Erstwhile readers of A.A. Milne may remember the bullying local squire, Sir Brian Botany: Brian had a pair of boots with great big spurs on; A fighting pair of which he was particularly fond.
On Wednesday and on Saturday, Especially on the latter day, He'd collect the passing villagers and kick them in the pond. Now, apart from the social and humanitarian consequences of the knight's deplorable behaviour, this twice weekly disturbance would render the pond meteorologically useless. Left to settle for a while its appearance might have provided the village with a useful predictor of the weather.
To understand why, it is necessary to recall that village shops, maybe in Sir Brian's time and certainly in mine, used to purvey paper containers of knick-knacks known as lucky-bags. For 2d one might acquire a whistle, a puzzle, a sticky sweet or two, and sometimes a little toy known as a Cartesian diver. It was a plastic figure of a deep-sea diver; when you put it in a bottle filled with water and replaced the cork, it was possible to make the figure rise to the top or sink into the depths by a slight manipulation of the cork.
The working principle is simple, and was allegedly discovered by the 17th century French philosopher, Rene Descartes. The figure's helmet has a cavity, in which a bubble of air is trapped when the diver is immersed. If the diver is floating near the surface of the water, supported by the bubble's buoyancy, then pushing downwards on the cork increases pressure everywhere inside the bottle; this causes compression of the air that forms the bubble, and the volume of the bubble is decreased. Since the diver's buoyancy, by the principle of Archimedes, depends on the combined volume of the diver and his bubble, the shrinking of the bubble ultimately overcomes the diver's tendency to float - and he sinks to the bottom of the bottle. To make him rise again, it is only necessary to adjust the cork so that pressure is reduced, the bubble expands, and the diver shoots to the surface once again.
Now the theory goes that the process of decay produces bubbles at the bottom of a village pond that cling to particles of mud. Rain is often preceded by a fall in atmospheric pressure, and as the pressure drops the bubbles in the pond increase in size, and shoot to the surface bringing with them thousands of tiny specks of adhering mud. Hence the observation that the sudden appearance of a scum on the village pond can be taken as a sure sign that rain is in the offing.