Plutarch's Natural Questions by Plutarch

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.