DISTRIBUTION OP EAINPAIiL. 273 



along the surface of smooth water. The same phenomenon is pro- 

 duced upon a greater scale when a cannon hall, aimed at a floating 

 target, strikes the water and rebounds into the air again and again 

 ere it disappears. I have often, at the Cape of Good Hope, seen 

 indications of the southeast wind advancing in waves, raising dust 

 where it seemed to descend and strike the ground, but leaving un- 

 disturbed alternate stripes over which it seemed to bound or rebound 

 at a higher level. Thus may it be with all currents, whether of 

 water or of air, descending and striking upon other matter, solid, 

 liquid, or gaseous, at an acute angle of inclination — as is indicated by 

 the undulations or waves raised by the breath on a cup of tea, 

 by the wind on a pond or placid stream, and by a storm upon 

 the sea. And, according to the supposition of Sir William Thomson, 

 a current of air striking upon a lower current, advancing in the same 

 direction but with less velocity, or advancing in another direction, or 

 upon a stratum of air in a state of quiescence,may rebound as does the 

 skimming stone or the rebounding cannon ball on striking the water. 

 And if the temperature of this current of air were near to that of the 

 dew-point, or point of saturation, when it rebounded to a higher 

 elevation, the temperature being thereby reduced, a condensation of 

 moisture would ensue ; but when the rebounding force was exhausted, 

 and it again descended to a lower level, it would acquire a higher 

 temperature, and the condensed vapour would be again dissolved in 

 the air, producing the phenomenon of alternating stripes of cloud 

 and of sky. 



It is to this difference in the power of air to sustain vapour in 

 solution at the different temperatures through which it passes at 

 different elevations that I refer as illustrative of the rainfall occa- 

 sioned by the wind-wave which has been spoken of. I have had 

 occasion to refer to an iUustration of the same kind afforded by 

 the passage of the southeast wind over Table Mountain, producing 

 dense clouds and even drizzling rain on the mountain top, but the 

 clouds disappearing as the current pouring over the front of the 

 mountain reached a lower level but a higher temperature ; so is it 

 with the wind-wave deluging the mountains with rain but passing 

 over the lower-lying plains without any such effect. 



Such wind waves, varying in their sweep and in their dimensions, 

 but essentially the same, are not of unfrequent occurrence. Most of 

 those, of which the phenomena have been studied, have swept along 

 the surface of the earth and the ocean, and even they may have 



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