Handbook of Railroad Construction; For the use of American engineers. / Containing the necessary rules, tables, and formulæ for the location, construction, equipment, and management of railroads, as built in the United States. - George L. Vose

For spans under six feet the depth should be thirteen inches.

From six to fifty feet, 13 inches plus 1
48 of the span.

From fifty to one hundred feet, 1
24 of the span.

For over one hundred feet, 1
24 of 100 plus 1
48 of the remainder.

Thus for a span of one hundred and ninety-six feet we have

100 × 12
24 + 96 × 12
48,

or, 50 + 24 equal in all to seventy-four inches, or six feet and two inches; whence the following table:—

Span of arch in feet.	Thickness of voussoir in inches.
6	13 +	0 = 13
8	13 +	2 = 15
10	13 +	3 = 16
12	13 +	3 = 16
15	13 +	4 = 17
18	13 +	5 = 18
20	13 +	6 = 19
25	13 +	7 = 20
30	13 +	8 = 21
35	13 +	9 = 22
40	13 +	10 = 23
45	13 +	11 = 24
50	13 +	13 = 26
60		= 30	inches.

254. The above depend upon the span and form of the arch, the height of the abutment, and character of the masonry.