178 ATMOSPHERIC PHENOMENA. 



ciiAr. XVL 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 tlie Orinoco commences about the end of 

 April. The sky begins to be overcast, its azure colour 

 Ileat iisappears, 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 atmospheric 

 Atmospheric electricity, which, during the period of the greatest 

 L-iecnidtj-. drought, from December to March, had been almost 

 constantly in the daytime from 1-7 to 2 lines to Volta's 

 electrometer, becomes extremely variable after March. 

 During whole days it appears null, and again, for some 

 hours, the pith-ljalls of the electrometer diverge from 

 three to four lines. The atmosphere, which in the 

 tori'id as in the temperate zone is generally in a state of 

 positive electricity, passes alternately, in tlie course of 

 eight or ten minutes, to the negative state. The rainy 

 season is that of tlmnder-storms ; and yet I have found, 

 from numerous experiments made during three years, 

 that at this season the electric tension is less in the 

 lower regions of the atmosphere. Are thunder-storms 

 the effect of this unequal charge of the different super- 

 imposed strata of the air ? What prevents the electri- 

 city from descending towards the earth in a stratum of 

 air which has become more humid since the month of 

 Accumuia- March \ At this period the electricity, in place of being 

 c/ccirkity ^^'^^scd through the whole atmosphere, would seem to 

 be accumulated on the outer envelope at the surface of 

 the clouds. According to M. Gay Lussac, it is the for- 

 mation of the cloud itself that carries the fluid toward 

 the surface. The storm rises in the phiins two hours 

 after tlie sun passes through the meridian, and therefore 

 shortly after the period of the maxinmm of the diurnal 

 heat in the tropics. In the inland districts it is exceed- 

 ingly rare to luar thunder at nii^lit or in the mornimr. 



