Note 15 — Chapter IV
Reading edition
The so-called sunspot numbers to which reference is made again and again in this book are based on a system devised by Wolf and revised by A. Wolfer. The number and size of the spots are both taken into account. The numbers from 1749 to 1900 may be found in the Monthly Weather Review for April, 1902, and from 1901 to 1918 in the same journal for 1920.