§ 787. STORMS, HURRICANES, AND TYriiOOXS 419 



forming ; the current of the air, however, will continue to ascend, 

 and grow colder about half as much as it would do if it had no 

 vapor in it to condense; and when it has risen high enough to 

 have condensed, by the cold of expansion from diminished press- 

 ure, one hundredth of its weight of vapor, it will be about forty- 

 eight degrees less cold than it would have been if it had no vapor 

 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 con- 

 tinue to ascend to the top of the atmosphere, spreading out in all 

 directions above as it ascends, overlapping the air in all the sur- 

 rounding regions in the vicinity of the storm, and thus, by in- 

 creasing 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 ponder- 

 able matter near the centre of the upmoving column to press on 

 the barometer below. The barometer thus standing below the 

 mean under the 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 vapor it contains with it, and as it ascends will become cold- 

 er by expansion from constantly diminishing pressure, and will 

 begin to condense its vapor in cloud at the height indicated be- 

 fore, 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, toward the eastward, probably a 

 little to the south of east; and as the upmoving column contain- 

 ing 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 toward 

 the eastward, the wind must change to some western direction, 

 and blow from that quarter till the end of the storm."" 



* The Fourth Meteorological Report of Prof. James P. Espy ; Senate Doc. 65, 

 34th Congress, 3d session. 



