A STUDY OF rOKUKLATION'S AMOXCi TERRESTRIAL TEMPERATURES. 311 



§ 15. Search for variations synchronous with the sun's synodic rotation by the method of time-correlation. "When 

 a tendencj' toward a periodic variation can be expected — choice of San Diego as a station — 

 time correlation through a period of 3.3 years from 1872 to 1904 ; result, onlj' a suspicion of a 

 periodic tendency, the amplitude bring two or three-hundredth.s of a degree — further illus- 

 tration of the method from the general ten-day departures — tendency toward a persistence 

 of temperature conditions through periods of more than 40 days. Pp. 375-379. 



CHAPTER VI. 

 Discussion of Results. 



§ 16. Summary of conclusions. Actuality of the sun spot fluctuation — uncertain evidence of fluctu- 

 ations having a shorter period — limitation within which the conclusions are to be inter- 

 preted. Pp. 379-381. 



§ 17. Relation between solar radiation and meteorological jrrocesses. The present study limited to thermal 

 radiation — the question whether other emanations producing aurorte and magnetic storms 

 have any appreciable thermal effect — relation between fluctuations of the solar radiation 

 and the fluctuations of temperature hence arising — possible causes of change in the solar 

 radiation — definitive outcome of the investigation. Pp. 381-384. 



§ 18. Comparison with results of Langley's ^vork of 190S. Pp. 384-387. 



INTRODUCTION. 



The view tliat the rate at which the sua radiates thermal energy is or may be 

 variable finds frequent expression in scientific literature. The inference of such vari- 

 ability may be drawn from two sources ; one direct measures with the bolometer, 

 the other, meteorological phenomena, especially variations of temperature at the earth's 

 surface. Many years ago Lockyer pointed out that a cycle corresponding to that of 

 the solar spots was indicated in the agricultural productions of India. A similar cycle 

 has been sought for in the variations of temperature at special places, and in a variety 

 of meteorological phenomena. Briickner has in an elaborate work adduced evidence 

 to show a cycle of about 35 years in meteorological changes generally, those of temper- 

 ature included. Although the fluctuations here described are not always expressly 

 attributed to the action of the sun, it would be difficult to account for them in any 

 other way than by fluctuations in the sun's radiant enei'gy. 



Bigelow's many and long-continued researches on meteorological phenomena, with 

 the view of determining their laws and periods of variations and their relation to the 

 activity of the sun, have also led him to an affirmative conclusion. The best marked 

 period he has sought to establish is one corresponding to the period of the sun's syn- 

 odic rotation. But the actual conclusions deducible from his work seem to relate to 

 the electric and magnetic efifects of the solar activity, rather than to purely thermal 

 effects, which alone are studied in the present work. 



