Here bolt refers to a roll of fabric. Today the length of fabric on a bolt varies, though usually (in the United States) in round numbers of yards. A survey of fabric retailers finds bolts of 10, 20, 25, 40 and 50 yards being sold. But at certain times and places a bolt meant a definite length, a practice that seems to have originated with canvas. See the examples.
BOLT; This is eight and twenty Ells of canvas.
Twenty-eight ells is 35 yards.
Bolt, a compact parcel or roll of canvas containing about 40 yards, 24 inches in width, or 28 ells, and weighing about 28 lbs.; a bolt of silk is a long narrow roll.
40 yards = 1 bolt (cloth).
Commonwealth of Massachusetts. Dept. of Weights and Measures.
Measurements for the Household. 2nd ed.
Boston: State House, 1917.
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Last revised: 26 September 2016.