148 THE SUlSr-SPOT PERIOD. 



the obiservations at Hohenpeissenberg, extending from 1792 to 1850, which he himself 

 pubhshed in 1870. This discussion yielded the result that an increase of 100 Wolf 

 numbers in the sun-spot frequency was attended on the average by a decrease of about 

 1° C. in the mean annual temperature for this station. He refers also to Koppen's 

 extensive investigation, which yielded the result that an increase of 100 sun-spot 

 numbers was attended with a decrease of temperature of 0°.54 C. for equatorial 

 stations, but with more complex effects for stations in temperate latitudes. But while 

 this statistical evidence thus tends cliiefly in the same direction. Professor Abbe is 

 not convinced that we can certainly ascribe tliis apparent temperature periodicity to 

 solar influences. He says that although for a long time he "believed that it might be 

 possible to establish an intimate connection between solar radiation and solar spots, 

 yet the steady development of our knowledge of the selective absorption of the 

 earth's atmosphere has shown that we can not argue by crude statistical methods 

 from terrestrial temperatures up to solar radiation. We may speak of periods and 

 variations in our temperatures, but these do not demonstrate a similar period in the 

 solar temperatures or solar radiations, since unsuspected periodic variations in the 

 constituents of the earth's atmosphere may be the cause of the variations w'e should 

 otherwise attribute to the sun itself. * * * The mere fact that there is a decrease 

 of temperature in the Tropics at sun-spot maximum argues nothing as to the direct 

 relation of cause and effect between the two phenomena. I have on hand a collec- 

 tion of monthly charts of temperature departures for the whole globe for several suc- 

 cessive years, which tend to show clearly that the sun-spot period in the earth's 

 temperature is a purely local, terrestrial matter, moving round from one part of the 

 world to another, just as do our droughts and our rains, our barometric waves and 

 our cold waves; analogous to the movement of an earthquake wave over the ocean, 

 going sometimes rapidly and sometimes slowly, reflected from a continent, exaggerated 

 in some arm of the ocean, breaking in waves on a shore, but scarcely felt on an 

 island in midocean, and finally dying out by virtue of innumerable interruptions, as 

 all forced waves must do unless they happen to be reenforced by a process similar 

 to that of resonance in sound waves." 



A word may be added in connection with Nordmann's discussion of the direct effect 

 of sun spots on temperature, which the diminished radiation of sun spots as com- 

 pared with the photosphere would lead us to expect. Substantially the same argu- 

 ment, based on Newton's law of cooling, was published by Professor Langley in 1876 

 (see Monthly Notices British Astronomical Society, November, 1876), and he reached 

 the conclusion that the presence of sun spots in a period of maximum solar activity 

 might reduce the mean temperature of the earth not exceeding 0°.29 C. by their 

 direct effect in diminishing solar radiation, but he did not decide whether terrestrial 

 temiwrature may not be quite otherwise affected by some varying solar action of 

 which spots are merely accompaniments. 



Within the last twenty years it has been shown that Newton's law of cooling does 

 not apply to bodies losing heat solely by radiation, and it has been experimentally 

 verified that, in accordance with Stefan's law, the perfect radiator or so-called 

 "absolutely black body" emits an amount of radiation proportional to the fourth 

 power of its temperature above absolute zero. All other bodies radiating by virtue 

 of their temperature emit less than the perfect radiator at any given temperature, but 

 at low temi^eratures imperfect radiators are found to depart from Stefan's law and 

 to emit amounts more nearly i:)roportional to the fifth jiower of their temperature. 

 Since the earth is losing heat almost solely by radiation and is kept at substantially a 

 constant mean temperature of about 290° absolute by the solar rays, the earth's total 

 radiation is proportional to (290)*+ and is equal to that received from the sun if we 

 neglect the small amount received from space. If now the sun's radiation were 

 reduced by o Jo, as supposed by the author, on account of the presence of sun spots, 



