138 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1914. 
As viewed through the telescope, the sun at first sight is a very 
disappointing object as compared with the moon. Nevertheless, 
there is much of interest to be seen there. In the illustration (pl. 1, 
fig. 1) we see a direct photograph of the sun as obtained te 
pibea of the Yerkes Observatory, on May 18, 1910. Several 
interesting features may be pointed out. In the Abs place, note 
the falling off of the brightness of the disk toward the edges of the 
sun. In he second place, one sees in the original photograph all 
over the sun’s surface a sort of mottled appearance, not very dis- 
tinct, but yet interesting. In the third place, in this particular pho- 
(een appear some dark spots, called sun spots. Sun spots were 
discovered by Galileo in the year 1610, soon after the invention of 
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SuN-sPoTs AND TERRESTRIAL TEMPERATURES AND MAGNETISM. 
I. Temperature departures, United States inland stations. 
II. Sun-spot relative numbers (Wolf's). 
III. Magnetic declination { mean diurnal renge}} 
IV. Magnetic horizontal force { Ellis, Chree. 
Fig. 1. 
the telescope.'| They are distinguished by dark central parts, called 
the umbra, surrounded by a ene of less darkness, called the pen- 
umbra. The spots shown in the illustration are very large ones, 
although they seem very small upon the surface of the sun. This is 
because of the immense diameter of the sun itself. The earth might 
be dropped into one of these sun spots without much more than filling 
the umbra, leaving a generous space for the penumbra outside of it. 
It was fang by Schwabe, about the middle of the nineteenth 
century, that sun spots occur most plentifully in periods of about 11 
years between maxima. This may be seen by the diagram (fig. 1), 
in which the second curve represents the prevalence of sun spots 
according to the so-called sun-spot numbers published by Wolter. 
The two donee curves represent, ‘Tespectively, variations in a the earth’s 
1Sun aod had beeasionalty been seen with the naked eye earlier, put it was not until 1610 ‘that they 
were definitely regarded as a solar phenomenon. 
