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FORE:A pressure wheel, like a steam-engine, must include running contact between water-tight surfaces, and like a rotary steam-engine, this contact is between surfaces which move at different rates of speed in the same joint, so that the wear is unequal, and increases as the speed or the distance from the axis. When it is considered that the most careful workmanship has never produced rotary engines that would surmount these difficulties in working steam, it can hardly be expected they can be overcome in using water, which is not only liable to be filled with grit and sediment, but lacks the peculiar lubricating properties [41] of steam. A rotary steam-engine is in effect the same as a pressure water-wheel, and the apprentice in studying one will fully understand the principles of the other.We have now to consider how the philosophy of the empire was affected by the atmosphere of supernaturalism which surrounded it on every side. Of the Epicureans it need only be said that they were true to their trust, and upheld the principles of their founder so long as the sect itself continued to exist. But we may reckon it as a first consequence of the religious reaction, that, after Lucretius, Epicureanism failed to secure the adhesion of a single eminent man, and that, even as a popular philosophy, it suffered by the competition of other systems, among which Stoicism long maintained the foremost place. We showed in a former chapter how strong a religious colouring was given to their teaching by the earlier Stoics, especially Cleanthes. It would appear, however, that Panaetius discarded many of the superstitions accepted by his predecessors, possibly as a concession to that revived Scepticism which was so vigorously advocated just before his time; and it was under the form imposed on it by this philosopher that Stoicism first gained acceptance in Roman society; if indeed the rationalism of Panaetius was not itself partly determined by his intercourse with such liberal minds as Laelius and the younger Scipio. But Posidonius, his successor, already marks the beginning of a reactionary movement; and, in Virgil, Stoical opinions are closely associated with an unquestioning acceptance of the ancient Roman faith. The attitude of Seneca is much more independent; he is full of contempt for popular superstition, and his god is not very distinguishable from the order of Nature. Yet his tendency towards clothing philosophical instruction in religious terms deserves notice, as a symptom of the superior facility with which such terms lent themselves to didactic purposes. Acceptance of the universal order became more intelligible under the name of obedience to a divine decree; the unity of the human race and the obligations resulting therefrom242 impressed themselves more deeply on the imaginations of those who heard that men are all members of one body; the supremacy of reason over appetite became more assured when its dictates were interpreted as the voice of a god within the soul.375