When the first rain gauge was set up, who it was that had the bright idea, or even in what country the whole rigmarole began, we just have no idea.
We know, however, that the Greeks kept systematic weather records of a kind as early as the 5th century BC, and that quantitative rainfall measurements were made in Palestine in the first century AD.
Since the very earliest times, the design of the rain-gauge has varied very little. Those used nowadays have a funnel 5 inches in diameter to catch the water. The catch drains into a bottle, which is then assumed to contain the amount of rain which has fallen on the 5-inch circle above. Elementary mathematics yields the required figure, which it is hoped will approximate to "the depth to which a flat and impermeable surface would be covered by water in a given period, assuming that none of the liquid disappears by run-off or evaporation".
It is all, however, highly theoretical, since several factors diminish the accuracy of measurement. Some of the rain adheres to the sides of the funnel and may evaporate without entering the bottle at all.
Then, if the rain is heavy, some of it is lost by splashing out of the funnel, or rain which falls on the ground nearby may splash in.
And by far the greatest error arises from the wind; the rain-gauge itself, projecting above the ground, causes swirls and eddies in the local airflow which result in a loss of catch.
This could be avoided by sinking the instrument into the soil with the top of the funnel flush with the ground, but then the "splashing" problem becomes worse.
Moreover the contents of a rain-gauge are conceptually, in Cicero's words, fluctus in simpulo, "a storm in a teacup".
Although the gauge is not much bigger than a teacup, its contents must be assumed to have captured the whole character of any rainstorm extending over an area of several square miles.
Despite its faults, however, the common rain-gauge has the twin advantages of being both cheap and simple, two very important factors because of the large number of sites required to get an adequate picture of the national rainfall pattern.
The statistics obtained may not be accurate to the nearest half a millimetre, but they give a good idea of the variation of rainfall from place to place, and an approximation to the actual rainfall which is acceptable for most purposes.
Die Politik, said Bismarck, ist die Lehre von M÷glichen: "Politics is the art of the possible". The same applies in meteorology.