Note 138 — Chapter XXI
Reading edition
In all hurricanes hitherto observed, the sinking of the mercury, and the increase of the wind, have been more or less regularly progressive till within three or four hours’ sail of the centre of the storm; and in one class they have continued so even to the centre; while in another class, and by far the most terrible, the depression of the mercury has been sudden and excessive when within that distance of the centre, and the violence of the tempest far beyond the average. When a ship is within 50 or 60 miles of the centre, the storm has the mastery, and seamanship is of little avail. Rules for avoiding this calamity, and for managing a ship when involved in a hurricane, are fully explained in the “Sailor’s Horn-Book for the Laws of Storms,” by H. Piddington, Esq., President of the Marine Courts of Inquiry at Calcutta. The following approximate table is given by him, to serve as a guide till better data shall be obtained:—
Average fall of the barometer per hour. Distance of a ship from the centre of the storm, in miles.
From 0·020 to 0·060 From 250 to 150
From 0·060 to 0·080 From 150 to 100
From 0·080 to 0·120 From 100 to 80
From 0·120 to 0·150 From 80 to 50
The rate of fall per hour doubles after the storm has lasted six hours, and within three hours of the centre of the hurricane the mercury will fall four times as fast, if it be of the violent class.
Colonel James Capper discovered the rotatory motions of storms, and W. C. Redfield, Esq., of New York, was the first who determined their laws. Colonel Reid, Governor of Barbadoes, and Dr. Thom, of the 86th regiment, have also written on the subject.