418 PHYSICAL GEOGRAPHY OF THE SEA, AND ITS METEOROLOGY. 



degrees less cold than it would liiive been if it liad 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 wdth it, and as it ascends 

 will become colder by expansion from constantly dimini.«<hing 

 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 tbe 

 same direction. And over whatever region the storm-cloud 

 appears, to that region wdll the wdnd 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 Dove's *' Law of dotation," 

 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 



* The Fourth Meteorological Eeport of Prof. James P. Espy; Senate Doc. 

 65, 34th Congress, 3rd session. 



