220 SCIENCE PROGRESS 



or reduction of radiation of sufficient magnitude. How can 

 these moods be detected ? Has it been possible to measure 

 any variation in the sun's radiating power? In spite of the 

 rapid strides made in the designing and construction of 

 instruments of precision, I do not think it can be said for 

 certain that any changes that have been recorded can be 

 positively attributed to the variation of solar radiation. Our 

 atmosphere, the very movements and changes of which we 

 wish to investigate, is responsible for this deficiency of our 

 knowledge, for it masks or mitigates all effects which, if it 

 were absent, might be recorded. 



By carefully studying the sun, and observing the many 

 phenomena rendered visible by means of telescopes and spectro- 

 scopes, it has been found that the sun appears very much 

 more disturbed at some times than at others. Failing absolute 

 measurements of solar radiation, we are therefore driven to em- 

 ploy deductions. Thus, for instance, we know that sunspots are 

 indications of solar disturbance ; so we can say that the greater 

 the number or size of the spots the larger will be the areas 

 of disturbance. From this it is deduced that the radiation must 

 be greater, and consequently the sun hotter. It has been found 

 further that there is a periodicity in spot activity ; that is, during 

 some years only a few spots are visible on the solar disc, while 

 in other years they are very numerous. In fact, they are most 

 or least numerous about every eleven years. We are thus led 

 finally to infer that the sun becomes hotter and colder alternately 

 every eleven years. 



From sunspot and other solar data it has also been concluded 

 that changes which extend over about four years and thirty-five 

 years are also in operation. 



We are thus brought into the presence of three variations in 

 solar activity, the oscillations of which cover approximately four, 

 eleven, and thirty-five years each. 



It is needless to ask the reader whether we have in our 

 atmosphere any changes which correspond to these solar fluc- 

 tuations, for he must now be quite familiar with them. 



The fact that the only three solar pulses known should have 

 their terrestrial equivalents in our atmosphere is, I think, a very 

 strong case in favour of the solar origin of these barometric 

 surges. 



There seems reason also to believe that, if these solar 



