SOME folk tend to look perennially blank. Or to put it another way, as Shakespeare did when writing the Merchant Of Venice
There are a sort of men whose visages Do cream and mantle like a standing pond And do a wilful stillness entertain
Perhaps he could have worked his simile even harder, since it is said that sometimes the appearance of the village pond predicts the future. In particular, if a pond acquires a temporary scum it is said to be a sign of rain.
To understand why, cast your mind back to Christmas and to the little knickknacks found in Christmas crackers. Among them, sometimes, you will find a little plastic figure of a deep sea diver. When you put this in a bottle filled with water and replace the cork, it is possible to make the figure rise to the top or sink into the depths by a slight manipulation of the cork.
The device is called a "Cartesian diver" and is named, for some reason that I do not know, after the 17th century French philosopher Renew Descartes.
The working principle is simple. The figure's helmet has a little hole, in which a bubble of air is trapped when the diver is immersed. If we assume that, to start with the diver is floating near the surface of the water and 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.
Now if we recall that famous principle that caused Archimedes to jump from his bath and shout "Eureka", we know that the upward thrust on a submerged object is equal to the weight of the volume of liquid replaced.
When the diver's bubble shrinks in size, the combined volume of diver and bubble has decreased. The figure's buoyancy, therefore, is less than it was previously. With increasing pressure, the bubble shrinks to the extent that the weight of the diver overcomes his 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 "effervescence 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. As the pressure drops, the bubbles in the pond increase in size, become more buoyant and shoot to the surface, bringing with them thousands of tiny specks of adhering mud to cause a scum.