91 
ANALOGOUS C'ONHGUHATION. 
of three ;r four degrees alternate with others which are 
inclined from 25 to 30 degrees ; and the latter only strike 
our imagination, because we think all the slopes of moun- 
tains more steep than they really are. I may cite in sup- 
port of this consideration the example of the ascent from 
the port of Vera Cruz to the elevated plain of Mexico. On 
the eastern slope of the Cordillera a road has been traced, 
which for ages has not been frequented except on foot, or 
on the backs of mules. From Encero to the small Indian 
village of Las Vigas, there are 7500 toises of horizontal 
distance; and Encero being, according to my barometric 
measurement, 740 toises lower than Las Vigas, the result, 
for the mean slope, is only an angle of 5° 40'. 
In the note at the foot of this page will be seen the 
results of some experiments 1 have made on the difficulties 
arising from the declivities in mountainous countries.* 
Isolated volcanoes, in the most distant regions, are very 
analogous in their structure. At great elevations all have 
considerable plains, in the middle of which arises a cone 
perfectly circular. Thus at Cotopaxi the plains of Suni- 
guaieu extend beyond the farm of Pansache. The stony 
summit of Antisana, covered with eternal snow, forms an 
islet in the midst of an immense plain, the surface of which 
is twelve leagues square, while its height exceeds that of 
the peak of Teueriffb by two hundred toises. At Vesuvius, 
* In places where there were at the same time slopes covered with 
tufted grass and loose sands, I took the following measures : — 
5°, slope of a very marked inclination. In Fi ance the high roads 
must not exceed 4° 46' by law; 
15°, slope extremely steep, and which we cannot descend in a 
carriage; 
3/ slope almost inaccessible on foot, if the ground be naked 
rock, or turf too thick to form steps. The body falls backwards 
when the tibia makes a smaller angle than 53° with the sole 
of the foot; 
42®, the steepest slope that can he climbed on foot in a ground that 
is sandy, or covered witli volcanic ashes. 
When the slope is 44°, it is almost impossible to scale it, though the 
ground permits the forming of steps by thrusting in the foot. The cones 
of volcanoes have a medium slope from 33° to 40°. The steepest parts 
of these cones, either of Vesuvius, the Peak of Teneriffe, the volcano of 
Pichincha, or Jorullo, are from 40° to 42°. A slope of 55' is quite ina?. 
cejbihle. If seen from above it would be estimated at 75°. 
