A natural register thus re-enforces the written records, and both together compose a body of evidence not to be gainsaid. Thus the depth of muddy silt accumulated round the pedestals of monuments is an irreproachable index of their age. In the eminent position he occupied, Eusebius might succeed in perverting the received book-chronology; but he had no power to make the endless trade-wind that sweeps over the tropical Pacific blow a day more or a day less; none to change the weight of water precipitated from it by the African mountains; none to arrest the annual mass of mud brought down by the river. It is by collating such different orders of evidence together--the natural and the monumental, the latter gaining strength every year from the cultivation of hieroglyphic studies--that we begin to discern the true Egyptian chronology, and to put confidence in the fragments that remain of Eratosthenes and Manetho. [Sidenote: Astronomy of Eratosthenes.] [Sidenote: Attempts of Aristarchus to find the distance of the sun.] At the time of which we are speaking--the time of Eratosthenes--general ideas had been attained to respecting the doctrine of the sphere, its poles, axis, the equator, arctic and antarctic circles, equinoctial points, solstices, colures, horizon, etc. No one competent to form an opinion any longer entertained a doubt respecting the globular form of the earth, the arguments adduced in support of that fact being such as are still popularly resorted to--the different positions of the horizon at different places, the changes in elevation of the pole, the phenomena of eclipses, and the gradual disappearance of ships as they sail from us. As to eclipses, once looked upon with superstitious awe, their true causes had not only been assigned, but their periodicities so well ascertained that predictions of their occurrence could be made. The Babylonians had thus long known that after a cycle of 223 lunations the eclipses of the moon return. The mechanism of the phases of that satellite was clearly understood. Indeed, Aristarchus of Samos attempted to ascertain the distance of the sun from the earth on the principle of observing the moon when she is dichotomized, a method quite significant of the knowledge of the time, though in practice untrustworthy; Aristarchus thus finding that the sun's distance is eighteen times that of the moon, whereas it is in reality 400. In like manner, in a general way, pretty clear notions were entertained of the climatic distribution of heat upon the earth, exaggerated, however, in this respect, that the torrid zone was believed to be too hot for human life, and the frigid too cold. Observations, as good as could be made by simple instruments, had not only demonstrated in a general manner the progressions, retrogradations and stations of the planets, but attempts had been made to account for, or rather to represent them, by the aid of epicycles. [Sidenote: Biography of the Ptolemies.]