1428. Respecting the words engraved on the register plate of the barometer, it may be observed, that their exact correspondence with the state of the weather cannot be strictly relied upon, though they will in general agree with it as to the mercury rising and falling. The engraved words are to be regarded only as indicating probable consequences of the varying pressure of the atmosphere. The barometer, in fact, only shows the pressure of the aerial column; and the precipitation of rain, or the agitations of the atmosphere, are merely events which experience has shown usually to accompany the sinking of the mercurial column, but are not necessarily connected with fluctuations of pressure. The words deserve to be particularly noticed when the mercury removes from 'changeable' upwards; as those on the lower part should be adverted to, when the mercury falls from 'changeable' downwards. In other cases, they are of no use: for, as its rising in any part forebodes a tendency to fair, and its falling to foul, weather, it follows that, though it descend in the tube from 'settled' to 'fair,' it may nevertheless be attended with a little rain, and when it rises from the words 'much rain' to 'rain' it shows only an inclination to become fair, though the wet weather may still continue in a less considerable degree than it was when the mercury began to rise. But if the mercury, after having fallen to 'much rain,' should ascend to 'changeable,' it foretells fair weather, though of a shorter continuance than if the mercury had risen still higher; and so, on the contrary, if the mercury stood at 'fair' and descends to 'changeable,' it announces foul weather, though not of so long a continuance as if it had fallen lower.
