384 A STUDY OF COKKKLATIONS AMON(; TKKKESTRIAL TEMPKRATURES. 



preclude the possibility of much larger fluctuations in shorter periods, because it 

 would take time for temporary increase in the sun's radiation to produce its full 

 effect. The shorter the time that we suppose an increase or decrease to last, the 

 greater it must be. It is mainly a question of judgment and probabilities whether 

 changes of such very short period in the radiation can exist. The probabilities against 

 them are based mainly on the fact that it is scarcely conceivable that any cause affect- 

 ing the totality of the sun's radiation should act simultaneously over the entire photo- 

 sphere. The most plausible cause of such fluctuations would be looked for in the 

 faculse and spots. These, and the phenomena connected with them are mainly local, 

 never covering any important fraction of the sun's disc. 



A yet more plausible source of change is found in possible fluctuations in the 

 transparency of the solar envelopes. But these would take a long time to extend 

 themselves over the entire photosphere. By allowing them a period of several weeks 

 to spread over the sun, we bring them within the range of the present studies which 

 then seem to establish their non-existence, except within the limits already several 

 times mentioned. 



A collateral question which is not included in the present research is whether the 

 conclusions which have been drawn as to the constancy of the sun's radiation can be 

 applied to other meteorological changes than those of temperature. The writer con- 

 ceives that fluctuations of temperature are the primary cause of changes in precipita- 

 tion, rainfall or great movements of the air, and fluctuations of the barometer. Con- 

 fining ourselves within the limit of reasonable probability, the totality of rainfall 

 must in the long run balance the evaporation. The rate of evaporation is, so far as is 

 known, not influenced by electrical or magnetic conditions of any kind, but dependent 

 solely upon the temperature and physical condition of the evaporating surface, and the 

 temperature and motion of the air in contact with it. If the motions of the air are 

 not affected by changes in the sun's radiation it would therefore follow that the rate 

 of evaporation is determined solely by ten-estrial conditions. This being granted it 

 follows also that the total rainMl is determined in the same way. The total mass of 

 the atmosphere being a constant, the integrated barometric pressure through the whole 

 globe must also be a constant. Fluctuations in its amount in any region must there- 

 fore be balanced by opposite fluctuations in other regions and must be due to motions 

 of the air which are determined only by conditions of temperature. If these views are 

 correct it follows as the final result of the present investigation that all the ordinary 

 phenomena of temperature, rainfall and winds are due to purely terrestrial causes and that 

 no changes occur in the sun's radiation which have any influence upon them. 



