The pressure exerted by the atmosphere was but little understood at that time, but Torricelli's discovery aided materially in solving the mystery.

The whole class of similar phenomena of air pressure, which had been held in the trammel of long-established but false doctrines, was now reduced to one simple law, and the door to a solution of a host of unsolved problems thrown open.
It had long been suspected and believed that the density of the atmosphere varies at certain times.

That the air is sometimes "heavy" and at other times "light" is apparent to the senses without scientific apparatus for demonstration.

It is evident, then, that Torricelli's column of mercury should rise and fall just in proportion to the lightness or heaviness of the air.

A short series of observations proved that it did so, and with those observations went naturally the observations as to changes in the weather.