longitudinal measurement on the same basis. The measure of the hour was the parasang (3/4 of a geographical mile), and the thirtieth part of the parasang was the stadium. Till we obtain help from the inscriptions we must remain acquainted only with the Persian name of the first measure and the Greek name of the second. At the equator the sun was supposed in every hour to traverse a distance of thirty stadia. On this system also the Chaldæans fixed the length of their cubit. The stadium was divided into 360 cubits, and the sixth part of the stadium, or plethron, into sixty cubits, and the foot was fixed at 3/5 of this cubit. Consequently the Babylonian cubit was fixed at twenty-one inches of our measure (525 millimeters).[395] From this division of the sphere the Babylonians, though aided by very simple instruments, the polus and gnomon,[396] arrived at very exact astronomical observations and results. They discovered a period of 223 months, within which all eclipses of the moon occurred in a similar number and equal extent. By means of this period they fixed the average length of the synodic and periodic month with such accuracy that our astronomers here found the first to be too large by four seconds only, and the last by one second. Their observations of ten lunar eclipses, and three conjunctions of planets and fixed stars, have come down to us. The oldest of these observations is that of a lunar eclipse of the year 721 B.C., which took place "a good hour after midnight." The second took place in 720 B.C., "about midnight;" the third in the same year "after the rising of the moon." In these observations also our astronomers have found but little to correct. As the Chaldæans brought their measures of the sphere, of time and length into correlation, so also they attempted to preserve the same relation in their cubic measures and weights. For their weights and cubic measures the division of the units into sixtieths (minæ, _i.e._ parts) was retained. The quadrantal, or _Maris_, contained one Babylonian cubic foot, and the sixtieth part of this was the _Log_. The weight in water of a Babylonian cubic foot was, according to the statistics of our physicists, about sixty-six pounds (32,721 kilogrammes), but the Chaldæans reckoned it at only 60-3/5 pounds (30,300 kilogrammes).[397]