402 PHYSICAL GEOGRAPHY OF THE SEA, AND ITS METEOROLOGY. 



and, in support of these positions, he advanced this theory: 

 " When the air in any locality acquires a higlier temperature or 

 a higher dew-point than that of the sorrounding regions, it is 

 specifically lighter, and will ascend ; in ascending, it comes under 

 less pressure, and expands ; in expanding from diminished 

 pressure, it grows colder about a degree and a quarter for every 

 hundred yards of ascent ; in cooling as low as the dew-point 

 (which it will do when it rises as many hundred yards as the 

 dew point at the time is below the temperature of the air in 

 degrees of Fahrenheit), it will begin to condense its vapour into 

 cloud ; in condensing its vapour into water or cloud, it will 

 ■evolve its latent caloric ; this evolution of latent caloric will 

 prevent the air from cooling so fast in its farther ascent as it did 

 in ascending below the base of the cloud now 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 vapour in 

 it to condense ; and when it has risen high enough to have 

 condensed, by the cold of expansion from diminished pressure, one 

 hundredth of its weight of vapour, it will be about forty-eight 

 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 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 

 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 



