‘Mahmd Ibn Mesoud says that in the time of Almamon (the learned Calife of Babylon) by the elevation of the pole of the equator, they measured the quantity of the degree upon the globe of the earth, and found it to be 56-2/3 miles, every mile containing 4000 cubits, and each cubit 24 digits, and every digit 6 barleycorns, and every barleycorn 6 hairs of a camel’ (‘A Discourse of the Romane Foot and Denarius,’ by John Greaves, Professor of Astronomy in the University of Oxford, 1647). From this determination of 56-2/3 meridian miles to the degree of longitude it would appear, (1) that the measurement was made at about 20·1°; south of Mecca, (2) that the meridian mile was still of 4000 Egyptian common cubits or 1000 Egyptian fathoms. It was then probably after this measurement that Al-Mamūn instituted his new Cubit, sometimes known as the Black cubit, so named from the black banner and dress adopted by the Abbaside caliphs. This new cubit was not, directly at least, of geodesic basis. The caliph was probably inspired by the idea of making in a reasonable manner the alteration which the ancient Egyptians had done badly in making their seven-palm cubit out of simple proportion to the common cubit. So the new cubit had palms and digits of the same length as the common cubit. But it had all the inconveniences of the factor seven. Perhaps Al-Mamūn may have thought that the addition of a seventh palm was not only a homage to the seven planets but that it was satisfactory to lengthen the common cubit in the ratio of the degree of latitude to that of longitude in a part of his dominions where the ratio was exactly 7 to 6. This is the ratio at Alexandria, in 31° N. The Common cubit being = 18·24 inches = 6 × 3·04 in. The Black cubit was = 21·28 „ = 7 × 3·04 in. Two-thirds of this cubit were taken for The Black foot = 14·186 inches, divided into 16 digits of the 24 digits or qiráts of the cubit. This cubit and foot are still in use. The old nilometer on the island of Al-Rauzah (Rode) near Cairo has its scale in cubits of this standard, and measurement of the worn scale gives 21·29 inches for the cubit. The cubit and foot of Al-Mamūn are the basis of measures and of weights which spread from Egypt to every country in Europe. The story of the five cubits, ancient and medieval, has shown that they were all derived, directly or indirectly, from the meridian measurement of the earth, some of them being probably instituted with the desire to make them representative of the relation of latitude and longitude.