Seneca begins his discussion of the various forms of water by grouping them into two chief classes, standing in collected sheets, as in lakes, or running in channels, as rivers above ground and springs underneath. After a brief enumeration of various qualities of water, he inquires whence the vast volume of water comes that is carried down by rivers to the sea, and how it happens that neither is the earth sensible of this daily loss, nor does the ocean show any perceptible gain. He merely notices the opinion which some philosophers had expressed that the sea does not get larger because it restores to the earth as much water as it receives, allowing its own saline water to sink through endless subterranean winding passages wherein it is purged of its saltness and rises on the land as pure fresh water.[112] Another view, that most of the water supplied by rain eventually finds its way into the rivers, is approximately that at which modern research has arrived, but it meets with our philosopher’s strong opposition. His first objection is derived from his own observation. He tells us that, as a diligent digger among his vines, he can confidently affirm that even the heaviest rain does not penetrate to a depth of more than ten feet from the surface. What is not absorbed by the upper crust of the ground runs at once into river channels, and thence into the sea. He next asks how rain, which immediately flows off the surface of naked rocks, can possibly be the source of the springs and rivers that issue from bare crags, or how springs that appear on the very summit of mountains can be due to rain. Though he could not but be aware of the close connection everywhere observable between evaporation, rainfall, and the volume of springs and rivers, he does not seem to have reflected on its meaning--how in seasons of drought the surface waters fail first, how by degrees the springs begin to lessen and even to cease, how the rivers dwindle until in many cases their beds become almost or quite dry, and yet how, when welcome rains set in, the springs and rivers gradually resume the bulk they had before the dry weather impoverished them. He had made no study of the way in which rain percolates through the soil, subsoil, and rocks underneath, though there are places, such as his vineyard may have been, where, from some impervious material, only a feeble or inappreciable flow of moisture descends beyond a few feet from the surface. Nor was he aware of the innumerable lines of joint by which the most solid rocks are traversed, and which serve as passages for the descent and ascent of water. Had he climbed many mountains, he would have failed to find a spring on the summit of any one of them, unless there had been a sufficient area of higher ground at hand to serve for the supply of the water. [112] This is the view expressed by Lucretius: