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The 150-Foot Solar Tower Sunspot Drawing References  | 
Hale, George E. and Nicholson, Seth B.: 1938, Carnegie Institution of Washington Publication, No. 498, 'Magnetic Observations of Sunspots 1917 - 1924,  Part 1.' 
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 Bray, R. J. and Loughhead, R. E.: 1965,  John Wiley and Sons, Sunspots.  (Also available as a Dover reprint.) 
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