ViOMnICeAN LC ATCT OUN, HAW ATI. 217 
violence. It is perhaps difficult for the mind to dissociate from the 
idea of a volcano, noise and earthquake; yet there is no doubt that 
they are no necessary attendants. For both Kilauea and Mokua- 
weo-weo, neither of which is exceeded in its crater by any known 
volcano, have emptied out their lavas in vast eruptions, without such 
accompanying phenomena. We have evidence that Mount Loa has 
been rent through its sides for miles,—even for twenty-five miles or 
more at the eruption of 1843,—without a murmur reaching the resi- 
dents at Hilo, on the eastern shores. It is plain that the mode of 
action at the summit is similar to that in Kilauea. For years there is 
an accumulation of lavas in progress,—a rising within and a gradual 
increase of pressure from this source, as is so well exhibited before 
the eye in Kilauea; and this increase goes on until the sides of the 
mountain give way, and the lavas run out. Noise need not attend 
such a rupture, and in many cases it does not. ‘The mere force of 
pressure alone, without any unusual discharge of vapours, may cause 
the fracture and discharge. 
The amount of this pressure is easily calculated. In the ascent of 
lavas, every additional twelve feet in height adds one atmosphere or 
fifteen pounds per square inch to the pressure exerted. Consequently 
every 1600 feet will make an addition of a ton or 2000 pounds to the 
square inch, and 13,760 feet, the height of Mount Loa, a pressure 
of 17,200 pounds to the square inch, or 2,500,000 pounds to the 
square foot. This is the amount of pressure at the level of the ocean, 
and also the force exerted at that level against the sides of the moun- 
tain, tending to cause a rupture. As the mountain is much narrower 
above, or the walls of the central cavity thinner,—and the tension in 
the walls of the conduit below would be communicated upward,—the 
resistance to external fracture, other things the same, would be least 
in the upper part of the mountain. The rupture actually appeared 
near the summit at an elevation of 13,000 feet, and the lavas conse- 
quently had risen nearly or quite to the summit before the outbreak. 
A crater in violent action was afterwards seen there by Mr. Coan. 
Whether there was anything in the nature of the mountain structure 
on the side of the eruption to favour its breaking there, could not of 
course be ascertained. On this point we can only say that the same 
region has been a frequent place of eruption: and moreover it is in a 
direction nearly transverse to the trend of the group, like the bearing 
of Kea and Loa, and the course of the longer diameter of Kilauea. 
395 
