RIVERS. 



297 



the air. At the end of March the southern region of the atmosphere is illuminated by 

 small electric explosions, like phosphorescent gleams, confined to a single group of 

 vapours. From this period the breeze shifts at intervals, and for several hours, to the 

 west and south-west, affording a sure indication of the approach of the rainy season, 

 which, on the Orinoco, commences about the end of April. The sky begins to be over 

 cast, its azure colour disappears, and a gray tint is uniformly diffused over it. At the 

 same time the heat of the atmosphere gradually increases, and, instead of scattered 

 clouds, the whole vault of the heavens is overspread with condensed vapours. The 

 howling monkeys begin to utter their plaintive cries long before sunrise. The atmo 

 spheric electricity, which, during the period of the greatest drought, from December to 

 March, had been almost constantly in the day-time from 1'7 to 2 lines to Volta's electro 

 meter, becomes extremely variable after March. During whole days it appears null, and 

 again, for some hours, the pith-balls of the electrometer diverge from three to four lines. 

 The atmosphere, which in the torrid as in the temperate zone is generally in a state of 

 positive electricity, passes alternately, in the course of eight or ten minutes, to a negative 

 state. The rainy season is that of thunder-storms. The storm rises in the plains two 

 hours after the sun passes through the meridian, and therefore shortly after the period of 

 the maximum of the diurnal heat in the tropics. In the inland districts it is exceedingly 

 rare to hear thunder at night or in the morning, nocturnal thunder-storms being peculiar 

 to certain valleys of rivers which have a particular climate." The substance of the 

 explanation of the preceding phenomena, by the philosophic writer of the statement, may 

 be briefly given : The season of rains and thunder in the northern equinoctial zone coincides 

 with the passage of the sun through the zenith of the place, the cessation of the breezes, 

 or north-east winds, and the frequency of calms, and furious currents of the atmosphere 

 from the south-east and south-west, accompanied with a cloudy sky. While the breexe 

 from the north-east blows, it prevents the atmosphere from being saturated with moisture. 

 The hot and loaded air of the torrid zone rises, and flows off again towards the poles, 

 while inferior currents from these last, bringing drier and colder strata, take the place of 

 the ascending columns. In this manner the humidity, being prevented from accumulating, 

 passes off towards the temperate and colder regions, so that the sky is always clear. 

 When the sun, entering the northern signs, rises towards the zenith, the breeze from the 

 north-east softens, and at length ceases ; this being the season at which the difference of 

 temperature between the tropics and the contiguous zone is least. The column of air 

 resting on the equinoctial zone becomes replete with vapours, because it is no longer 

 renewed by the current from the pole ; clouds form in this atmosphere, saturated and 

 cooled by the effects of radiation and the dilatation of the ascending air, which increases 

 its capacity for heat in proportion as it is rarefied. Electricity accumulates in the higher 

 regions, in consequence of the formation of the vesicular vapours, the precipitation of 

 which is constant during the day, but generally ceases at night. The showers are more 

 violent, and accompanied with electrical explosions, shortly after the maximum of the 

 diurnal heat. These phenomena continue until the sun enters the southern signs, when 

 the polar current is re-established, because the difference between the heat of the equi 

 noctial and temperate regions is daily increasing. The air of the tropics being thus 

 renewed, the rains cease, the vapours are dissolved, and the sky resumes its azure tint. 



The Orinoco, when in flood, inundates a vast extent of country, six hundred miles in 

 length and from sixty to ninety in width. Its waters cover the savannahs along its banks 

 to the depth of twelve or fourteen feet, giving to them a lake-like appearance, in the midst 

 of which farm-houses and villages are seen rising on islands but little elevated above the 

 surface. The wild cattle perish in great numbers, and fall an easy prey to the carrion- 

 vultures and alligators. In one part of the river Humboldt found marks of recent inun- 



