Page:Physical Geography of the Sea and its Meteorology.djvu/480

454 the poles, is, in the northern hemisphere, between the parallels of 25° and 50°. In the southern the field of battle is narrowed down to a single belt (between 35° and 40°); here the two winds exactly counterbalance each other. As the seaman proceeds from this medial belt, the winds increase belt for belt very nearly pari passu on the polar side, the polar winds—on the equatorial, the equatorial winds, gaining more and more in days of annual duration, and more and more in average velocity each.

853. Extent of the polar indraught.—The fact that the influence of the polar indraught upon the winds should extend from the antarctic to the parallel of 40° S., while that from the arctic is so feeble as scarcely to be felt in 50° N., is indicative enough as to difference in degree of aerial rarefaction over the two regions. The significance of this fact is enhanced by the "brave west winds," which, being bound to the place of greatest rarefaction, rush more violently and constantly along to their destination than do the counter-trades of the northern hemisphere. Why should these polar-bound winds of the two hemispheres differ so much in strength and prevalence, unless there be a much more abundant supply of caloric, and, consequently, a higher degree of rarefaction, at one pole than the other?

854. The rarefaction of the air over polar regions.—In the southern hemisphere—and our attention is now directed exclusively to that—the polar winds on the south side of 40° are very much stronger than are the equatorial winds on the north side of 35°: a fact indicative of a greater degree of rarefaction about the place of polar calms than we have in the equatorial calm belt.

855. Barometrical observations.—That such is the case is also suggested by the fact that the indraught into the antarctic calm place is felt (§ 854) at the distance of 50° from the pole all round, while the equatorial indraught is felt no farther than 35° from the equator; and that such is the case is proved by the barometer. Lieutenant Andrau, of the Meteorological Institute of Utrecht, has furnished us from the Dutch logs with 83,334 observations on the height of the barometer between the parallels of 50° N. and 36° S. at sea. Lieutenants Warley and Young have extracted from the log-books in the Washington Observatory, taken at random, 6,945 observations on the barometer south of the parallels of 40° at sea. Dr. Kane has furnished us with the mean height of the barometer in lat. 78° 37' N., according to 12,000 hourly observations made during his imprisonment of 17 months