148 rnYSiCAL CEOGKArnY op the sea, and its meteorology, 



the eartli in this region, strewed with impalpable and feather- 

 light remains of animal and vegetable organisms, is swept over 

 by whirlwinds, gales, and tornadoes of terrific force : this is the 

 period for the general atmospheric disturbances which have 

 made characteristic the equinoxes. Do not these conditions 

 appear sufficient to afford the " rain dust " for the spring showers ? 

 At the period of the autumnal equinox, another portion of the 

 Amazonian basin is parched with drought, and liable to winds 

 that fill the air with dust, and with the remains of dead animal 

 and vegetable matter: these impalpable organisms, wliieh each 

 rainy season calls into being, to perish the succeeding season of 

 drought, are perhaps distended and made even lighter by the 

 gases of decomposition which has been going on in the period of 

 drought. May not, therefore, the whirlwinds which accompany 

 the vernal equinox, and sweep over the lifeless plains of the 

 Lower Orinoco, take up the " rain dust" which descends in the 

 northern hemisphere in April and May ? and may it not be the 

 atmospherical disturbances which accompany the autumnal 

 equinox that take up the microscopic organisms from the Upper 

 Orinoco and the great Amazonian basin for the showers of 

 October ? 



326. Humholclfs description of the dust-icliirlwinds of the Orinoco. — 

 The Baron von Humboldt, in his Aspects of Nature, thus contrasts 

 the wet and the dry seasons there : " AYhen, under the vertical 

 Tn.ys of the never-clouded sun, the carbonized tuify covering 

 falls into dust, the indurated soil cracks asunder as if from the 

 shock of an earthquake. If at such times two opposing currents 

 of air, whose conflict produces a rotary motion, come in contact 

 with the soil, the plain assumes a strange and singular aspect. 

 Like conical-shaped clouds, the points of which descend to the 

 earth, the sand rises through the rarefied air on the .electrically- 

 charged centre of the whirling current, resembling the loud 

 water-spout, dreaded b}' the experienced mariner. The lowering 

 sky sheds a dim, almost straw-coloured light on. the desolate 

 plain. The horizon draws suddenly nearer, the steppe seems to 

 contract, and with it the heart of the wanderer. The hot, dusty 

 particles which fill the air increase its suffocating heat, and the 

 east wind, blowing over the long-heated soil, brings with it no 

 refreshment, but rather a still more burning glow. The pools 

 which the yellow, fading branches of the f\\n-palm had protected 

 from evaporation, now gradually disappear. As in the icy north 



