It is a matter of high moment that the English equivalents of the Arabic weights and measures should be determined exactly, but the task is very difficult after so long a lapse of time and so man changes of standards. A surprisingly large number of measure used in the Orient have been called the cubit, and we cannot b categorical as to the one which was in the mind of our author. It is probable that the following citation indicates the cubit for which we are searching. It is abbreviated from the authoritative Men and Measures (1917) by Dr. Edward Nicholson, a British physician of wide Asiatic experience.
"Many centuries after the institution of the Assyrian great cub of 25.26 inches, and of the Persian Beládi cubit of 21.88 inche another important cubit became a standard of measure in the Moslem caliphate which reigned over the lands of the Eastern great kingdoms. Under Al-Māmūn, son of Harūn al-Rashid, science was flourishing in the East, while the West was in the dark ages, at leas in all countries unenlightened by the civilization of the Moors in Spain. The cubit which was legally prescribed by Al-Māmūn was called the Black Cubit, and was so named from the black banner and dress adopted by the Abbaside caliphs.
"The Black Cubit equaled 21.28 inches and was derived from the common, or original, cubit of
18.24 inches as being equal to seven handbreadths of 3.04 inches whereas the latter is equal to six,
Common Cubit = 18.24 = 6 X 3.04
Black Cubit = 21.28 = 7 X 3.04.
"This Black Cubit is still in use and is the basis of measures an of weights which spread from Egypt to every country in Europe The old Nilometer (built 861 A.D.) on the island of Al-Rawḍah (the garden) has its cubits in this scale and measurements of the worn scale give 21.29 inches for the cubit."
We cannot ignore the fact, however, that the slightly long cubit of 21.88 inches was also used in
North Africa, as is attested by the following words from the same authority:
"The Persian cubit known as the Beládi, (from belád, country was one ten thousandth of a meridian league, or 21.88 inches passed to Spain with the Moors and is still sound in the East."
The short cubit of 18.24 inches—also called the common cub and original cubit—may probably be discarded by us because seems to make the archery tackle too short for practical use, but was must confess that in some instances there is ample room for doubt.
In our calculations we will use the Black Cubit, which seems figure out fairly well. However, in mathematical data our author is often far from modern standards of accuracy; for instance, he says that forty-five times three cubits and a finger equal one hundred and forty cubits, whereas they are obviously nearer to one hundred and thirty-seven cubits.
The approximate lengths given above will then be: 25 cubits = 14.8 yards ; 125 cubits = 74 yards ; 140 cubits = 82.75 yards ; 300 cubits = 177.6 yards.
One bow-length = 65 inches (64.84) ; 45 bow-lengths = 81 yards.