ON THE BATE OF INCEEASE OF UNDERGROUND TEMPERATURE. 79 



deep at Greenwich and at Edinburgh. These observations point to an 

 excess of surface temperature above air temperature, ranging from half a 

 degree to nearly two degrees, and having an average value of about one 

 degree. 



Dr. Schwartz, Professor of Physics in the Imperial School of Mines 

 at Schemnitz, in sending his observations made in the mines at that 

 place (X. 195), remarks on this point : — 



' Observations in various localities show that in sandy soils the excess 

 in question amounts, on the average, to about half a degree Centigrade. 

 In this locality the surface is a compact rock, which is highly heated by 

 the sun in summer, and is protected from radiation by a covering of 

 snow iu winter ; and the conformation of the hills in the neighbourhood 

 is such as to give protection against the prevailing winds. Hence the 

 excess is probably greater here than in most places, and may fairly be 

 assumed to be double the above average.' 



Some excellent observations of underground temperature at small 

 depths were made at the Botanic Gardens, Regent's Park, London, for 

 the six years 1871-76, along with observations of air temperature, and 

 have been reduced by Mr. Symons. They are at depths of 3, 6, 12, 24, 

 and 48 inches beneath a surface of grass, and their joint mean derived 

 from readings at 9 a.m. and 9 p.m. for the six years is 49 '9, the mean for 

 the 48-inch thermometer being 60'05. The mean air temperature derived 

 in the same way from the readings of the dry-bulb thermometer is 

 49 '6. Hence it appears that the excess of soil above air is in this case 

 about 0°-4. 



Quetelet's observations for thi-ee years at Brussels (p. 48 of his 

 'Memoire') make the earth, at depths less than 1^ foot, colder than 

 the air, and at greater depths warmer than the air. 



Caldecott's observations for three years at Trevandrum, in India, 

 make the ground at the depth of three feet warmer than the air by 5°'7 F. 



Dr. Stapff, in his elaborate publications on the temperature of the 

 St. Gothard tunnel, arrives at the conclusion that the mean temperature 

 of the soil on the surface of the mountain above the tunnel is some degrees 

 higher than that of the air, the excess increasing with the height of the 

 surface and ranging from 2° or 3° C. near the ends of the tunnel to 5° or 

 6'^ C. in the neighbourhood of the central ridge. 



Connected with this is the question — 



2. Whether the mean annual temperature of the soil increases down- 

 wards from the surface itself, or whether, as is sometimes asserted, the 

 increase only begins where annual range ceases to be sensible — say at a 

 depth of 50 or 60 feet. 



The general answer is obvious from the nature of conduction. Start- 

 ing with the fact that temperature increases downwards at depths where 

 the annual range is insensible, it follows that heat is travelling upwards, 

 because heat will always pass from a hotter to a colder stratum. This 

 heat must make its way to the surface and escape there. But it could 

 not make its way to the surface unless the mean temperature diminished 

 iu approaching the surface ; for if two superposed layers had the same 

 mean temperature, just as much heat would pass from the upper to the 

 lower as from the lower to the upper, and there would not be that excess 

 of upward flow which is necessary to carry off the perennial supply from 

 below. 



This reasoning is rigorously true if the conductivity at a given depth 



