ON SEISMOLOGICAL INVESTIGATIONS. 99 
For Switzerland, Germany, Austria, Central North America, which 
are distant from the sea, the number of gradients below 1° in 64 feet is 
32, while of those above 64 feet the number is twelve. In this case the 
ratio of the gentle gradients to those which are steep is as 1 to 2°6. 
Here again the inference is that steep gradients increase in frequency 
as we approach the seaboard. In this latter catalogue I find no less 
than twenty localities where the gradients are 33 feet or less. The 
steepest of this group is at the Dolcoath Mine, in Cornwall, where we 
have an increase of 19 F. for 18 feet of depth. For five mines 
beneath the sea the average gradient is 1° for 38 feet of depth. 
The only other materials bearing upon this subject with which I 
am acquainted are lists of heat gradients drawn up by Messrs. Koenigs- 
berger and Mihlberg.® When these are combined with those given by 
Professors Prestwich and Everett, the following two tables are obtained : 
Tasxe I. 
This gives the average heat gradients in feet per 1° F. for inland districts and highlands 
together with the number of stations at which observations have been made. 
No. of Average 
ay Stations Gradient 
§.E. Lancashire, S. Yorkshire, Nogpmepenshis 9 65 
Wales, Inland , 2 70 
S. Germany, Bohemia, astra g 3 - 5 5 12 62 
Central France : p : F . i 6 51 
Victoria and New South Wales. . anpete 2 78 
Central United States and Central Canada ; aaa 13 92 or 79 
Witwatersrand, high ground ope is 1 207 
South America, high ground 5 113 
The average gradient deduced from these figures is 1° F. for 75 feet descent. From 
Everett’s tables the estimate was also 1° F. in 75 feet. 
TasieE II. 
For low ground, and localities near the sea, the gradients run as follows :-— 
es Stations Gradient 
Newcastle and Durham District 10 52 
West Cumberland 2 43 
S. Wales, near the coast 2 49 
Cornwall and Devon : 14 44 
Between Glasgow and 1 Edinburgh 4 48 
N. Germany 4 52 
West France 4 50 
N. France and Belgium 8 47 
Africa, the Sahara 3 45 
N. America, E. Coast 3 59 
Mexico, Central 2 50 
From these figures it would appear that the average gradient of these localities is 
an increase of 1° F. for 52 feet of descent. From Everett’s tables this becomes 1° F. 
in 60 feet of descent. 
® See Vrans. Institute Mining Engineers, vol. 39,.1909-10, p. 617. 
H 2 
