VII. WHY IN WINTER DO SHIPS SAIL SLOWER IN RIVERS, BUT DO NOT SO IN THE SEA? WHETHER, because the river-air, which is at all times heavy and slow, being in winter more condensed by the cold, does more resist sailing? Or is it long of the water rather than the air? For the piercing cold makes the water heavy and thick, as one may perceive in a water-clock; for the water passes more slowly in winter than in summer. Theophrastus talks of a well about Pangaeum in Thrace, how that a vessel filled with the water of it weighs twice as much in winter as it does in summer. Besides, hence it is apparent that the grossness of the water makes ships sail slower, because in winter river-vessels carry greater burthens. For the water, being made more dense and heavy, makes the more renitency; but the heat hinders the sea from being condensed or frozen.