OBSERVATIONS OF UNDERGROUND TEMPERATURE. 425 
geneous substance, at a temperature 1° lower, be continually placed in contact with 
the upper surface, and removed to be replaced by other homogeneous portions at 
the same lower temperature, as soon as the temperature of the matter actually 
thus applied rises in temperature by ;, of a degree. If this process is con- 
tinued for a year, the whole quantity of the refrigerating matter thus used to 
carry away the heat conducted through the stratum must amount to 235,000 
cubic feet for each square foot of area, which will be at the rate of -00745 of a 
cubic foot per’ second. We may therefore imagine the process as effected by 
applying an extra stratum ‘00745 of a foot thick every second of time. This 
extra stratum, after lying in contact for one second, will have risen in tempera- 
ture by ;., of a degree. By means of the information contained in this apparently 
unpractical statement, many interesting problems may be practically solved, as 
I hope to show in a subsequent communication. 
38. The value of < derived from the whole eighteen-years period of obser- 
vation (-1156), differs so little from that (-1154) found previously (§ 16) from 
Professor Forses’s observations and reductions of the first five of the years, that 
we may feel much confidence in the accuracy of the values 1098 and -06744, 
which, from his five years’ data alone, we found (§ 16) for the corresponding 
constant with reference to,the sand at the Experimental Garden and the sand- 
stone of Craigleith Quarry. From them, calculating as above (§ 36), we find 
260°5 and 690°7 as the values of £ for the terrestrial substances of these localities 
respectively; results of which the meaning is illustrated by the statements of 
§§ 36 and 37. 
39. To deduce the conductivities of the strata, in terms of uniform thermal units, 
Professor Forses had the “ specific heats” of the substances determined experi- 
mentally by M. Reanautt. The results, multiplied by the specific gravities, gave 
for the thermal capacities of portions of the three substances, in terms of that of an 
equal bulk of water, the values ‘5283, -3006, and ‘4623 respectively. Now, these 
must be the values of ¢, if the thermal unit in which é is measured is the thermal 
capacity of a French cubic foot of water. Multiplying the values of - found 
above by these values of c, we find for & the following values :— 
Trap-rock of Calton Hill. Sand of Experimental Gardens. Sandstone of Craigleith. 
124-2 78'31 319°3 
The values found by Professor ForBEs were— 
111-2 82:6 298°3 
Although many comparisons have been made between the conducting powers 
of different substances, scarcely any data as to thermal conductivity in absolute 
measure have been hitherto published, except these of Professor Forbes, and pro- 
WOle KL; PART WI: 5R 
