98 REPORTS ON THE STATE OF SCIENCE.—1912. 
oceans. It appears from this that for this particular period there was 
four times as much seismic activity beneath the cold waters of particu- 
lar parts of our oceans as there was beneath continental areas. 
This activity is represented by megaseisms, which usually occur 
in groups, and the periods of rest which follow the groups 
are found to be roughly proportional to the intensity of the 
groups by which they are preceded. This suggests that the strain 
- which finds relief in world-shaking disturbances accumulates uniformly, . 
and it may therefore be associated with uniformity in the rate of earth- 
cooling.* Should such a relationship exist, it seems likely that sub- 
oceanic thermometric gradients may be considerably steeper than those 
which exist beneath continental areas. This led me to examine such 
material as we have at our disposal relating to heat gradients in different 
parts of the world. In 1882, in the Fifteenth Report of the Under- 
ground Temperature Committee of the British Association, the late 
Professor Everett gives a summary of the results of their investigations. 
From a list of thirty-one localities in various parts of the world where 
observations have been made the conclusion is that the thermometric 
gradient is on the average 1° I’. for 64 feet of descent, or 0°000285° C. 
per cm. of depth, which, with a rock conductivity of 0°0058, means an 
average escape of heat annually from each square centimetre of the 
surface of our world of 41°4 gramme-degrees of heat. Sufficient 
materials to make a complete map of the world, showing the heat 
gradients, do not exist, but Professor Everett’s table may be split into 
two parts, one of which refers to highlands and the interior of con- 
tinents. and the other to lowlands or localities which are near the sea. 
In the former we find the following seven localities, viz.: Przibram, in 
Bohemia, St. Gothard Tunnel, Mont Cenis Tunnel, Schemnitz, in Hun- 
gary, Manegaon, in India, Yakutsk, in Siberia, and Sperenberg, near 
Berlin. The average gradient for these I find to be 1° F. for 75 feet of 
descent, or 0°000239° C. per cm. of depth. For the remaining twenty- 
four districts, which are comparatively near to the sea, the average 
gradient is 1° F. for 60 feet of descent, or 0°000303° C. per cm. of 
depth. With the latter gradient the number of gramme-degrees of heat 
which escape annually through each square cm. of the earth’s crust 
would be 42°4, but with the gradient for the highlands this number 
becomes 35°5. This means that from the lowlands one-fifth, or 20 per 
cent., more heat escapes than that which escapes from the inland high- 
lands. 
I next turned to the tables of the late Professor Prestwich, published 
in 1886.4 In this register I found 329 sets of observations. Of these, 
283 referred to Great Britain and Ireland, France, Holland, Belgium, 
Italy, St. Petersburg, Algeria, and Buenos Aires. These I regarded 
as countries and places near to the sea. Out of this group, 217 have 
gradients below 1° in 64 feet, which is Professor Everett’s average, 
while the remaining sixty-six have gradients above 64 feet. The latter, 
which are gentle gradients, are to the former, which are steep, in the 
ratio of 1 to 3°3. 
* See Brit. Assoc. Report, 1910, p. 54. 
* See Proc. of the Royal Soc., vol. xli-, 1886. 
