298 LIGHT SCIENCE FOR LEISURE HOURS. 



itself, not in an increase of pressure over the south 

 tropical zone that is, not in an increase of height there 

 but in the extension of the surplus atmosphere into 

 the northern hemisphere. This would be shown by the 

 extension of the southern trade-winds to or beyond the 

 equator, so that the (so-called) equatorial zone of calms 

 should lie north of the equator. As this is really the 

 position occupied by the belt of calms, Maury's theory 

 appears to gain additional force by the coincidence. 



Another argument may be drawn from the analogy 

 of the low barometer in moist weather. In fact, it is 

 well known that Deluc explained this phenomenon in a 

 manner precisely accordant with the views expressed 

 by Maury. 



Despite the apparent force of these arguments, and 

 others that might be adduced, it will not be difficult, 

 I think, to show that neither is Maury's theory con- 

 sistent with known physical laws, nor (passing over this 

 objection) is the theory sufficient to account for the 

 grand phenomenon under consideration. 



It is quite true that a volume of aqueous vapour 

 weighs less than an equal volume of air ; it is equally true 

 that a volume of moist air weighs less than an equal 

 volume of dry air at the same tension.- But water, 

 quietly evaporating in the open air, does not displace 

 the air, but penetrates into its interstices, according 

 to the well-established law regulating the mixture 

 of vapours. The aqueous vapour which thus intimately 

 mixes itself with the air produces no effect whatever, 

 either by its weight or by its elasticity, on the move- 



