21G 



such condensation may, and no doubt does aid in producing 

 atmospheric movements. See Meteor. ^ Arts. 165^ 171. 



But as respects the statement that I abandon the Hadleian 

 theory of the winds, I can only say that nothing is farther 

 from my thoughts, and that I must protest against any such 

 mode of expressing my views. It is true that in describing 

 the modus oj^eraildi in which the sun's heat, acting on the 

 equatorial regions of the earth, ocean, and air, produces that 

 ascentional movement and overflow of the atmosphere which 

 is what Hadley assumes as the primum mobile of his theory, 

 I have {MeleoroL, Arts. 54, ^^, 58) brought expressly and 

 prominently into view the very considerable share Avhich the 

 generation and ascent of vapour has in producing that result, 

 but without ignoring or in any way unduly depreciating the 

 effect of the rarefaction of the air itself. On the contrary, it 

 is expressly said (Art. 55), " The general effect" (of the two 

 causes) '^ is similar, and as the sun cannot generate vapour 

 without at the same time heating the air, it is impossible to 

 separate their dynamical effects. Whether the air go forth 

 from its place proprio motu or be jostled out of it by the 

 introduction of a lighter medium, the local relief of pressure 

 is equally produced." When Hadley wrote, the distinction 

 between air and vapoi.r was not recognised. He took the 

 atmosphere en masse and attributed to it an ascentional move- 

 ment due to the heat of the sun, by the process of " rarefac- 

 tion;" and this, so worded, remains true, although such 

 rarefaction be a more complex process than he understood it 

 to be. 



The displacement of air by vapour, the disturbance of 

 statical equilibrium, and the dynamical effects consequent 

 thereon, are no new principles in meteorology. They have 

 been strongly insisted on, and, if I may venture to say so, 

 rather over insisted on, and with a premature air of reduction 

 to computative precision, by Daniell in his Meteorological 

 Essays, and perhaps hardly enough insisted on in my work 



