78 Mr. H. H. Howorth on 



Let there be produced a concentration of aqueous vapour 

 over a particular region, or let one region show a higher 

 temperature than what prevails around it, then from the 

 different densities and different pressures thereby produced 

 the equilibrium of the atmosphere is destroyed, and, as 

 might be expected from the laws of aerial fluids, movements 

 of the air or winds set in to restore the equilibrium." 

 (E.B., XVI., 143). This is incontestible. So is the following: 

 " All winds may be regarded as carried directly by dif- 

 ferences of atmospheric pressure, just as the flow of rivers 

 is caused by differences of level, the motion of the air and 

 the motion of the water being both referable to gravitation. 

 The wind blows from a region of higher towards a region 

 of lower pressure — in other words, from where there is a 

 surplus to where there is a deficiency of air" {id.). The next 

 argument is equally sound. "So far as is known, differences 

 of atmospheric pressure, and consequently all winds, 

 originate in changes occurring either in the temperature or 

 the humidity of the air over restricted regions. Thus, if 

 two regions contiguous to each other come to be of unequal 

 temperature, the air of the warmer region, being specifically 

 lighter, will ascend, and the heavier air of the colder regions 

 will flow in below to take its place. Of this class of winds 

 the sea and land breezes are the best examples. Again, if 

 the air of one region comes to be more highly charged with 

 aqueous vapour than the air of surrounding regions, the air 

 of the more humid region, being lighter, will ascend, while 

 the heavier air of the drier regions will flow in below, and 

 take its place. Some part of the vapour will be condensed 

 into cloud or rain as it ascends, heat will be thereby 

 disengaged, and the equilibrium still further disturbed" {id.). 

 The paragraphs I have quoted state clearly some of the 

 elementary postulates of general pneumatics. Let us now 

 apply them to the earth. As is familiar to everybody, the 

 .sun's rays exercise their most potent influence in the neigh- 



