168 _ Prof. J. Milne—Distribution of Volcanos. 
Urup, one of the Kuriles, the slope.is about 1 in 22:1. In an 
easterly direction, from the northern end.of Nipon, the slope is 
about 1 in 80:4. From the Sandwich Islands towards the north 
there is a slope of 1 in 23:5. . 
The general slope on the coast of Australia is about 1 in 91, but 
on the 8.E. shores, measured from the summit of Mount Kosciusko, 
a slope of 1 in 57 can be obtained. From Ben Nevis, the 120 mile 
slope, which will just reach to the 100-fathom line, is about 1 in 158. 
From the summit of the mountains of Southern Norway there is a 
slope of about 1 in 73. 
In these few examples it will be seen that the 120 mile slope of 
the volcanic districts is about twice as steep as the slope in the 
districts which are free from volcanos. 
Instead of taking a slope of 120 miles, we might regard the 
manner in which the land slopes beneath the sea more generally by 
drawing sections across the various continents, prolonging them 
beneath the sea, and then take their average slope. Suppose, for 
instance, we prolong the section of Aconcagua to the east coast, we 
shall find that whilst on the west coast we have a slope of about 
1 in 20, to the east coast there is an average slope of 1 in 243. 
Similar differences would be found’ between the slopes to the 
eastern and western shores of North America. 
Without attempting to examine the various slopes upon the surface 
of the globe in detail, a general idea of them may be obtained by 
glancing over a map or chart showing the depths of the ocean and 
the heights of the adjoining land. 
If this is done, it will be seen that volcanos are chiefly distributed 
along the borders of land which slopes steeply beneath the sea. 
Now let us see what phenomena we might expect in connexion 
with these slopes. 
First, going upon the assumption that although the world may in 
its interior be extremely hot, it is extremely rigid, a conclusion 
which is arrived at by Sir William Thomson and other eminent 
mathematicians and physicists, we see that we have downward 
pressures acting over the two ends of any of these 120 mile lines, of 
very different intensities. 
z Sea level, 
Fic. 1. 
_ Over the end which is beneath the sea the pressure is that of a 
column of water h, very much less than the pressure of the larger 
column of rock H, acting downwards at the land end of the slope 
