Plate XIII.—The data for this Plate are furnished by Maury's Storm and Rain Charts, including observations for 107,277 days in the North Atlantic, and 158,025 in the South; collated by Lieutenant J. J. Guthrie, at the Washington Observatory, in 1855.
The heavy vertical lines, 5°, 10°, 15°, etc., represent parallels of latitude; the other vertical lines, months; and the horizontal lines, per cents., or the number of days in a hundred.
The continuous curve line stands for phenomena in the North, and the broken curve line for phenomena in the South Atlantic. Thus the Gales' Curve shows that in every hundred days, and on the average, in the month of January of different years, there have been observed, in the northern hemisphere, 36 gales (36 per cent.) between the parallels of 50° and 55°; whereas during the same time and between the same parallels in the southern hemisphere, only 10 gales on the average (10 per cent.) have been reported.
The fact is here developed that the atmosphere is in a more unstable condition in the North than in the South Atlantic; that we have more calms, more rains, more fogs, more gales, and more thunder in the northern than in the southern hemisphere, particularly between the equator and the 55th parallel. Beyond that, the influence of Cape Horn becomes manifest.
Plate XIV. (§ 839) shows the limits of the unexplored area about the south pole.
Plate XV. shows by curves the prevalence of winds with northing as compared with winds with southing in them in each of the two hemispheres, north and south.
Plate XVI. shows the Barometric Curve projected according to actual observations at sea, from the parallel of 78° north to the parallel of 56° south, and carried thence to the poles, by conjecture and in conformity with indications.
