382 A STUDY OF CORRELATIONS AMONG TERRESTRIAL TEMPERATURES. 



The radiations which reach the earth or its atmosphere from the sun are of two 

 great classes. We have first radiance properly so called by which I understand 

 radiant energy in its ordinary acceptation. This includes not only the rays commonly 

 called light, but all other raj's of the same class which difler from light only in wave 

 length. It may here be remarked, parenthetically, that the use of the word "light" 

 in physics is rather unfortunate, since the distinction of light and dark rays is not an 

 objective one, but rests only upon the property of afltecting the optic nerve. Thus, 

 when we use the word light, we have one word for radiance between certain limits of 

 wave length and no special term for radiance of the same kind of wave length without 

 the visible limits. 



Besides radiance as thus defined, we have abundant evidence that the sun sends, 

 at least to the confines of our atmosphere, certain emanations which affect the magnetic 

 needle, and which do not reach us in a steady stream, but fitfully, at irregular inter- 

 vals. These emanations have, up to the present time, eluded all direct investigation. 

 They are made known only by their effect upon the terrestrial magnetic force, as 

 shown by magnetic storms. It therefore seems probable that those which reach the 

 atmosphere are entirely absorbed in its outer envelopes. 



The preceding study is practically limited to radiations of the first class. It is 

 still questionable whether the magnetic or radio-active emanations, whatever they may 

 be, appreciably affect the temperature. The recent researches of Maunder seem to 

 show that they come mainly from the solar spots. Now, it is known that the radi- 

 ance from the spots is less than fi-om the rest of the photosphere. It follows that, if 

 the emanations in question convey an appreciable amount of thermal energy, it does 

 not reach the earth, but is absorbed in the upper i-egions of the air, perhaps almost at 

 the surface of the atmosphere itself But, were this the case, the extreme rarity of 

 the air at high altitudes would result in a proportionately greater rise of temperature 

 through a given radiation of thermal energy. In a word it seems highly improbable 

 that emanations having radiant energy in considerable quantities could be absorbed 

 by so rare a medium as the air at great heights above the earth. 



The evidence afforded by the frequency of magnetic storms shows that the emana- 

 tions in question are greatest at the period of sun-spot minimum when the terrestrial 

 temperature is least. This affords an additional ground for believing that the thermal 

 effect of the magnetic radiation is too small to produce any directly observable 

 meteorological effect. 



So far as research has yet gone, the balance of evidence would seem to favor the 

 view that the phenomena of atmospheric electricity, especially of thunder storms, so 

 far as they are changeable, arise mainly from terrestrial causes, and are but slightly 



