The three circuits: a study of the primary forces . THE SOLAR ENIGMA. 141 the artist pictures something like an auroral dis-play. Professor Lockyers description of the sun spotsand his conclusions reads as follows : Some of the spots cover millions of square miles ; othersare visible only in power- jrIG. 15, ful instruments and areof very short is a great differ-ence in the number ofspots visible from timeto time; indeed, thereis a minimum periodwhen none are seen forweeks together, and amaximum period whenmore are seen than atother times. The inter-val between two maxi-mum or t
The three circuits: a study of the primary forces . THE SOLAR ENIGMA. 141 the artist pictures something like an auroral dis-play. Professor Lockyers description of the sun spotsand his conclusions reads as follows : Some of the spots cover millions of square miles ; othersare visible only in power- jrIG. 15, ful instruments and areof very short is a great differ-ence in the number ofspots visible from timeto time; indeed, thereis a minimum periodwhen none are seen forweeks together, and amaximum period whenmore are seen than atother times. The inter-val between two maxi-mum or two minimumperiods is about elevenyears. Now as we mustget less light from thesun when it is covered with spots than when it . n n +1 Total Eclipse of the Sun, Dec. 22,1870. is free from them, we r may look upon it as a variable star with a period of eleven outer portion of the corona, on the western limb of the sunconsisted of three projections of light striated or of radial. 142 TEE THREE CIRCUITS. years It is also known that the magnetic needle has a period of the same length, its oscillations occurring whenthere are most sun spots. Aurorse and the currents of elec-tricity which traverse the earths surface are affected by asimilar period. There seems, therefore, some connectionbetween these things and the solar spots, though what it iswe do not know.—Elements of Astronomy, sec. 125. Then, as our aurorse have a maximum and mini-mum period of the same length, we may also lookupon the earth as a variable star with a period of elevenyears. We are not situated so that we can perspec-tively view the earth as a planet in space, hence wecannot say that it has such spots on its surface, but wemay infer that it has. The same author says: The heat thrown out from every square yard of the sunssurface is greater than that which would be produced byburning six tons of coal on it every hour. Now we may takethe surface of the sun roughly at 2,284,000,000,000 squaremil
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