the Negro , being a defence against the sun's rays. 3 
ful ; and when the heat was further increased, I could not 
bear it. 
Experiment 3. 
I exposed the backs of my two hands to the sun's rays, 
with a thermometer upon each ; the one hand was uncovered, 
the other had a covering of black cloth, under which the ball 
of the thermometer was placed. After ten minutes, the 
degree of heat of each thermometer was marked, and the 
appearance on the skin examined. This was repeated at 
three different times. The 
1st time the thermometer under the cloth 91 0 , the other 85° 
and time - - - - - 94, - - 91 
3rd time ----- 106 98 
In every one of these trials the skin was scorched that was 
uncovered ; the other had not suffered in the slightest degree ; 
there was no appearance of perspiration on either hand. 
Experiment 4. 
The back of a Negro's hand was exposed to the sun with 
a thermometer upon it, which stood at ioo° ; at the end of ten 
minutes the skin had not suffered in the least. 
Experiment 5. 
During the eclipse of the sun on September 7, 1820, I ex- 
posed the back of my hand to the rays concentrated by a 
double lens of half an inch focus, at three different periods of 
the eclipse. When the heat to a thermometer was 75 0 , that is 
from 47 to 57 minutes past one'clock, (including three of the 
figures in the annexed drawing, made by Mr. Bauer, PI. I.) 
