8 POPULAR SCIENCE MONTHLY. 



An observer at the intersection of these cross-lines with the 

 central line will see the totality during the intervals given in the 

 table. 



The mode of the formation of the shadow cones of the moon, 

 called the penumbra for the partial shadow and the umbra for the 

 total shadow, are well illustrated in general works on astronomy, 

 and good geometrical pictures of them can there be found, together 

 with much useful information regarding the subject of eclipses. 

 As we are here concerned chiefly with certain practical points 

 about the eclipse of 1900, it will be well for the reader to consult 

 such works for many details regarding the astronomical features 

 attending an eclipse of the sun which must now be omitted. 



There are many existing theories to account for the phenome- 

 non of the sun's bright appendage, called the corona, which is vis- 

 ible only during eclipses, on account of the absorbing effects of 

 the earth's atmosphere on its light. Is it electrical, or is it mag- 

 netic? Is it composed of fine stuff ejected from the sun, or of 

 meteoric dust falling upon the sun? Is it merely an optical effect, 

 as some suppose, or is it a portion of the newly discovered radiant 

 matter streaming off to enormous distances into space? The an- 

 swer to these questions is eagerly sought through observation, pho- 

 tography, and every other possible means, on the occasion of each 

 total eclipse. 



The efforts of astronomers have thus far secured a series of 

 pictures of the solar corona, which, when compared together, show 

 very distinctly that the corona, as well as the spots, the protuber- 

 ances, and the faculse, are going through a series of changes which 

 seem to repeat themselves in the so-called eleven-year period. It 

 has also been proven, with entire distinctness, that the earth's 

 magnetic field, as marked by the changes in the intensity of the 

 magnetic elements, in the auroral displays, and the earth electric 

 currents show variations which synchronize closely with those oil- 

 served on the sun; also that the weather elements of pressure, tem- 

 perature, precipitation, and storm intensity all harmonize with the 

 solar and the earth's magnetism in the same synchronism. All at- 

 tempts of scientists to detect any variations in the sunshine which 

 falls upon the tropics have been entirely futile; on the other hand, 

 it has been shown that the magnetic forces having the character- 

 istics just mentioned impinge upon the earth in a direction per- 

 pendicular to the plane of the earth's orbit, just as if the sun, being 

 a magnet, throws out a field of force to the surface of the earth, 

 which, by its variation depending upon the internal workings of 

 the sun, produces the changes just enumerated in the earth's atmos- 

 phere and in its magnetic field, also throughout the planetary sys- 



