Page:Physical Geography of the Sea and its Meteorology.djvu/444

418 degrees less cold than it would have been if it had no vapour to condense nor latent caloric to give out—that is, it will be about forty-eight degrees warmer than the surrounding air at the same height; it will, therefore (without making any allowance for the higher dew-point of the ascending current), be about one tenth lighter than the surrounding colder air, and, of course, it will continue to ascend to the top of the atmosphere, spreading out in all directions above as it ascends, overlapping the air in all the surrounding regions in the vicinity of the storm, and thus, by increasing the weight of the air around, cause the barometer to rise on the outside of the storm, and fall still more under the storm-cloud by the outspreading of air above, thus leaving less ponderable matter near the centre of the upmoving column to press on the barometer below. The barometer thus standing below the mean under cloud in the central regions, and above the mean on the outside of the cloud, the air will blow on all sides from without, inward, under the cloud. The air, on coming under the cloud, being subjected to less pressure, will ascend and carry up the vapour it contains with it, and as it ascends will become colder by expansion from constantly diminishing pressure, and will begin to condense its vapour in cloud at the height indicated before, and thus the process of cloud-forming will go on. Now it is known that the upper current of air in the United States moves constantly, from a known cause, towards the eastward, probably a little to the south of east; and as the upmoving column containing the cloud is chiefly in this upper current of air, it follows that the storm-cloud must move in the same direction. And over whatever region the storm-cloud appears, to that region will the wind blow below; thus the wind must set in with a storm from some eastern direction, and, as the storm-cloud passes on towards the eastward, the wind must change to some western direction, and blow from that quarter till the end of the storm."

788. Doves law.—According to Dové's "Law of Rotation," which is said to hold good in the northern hemisphere, and is supposed to obtain in the southern also, the wind being N.W. and veering, it ought to veer by W. to S.W., and so on, against the hands of a watch. This "law" is explained thus: Suppose a ship be in S. lat. off Cape Horn as at a, with a low barometer