These boats were generally of very considerable size, and they carried, in addition to their cargo and crew, one or more beasts of burden--generally asses or mules. These animals were allowed the pleasure, if any pleasure it was to them, of sailing thus idly down the stream, for the sake of having them at hand at the end of the voyage, to carry back again, up the country, the skins, which constituted the most valuable portion of the craft they sailed in. It was found that these skins, if carefully preserved, could be easily transported up the river, and would answer the purpose of a second voyage. Accordingly, when the boats arrived at Babylon, the cargo was sold, the boats were broken up, the skins were folded into packs, and in this form the mules carried them up the river again, the boatmen driving the mules as they walked by their side. Babylon was a city of immense extent and magnitude. In fact, the accounts given of the space which it covered have often been considered incredible. These accounts make the space which was included within the walls four or five times as large as London. A great deal of this space was, however, occupied by parks and gardens connected with the royal palaces, and by open squares. Then, besides, the houses occupied by the common people in the ancient cities were of fewer stories in height, and consequently more extended on the ground, than those built in modern times. In fact, it is probable that, in many instances, they were mere ranges of huts and hovels, as is the case, indeed, to a considerable extent, in Oriental cities, at the present day, so that it is not at all impossible that even so large an area as four or five times the size of London may have been included within the fortifications of the city. In respect to the walls of the city, very extraordinary and apparently contradictory accounts are given by the various ancient authors who described them. Some make them seventy-five and others two or three hundred feet high. There have been many discussions in respect to the comparative credibility of these several statements, and some ingenious attempts have been made to reconcile them. It is not, however, at all surprising that there should be such a diversity in the dimensions given, for the walling of an ancient city was seldom of the same height in all places. The structure necessarily varied according to the nature of the ground, being high wherever the ground without was such as to give the enemy an advantage in an attack, and lower in other situations, where the conformation of the surface was such as to afford, of itself, a partial protection. It is not, perhaps, impossible that, at some particular points--as, for example, across glens and ravines, or along steep declivities--the walls of Babylon may have been raised even to the very extraordinary height which Herodotus ascribes to them.