An Illustrations-list has been added after the Contents-list.
A few illustrations didn't have a description. A description has been provided by the transcriber to make reference to it possible. To distinguish them from the other illustrations, the description is put in lower-case.
Several illustrations depict tables with numbers. These tables have been converted to text in the plain text version.
There are two "misprints" in the illustrations depicting tables:
1) The illustration of "The unsolved cipher" should show an extra seconds-mark. This becomes clear when you compare it with the illustration of "THE SOLVED CIPHER", where you can see an extra seconds-mark to the right of the second degrees-mark.
2) "THE SOLVED CIPHER" shows totals in the top-row and totals in the last column on the right. The total in the top-row, 6th square, should be "191" instead of "19".
A few cases of punctuation errors were corrected, but are not mentioned here.