. It may be noted that a light 

 year, which is the unit of distance often used by astronomers, is the 

 distance traveled by a ray of light in a year, and is about 10 13 km. 

 About 90 percent of the matter in the galactic system lies in this 

 central flattened disk, about half consisting of stars and about half 

 of diffuse matter and of gas. The earth is situated somewhere near 

 the central plane of the disk, but about 20,000 light years from its 

 center. This whole galactic system constitutes a huge nebula analo- 

 gous to, but probably rather larger than, the great spiral nebula in 

 Andromeda. There is a great central condensation of stars near 

 its center which lies toward the constellation of Sagittarius, this 

 constellation lying in about the thickest part of the Milky Way. In 

 recent years it has been discovered that the whole galactic system is 

 in rotation, making one complete revolution in about 250 million 

 years. Since the earth lies so far from the center, it is traveling 

 through space with a very large velocity, amounting approximately 

 to about 300 km per second. It can be shown that the effect of this 

 large velocity is to make the intensity of cosmic radiation slightly 

 greater on the side of the earth which faces the direction of motion, 

 as compared with the opposite side. Just in the same way as more 

 raindrops are found on the windscreen of a moving car than on the 

 back window, so the motion of the earth is revealed by the greater 

 intensity of cosmic rays on one side. This fact was predicted by 

 Compton, and has been recently found experimentally. The varia- 

 tion is quite small, being less than 1 percent in magnitude, but it seems 

 fairly clear from the measurements of Hess and Steinmaurer Schon- 

 land and of Kohlhorster, that the predicted variation does really exist. 

 This is a very important result, for it confirms the view, that I have 

 mentioned above, that the cosmic radiation cannot have its origiu 

 in our own galaxy. Figure 5 shows the results of the variation with 

 sidereal time which confirm Comp ton's predictions. 3 



a Note in proof, February 1939. — It is now thought that there may be a sufficiently large magnetic field 

 throughout the galaxy to deflect the rays very considerably. If this is the case, the above argument is in- 

 valid, and so the galaxy still remains a possible place of origin of the rays. 



* Note in proof, February 1939. — More recent measurements and calculations have thrown great doubt on 

 the existence of a variation with siderial time of the magnitude demanded by Compton's theory. 



