“It is remarkable,” said Mrs. Merton, “that it is not navigable in any part of its long course of forty-two miles; and that occasionally when the weather has been dry a long time, it disappears altogether. At the foot of Box-Hill, near Dorking, with regard to this phenomenon, it is supposed that there are cavities, or hollow places, under ground, which communicate with the bed of the river, and which are filled with water in ordinary seasons, but, in times of drought, become empty, and absorb the water from the river to refill them. When this is the case, the bed of the river becomes dry, and Burford bridge often presents the odd appearance of a bridge over land dry enough to be walked on. The river, however, always rises again about Letherhead, and suffers no further interruption in its course.” While Mrs. Merton was speaking, the train had continued whirling on, and they had long passed the sluggish Mole, and had caught a glance of the more useful Wey; a river of about the same length as the Mole, but which has the advantage of being navigable for a great part of its course; and Agnes had watched the inhabitants of the little cottages which bordered the line of the railway trimming their gardens, and spreading their seeds out to dry in the sun. She had been amused, in one place, observing the careful manner in which a stack of faggots had been thatched, to keep it from the rain; and, in another, by observing the delight of a number of pigs, which had been turned into a stubble field, from which the corn had just been carried; and which ran about, grunting and capering, in a manner which none but pigs could ever accomplish. The train now passed another stream; and Agnes asked what river it was. “It is not a river,” said Mrs. Merton, “but the Basingstoke canal.” “How do you know it is a canal, mamma?” asked Agnes. “Its banks are straight and regular,” said Mrs. Merton, “which shows that they have been formed artificially; and the water is as deep close to the bank as it is in the centre: whereas, in rivers, the banks are generally irregular, and the water is shallower near them. Besides, there can be no doubt about this being a canal, for there, you see, is a lock.” “Now, mamma,” said Agnes, “you have told me a great many things that I do not understand. I thought a canal had been only to supply the place of a river; and, if that is the case, I do not see why its banks should be different; and I do not know what you mean by a lock.”