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barometer measures exactly the weight of a perpendicular column of the air immediately above it, it in consequence rises and falls according as the atmospheric waves reach, pass over, and leave it.
The next thing worthy of notice is the fact that the barometer often rises just before the gale comes on; a fact which when properly understood will always put the seaman on his guard and give him timely warning, but a dangerous thing when not understood, as it tends to throw the seaman off his guard, and lull him to sleep, when he ought really to be wakeful and watching.
The cause of this phenomenon is evidently the air being banked up in front and by the pressure of the advancing gale; and can best be demonstrated by moving a large tub through a body of water, when it will be found that the water in front of the tub will be higher, and that the water behind the tub will be lower than that portion of the water which is not affected by the movement of the tub;—and this is just the case with the barometer, which as a rule is above in front of, and lower behind the gale than its average height, for the time being, in places not affected by the storm.
But we also find the barometer standing lowest at or near the centre, and this may be accounted for by the fact of a partial vacuum existing there from condensation of vapor, and the surrounding air rushing in to supply the vacancy, leaves room for neighbouring currents to expand and become lighter, a process which on this principle I suppose to be going on from the centre towards every part of the outer circumference; and the gradual fall of the barometer as a matter of course follows.
Now as the severity of a typhoon is measured by the velocity of the winds within the storm-disc—and we admit on the first principles that the greater the rapidity with which the currents revolve around the focus, the greater the condensation of vapour there, and hence the more perfect the vacuum at the centre—we shall have an explanation of the reasons why the barometer falls lower in a severe, than in a more moderate gale.
