502 Professor W. J. SoUas — Underground Temperature. 



III. — On the Eate of Increase of Underground Temperature. 

 By Professor W. J. Sollas, LL.D., D.Sc, F.R.S. 



IN the 22nd Eeport of the Committee appointed to investigate the 

 rate of increase of underground temperature, read this year 

 before the British Association in Glasgow, some remarks previously- 

 made by me are animadverted upon ; and as the Secretary, Professor 

 Everett, has invited me to discuss the matter with him, I take the 

 opportunity of entering somewhat more fully into the question of 

 conductivity than has hitherto seemed necessary. We read in the 

 Keport " .... in view of the fact that the President of 

 Section C last year characterised the variation in the British Isles 

 ' from 1° in 34 feet to 1° in 92 feet ' as ' a surprising divergence 

 of extremes from the mean,' it is well to emphasise the connection 

 between gradient and conductivity. If there is anything like 

 uniformity in the annual escape of heat from the earth at different 

 places, there must necessarily be large differences in geothermic 

 gradients, since the rate of escape is jointly proportional to the 

 gradient and the conductivity." 



So well known a fact as the statement in the last sentence seems 

 to me scarcely to require emphasis, since it must assuredly be 

 present in the mind of everyone capable of discussing the question : 

 but it is not sufficient to make general statements of this kind ; it 

 must also be shown, if the argument is to be of any real value, that 

 the known divergences of extremes from the mean may be definitely 

 connected with known differences in conductivity. Hitherto this 

 has not been attempted, and in the Address to Section C last 

 year it was expressly stated that " many cases exist which cannot 

 be explained in such a manner, but are suggestive of some 

 deep-seated cause, such as the distribution of molten matter below 

 the ground," Before proceeding to enter into calculations which 

 may illustrate this statement, it may be worth while to observe 

 that we have no evidence to suggest, much less to prove, that 

 " there is anything like uniformity in the annual escape of heat 

 from the earth at different places " ; the indications are altogether 

 to the contrary : the mere existence of volcanos obviously invalidates 

 the statement as an absolute affirmative, and ancient laccolites show 

 that in past time at least concealed sources of heat have existed not 

 very remote from the surface. 



If, then, there is not uniformity in the annual escape of heat from 

 the earth at different places it may be thought unnecessary to labour 

 the question in greater detail, yet in view of the importance of the 

 subject to geological inquiry it may be worth while to consider 

 some special cases. If we turn to the " Summary of the Eesults 

 in the first 15 Eeports by Professor Everett " (1882) we shall find 

 a table of mean conductivities from several kinds of rocks given in 

 C.G.S. measure, from determinations made by Professor Herschel, 

 but these, following the direction of Professor Everett, must be 

 multiplied by a correcting factor 1-4 for use in calculation. For 

 our purpose we select the following : — 



