401 



The electrometer, armed with a smoking match, 

 gave no sign of electricity. As the storm ga- 

 thered, the blue of the sky changed at first to 

 deep azure, and then to gray. The vesicular 

 vapour became visible, and the thermometer 

 rose 3°, as is almost always the case within the 

 tropics from a cloudy sky, that sends back the 

 radiant heat of the soil. A heavy rain fell. 

 Being sufficiently habituated to the climate not 

 to fear the effect of tropical rains, we remained 

 on the shore, to observe the electrometer. I 

 held it more than twenty minutes in my hand, 

 six feet above the ground, and observed, that 

 in general the pith balls separated only a few 

 seconds before the lightning was seen. The se- 

 paration was four lines. The electric charge 

 remained the same during several minutes ; and 

 having time to determine the nature of the elec- 

 tricity, by approaching a stick of sealingwax, I 

 saw here in the plain what I have often observed 

 on the back of the Andes during a stormy that 

 the electricity of the atmosphere was first posi- 

 tive, then null, and then negative. These oscil- 

 lations from positive to negative (from the 

 vitreous to the resinous state) were often re- 

 peated. Yet the electrometer constantly denoted 

 a little before the lightning only no E., or + E., 

 and never — E. Toward the end of the storm, 

 the west wind became very impetuous. The 

 clouds dispersed, and the thermometer sunk to 



VOL. IV. 2 D 



