Even to us, with nineteenth-century science within reach, how mysterious a river seems, as, in the quiet gloaming or in the grey dawn, it glides along beneath overhanging trees, and how full of life it is when, swollen by rain, it rushes forward in a resistless flood! How much more awe-inspiring it must have been to men ignorant of the commonest laws of Nature! Well might its channel be regarded as the home of a spirit eager to waylay and destroy the too-venturesome passer-by. Rivers, however, were not always reckoned the enemies of man, for experience showed that they were helpful, as well as hurtful, to him. The Tiber, for instance, was regarded with reverence by the ancient inhabitants of Rome. Who does not remember the scene in one of Macaulay's Lays, where, after the bridge has been hewn down to block the passage of Lars Porsena and his host, the valiant Horatius exclaims-- "O Tiber! father Tiber! To whom the Romans pray; A Roman's life, a Roman's arms, Take thou in charge this day?" Then with his harness on his back he plunges headlong into the flood, and reaches the other side in safety. In Christian art pagan symbolism continued long to flourish. Proof of this bearing on the present subject is to be found in a mosaic at Ravenna, of the sixth century, representing the baptism of Christ. The water flows from an inverted urn, held by a venerable figure typifying the river god of the Jordan, with reeds growing beside his head, and snakes coiling around it.