Prof. John Milne — On the Form of Volcanos. 511 



X 



dv 



n feet. 



dx 







1-2500 



500 



1-1560 



700 



1-0690 



1000 



09140 



2000 



03580 



3000 



0-0751 



5000 



0-0005 



A curve was drawn with, these numbers as co-ordinates, and it is 

 evident that the area of this curve up to any value of x is the 

 difference between the temperature at that point and 2000° Fahr. 

 In this way the following table showing the temperature at various 

 distances from the hot surface was obtained. 



Distance from Hot Surface in feet. Eise of Te ™f n r ft at v re Produced in 



2000 Years. 







2000° Fahr 



600 



1275° „ 



800 



1062° „ 



1000 



862° „ 



1200 



667° „ 



1400 



548° „ 



1600 



423° „ 



1800 



321° „ 



2000 



244° „ 



2200 



178° „ 



2400 



129° „ 



2600 



92° „ 



2800 



64° „ 



3000 



45° „ 



On looking at these results, it will be observed that they are far 

 greater than might have been anticipated. Perhaps it may be 

 objected that the conditions which have been taken as compared 

 with those which occur in nature have been overrated. I think quite 

 differently. 



First, 2000 years is hardly an overestimate of the length of time 

 during which a volcano may remain in activity. Vesuvius is recorded 

 as having erupted in the year a.d. 79, and it still continues in activity. 

 Before that time it was a recognized volcano, but how many 

 thousands of years it may have been in existence we do not know. 

 In Ischia and neighbouring places there were eruptions 400 years 

 previously. All that we can say is that, as evidenced by the lai-ge 

 amount of ejected matter which was in existence before the eruption 

 of 79, it was even then a mountain of some antiquity. 



Secondly, a temperature of 2000° Fahr. for molten matter, such as 

 that about which we speak, is an estimate which is extremely low. 



Sir William Thomson, in his calculations relative to the age of 

 the earth, takes the temperature of molten rocks from 7000° to 10,000° 

 Fahr., but these temperatures are purposely taken as large as possible. 

 Bischof, in his " Geologie," p. 98, takes as the melting point of lava 

 2250 °Fahr. 



When speaking of this temperature, it must be remembered that it 

 is not supposed to exist in molten rocks near the surface, but in 



