When from his ethical lecture he turns to resume his physical disquisitions, it is the mysterious Nile to which he devotes attention. After a brief contradiction of the statement of some philosophers that the Nile and the Danube are similar in their characters, he enumerates some of the well-known peculiarities of the river of Egypt. A problem which greatly exercised the minds of the philosophers of antiquity, and which has only been finally solved in our own day, was the cause of the annual rise of the Nile on which the fertility of Egypt depended. Seneca says with justice that if the point of the river could be ascertained where the rise begins the question would be settled. He does not appear to have known much about the river, for he believed that the water is for the first time collected into a single channel at Philae. In his account of that place and of the cataract there (168, 169), he speaks of the river’s egress from Ethiopia, and of deserts which are crossed by the trade route to the Red Sea. In a subsequent part of the treatise he gives the interesting and important information that he had himself seen and conversed with two centurions who had been despatched by Nero to discover the source of the Nile (235). From them he learnt that they had penetrated far into the heart of Africa, and had reached a region of illimitable marshes where the river was so covered and impeded with vegetation that neither on foot nor by boat could it be ascended. There can be no doubt that these enterprising explorers had come to the _sudd_, which in recent years has been found so serious an impediment to navigation. They informed Seneca that in the marsh region they had seen with their own eyes “two rocks from which an enormous body of the river came out.” There are apparently no rocks along the course of the Nile in the present marsh region, which is a vast flat, and it is therefore difficult to conjecture to what the two military surveyors allude. Possibly they saw the mouth of some affluent of the main stream such as the Khor Adar, or the _sudd_ may have extended further north than it does now.