702 



SCIENCE. 



[Vol. II., No. 4;i. 



uprising oil the desert, inasmuch as the ocean's 

 calm surface is too smooth to offer aii}' distinct 

 starting-point for the np-draught. There are, 

 however, several phmsible ways out of tlic 

 difRcultj^ It is possible that localized warmth 

 and expansion where tlic air is calmest' may 

 produce agentle up-current, which, once begun, 

 will be soon well established. Agaiu : an ex- 

 cess of evaporation will cause a rapid upward 

 diffusion of vapor. It will reach an altitude 

 where it must condense, and form a cloud-layer, 

 and thereby warm the surrounding air both by 

 its latent heat and by catching the warmth of 

 the sun's rays ; and, as this will go on at a 

 considerable altitude, it will be especially efl'ec- 

 tive. Finally, if after a time of calm a breeze 

 should opportunely penetrate the district from 

 an adjoining one of higher pressure, an ascend- 

 ing current would surety be started. In some 

 such waj- a gradual overturning of the inibal- 

 anced air must begin, and its further action is 

 now to be traced. 



The rising mass expands as it escapes from 

 the pressure of the air that it leaves below, and 

 in expanding it is mechanicallj- cooled. As it 

 cools, some of the vapor with which it is well 

 charged condenses into cloud, and. on accumu- 

 lating, soon begins to fall as r.ain. Here we 

 have the entrance of a new and potent cause of 

 disturbance, — the briuging-forth of a great 

 amount of energ}' in the form of heat from the 

 condensation of the vapor.- It is probable that 

 this aid to the up-draught seldom takes the ini- 

 tiative : it waits till some other cause begins 

 the upsetting, and then falls to with a will to 

 help it along. 



This effect of condensation is so im[)ortant 

 that it maj' well be considered a little more 

 closely. As water evaporates, its molecules are 

 spread widety apart, and take on a xnvy active 

 motion ; but in doing so they must be furnished 

 with energy in some form, for the}' cannot de- 

 velop out of nothing the energy needed for 

 their increased activity. As a general rule, 

 the desired supply is found in the sun's radi- 

 ant heat : so, when water evaporates I'rom 

 the sea-surface, it takes to itself nearly all tlu' 

 energy that comes down in tlie sun's rays, 

 and thereb}^ its molecules arc enlivened u|i to 

 the point of vaporization. It will be readily 

 understood, that, if heat-energy be taken by the 

 water and transformed into vapor-energy, it 

 can no longer make itself felt as heat ; and, so 

 far as our senses are concerned, it is lost or 

 hidden, and for this reason is called 'latent 

 heat.' The term is misleading and improper, 

 for it implies that the sun's enei'gy still remains 

 somewhere in the vapor as a kind of heat that 



we cannot feel ; but this is wrong, for as heat it 

 no longer exists. It will be further seen, that, 

 when the vapor is condensed back again into 

 water, all its vapor energ}- must take some otiier 

 form : it uuist abandon the vapor molecules, 

 and allow them to quiet down and approach 

 one another as they resume the liquid condi- 

 tion : and the energy thus thrown out of em- 

 ployment must make itself felt in some other 

 way. We are therefore prepared to lind that 

 condensation is attended with the i)roduction 

 of just as much heat-energj' as was lost in tlie 

 process of evaporation. This is of capital im- 

 portance in the understanding of storms. 



It has already been seen, that the cause of 

 continued action in a desert-whirl is found in 

 the excessive warmth of the lower strata ; iu 

 •\irtue of which the air in the ascending column 

 finds itself warmer, and hence lighter, than the 

 surrounding air, and consequentlj' is impelled 

 to rise as oil rises through water. It was fur- 

 ther noted, that the ascending whirl will con- 

 tinue as long as it is supplied with excessively 

 warm air at the base ; but, as soon as the bot- 

 tom air is not more than l.G° warmer than the 

 air three hundred feet above it, the whirl will 

 die awaj'. In the case of an ascending column 

 of air satni'ated with vapor, it would also, as 

 iu the previous case, expand as it rose to higher 

 levels of less pressure, and, in consequence of 

 this expansion, it would cool. But when satu-- 

 rated air is cooled, some of its vapor must 

 condense ; and when vapor condenses, heat is 

 evolved ; and the heat thus produced will partly 

 make up for the loss of heat by expansion, and 

 therefore the ascending column of moist air 

 will not be allowed to cool so fast as if it had 

 not been saturated with vapor. Several im- 

 portant consequences now follow. In the first 

 place, a less warming at the base is needed to 

 produce unstable equilibrium in saturated than 

 in dry air. In the latter, the turning-point is 

 reached when there is a 

 ' difference of 1.0° F. be- 



tween the temperatures of 

 the suiface-air and that 

 three hundred feet above. 

 In the former, if the sur- 

 face-temperature be 80°, as 

 is common in the Baj^ of 

 Bengal, a difference of only 

 0.0° is required. In other 

 words, if a mass of drj' air 

 Fig. 5. at 80° rise three hundred 



feet, its temperature falls 

 to 78.4° : if a mass of saturated air at the same 

 temperature (fig. 5) rise through the same dis- 

 tance, it is cooled only to 7!l.I°; and conse- 



Z9.4- 



M 



