Underscores “_” before and after a word or phrase indicate _italics_ in the original text. Equal signs “=” before and after a word or phrase indicate =bold= in the original text. Carat symbol “^” designates a superscript. Small capitals have been converted to SOLID capitals. Illustrations have been moved so they do not break up paragraphs. Old or antiquated spellings have been preserved. Typographical and punctuation errors have been silently corrected. FELLOW OF THE ROYAL ASTRONOMICAL SOCIETY, AND OF THE CAMBRIDGE PHILOSOPHICAL SOCIETY; PROFESSOR OF MATHEMATICS IN UNIVERSITY COLLEGE, LONDON.…
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