180 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1938 



the rays, we really want to know the intensity of the cosmic radiation 

 from sea level right to the top of the atmosphere at all latitudes, but 

 so far it has not been possible to do many experiments of this type. 



Apart from the free balloon ascents by the technique already 

 described, valuable results have been obtained by the use of manned 

 balloons. This technique was first developed by Professor Piccard . 

 It is well known that it is not possible to live at a height above, say, 

 20,000 feet, without the use of oxygen — at least, if one cannot spend 

 a long time acclimatizing oneself to the reduced pressure. One can 

 go to a height of 40,000 feet or so if one breathes oxygen instead of 

 air, but one cannot go very much higher, even breathing pure oxygen, 

 unless one keeps up the pressure of the body artificially. There are 

 two ways of doing this: one can place oneself, as did Professor Piccard, 

 in a metal gondola, generally of spherical shape, which is sealed up 

 and retains the pressure on the body above that of the air outside. 

 The other method is to use a pressure suit, that is a suit rather like a 

 diver's suit, but in which the pressure is kept above that of the atmos- 

 phere. This method was used in setting up the recent altitude record 

 for aeroplanes. With the former method Piccard and his collaborators 

 and also some other investigators in America have reached heights up 

 to 18 km, carrying elaborate and heavy apparatus with them. In 

 one such flight from Belgium, which ended up somewhere in Central 

 Europe, Dr. Cosyns measured the variation of the intensity of cosmic 

 rays as he floated across Europe at a height of some 12 km. He found, 

 much to his surprise, that the cosmic radiation remained constant 

 from about 51° north to about 49° north, and then dropped suddenly 

 as he went farther away from the poles. This critical latitude of 49° 

 north, above which the cosmic radiation remains constant, is of great 

 importance in all cosmic ray theory. We have to try and explain 

 exactly why the cosmic rays remain constant north of this latitude 

 both at sea level and at great heights. At present there is no satis- 

 factory explanation of these facts. 



The next part of the study of cosmic radiation to be described is how 

 the intensity varies with the time. Experiments have been made 

 over periods of years to see if the cosmic radiation is quite constant or 

 if, and how, it varies < Some results are shown in figure 4. The soft 

 components of radiation, that is the part of the radiation which has 

 not a very great penetrating power, does show an appreciable variation 

 with the time of day. There is a slight maximum about midday of 

 the order of a few percent of the whole intensity. But when the soft 

 radiation is filtered out by means of thick lead screens, the remaining 

 penetrating component is found to be almost constant. The figure 

 shows that this penetrating component does not vary more than a 

 fraction of 1 percent throughout the day; that is, when the results 

 are averaged over a very large number of days. This again shows, as 



