INTENSE HEAT—THUNDER. 203 
instead of ascending the course of the rivers, find it 
safer to cross the flats in their boats. 
San Fernando is celebrated for the excessive heat 
which prevails there during the greater part of the 
year. The travellers found the white sand of the 
shores, wherever it was exposed to the sun, to have 
a temperature of 126°5°, at two in the afternoon. 
The thermometer, raised eighteen inches above the 
sand, indicated 109°; and at six feet, 101:7°. The 
temperature of the air in the shade was 97°. These 
observations were made during a dead calm, and 
when the wind began to blow, the heat increased 
three degrees. 
On the 28th March, Humboldt and his compa- 
nion being on the shore at sunrise, heard the thun- 
der rolling all around, although as yet there were 
only scattered clouds, advancing in opposite direc- 
tions toward the zenith. Deluc’s hygrometer was 
at 53°, the thermometer stood at 74:-7°, and the elec- 
trometer gave no particular indication. As the clouds 
mustered, the blue of the sky changed to deep azure, 
and then to gray; and when it was completely overcast 
the thermometer rose several degrees. Although a 
heavy rain fell, the travellers remained on the 
shore to observe the electrometer. When it was 
held at the height of six feet from the ground, the 
pith-balls generally separated only a few seconds 
before the lightning was seen. The separation was 
four lines. The electric charge remained the same 
for several minutes, and there were repeated oscil- 
lations from positive to negative. Toward the end 
of the storm the west wind blew with great impe- 
tuosity, and when the clouds dispersed the thermo- 
meter fell to 716°. 
