[154], last line, for “tubular,” read “tabular.”

[156], l. 4 from bottom, for “washer band,” read “washer used.”

[164], l. 10 to 14, inclusive. The first number of ratios should be 20 instead of 15.

[166], l. 11, for “69 B,” read “69 A.”

[171], head of col. 5 of table, for “Rod of Arch,” read “Rad. of Arch.”

[173], l. 25, for “ability,” read “stability.”

[173], l. 32, for “Whence,” read “Where.”

[175], l. 8, for “triangular,” read “diagonal.”

[178], l. 3, for “article,” read “outside.”

[184], l. 4 from bottom, for “barriers,” read “voussoirs.”