o\3* 



3. On the Results of the most recent Experiments on the 

 Conducting Power for Heat of different Soils. By Pro- 

 fessor Forbes. 

 The author gn.\e, in continuation of a former paper (see Proceed- 

 ing's of the Royal Society of Edinburgh, 3d Dec. 1838), an account 

 of the continued systematic prosecution and annual reduction of 

 the observations of temperature at different depths (3, 6, 12, and 24 

 French feet) below the surface of the ground, in various geologi- 

 cal formations near Edinburgh, viz. the trap rock of the Calton 

 Hill, the incoherent sand of the Experimental Garden, and the 

 coal formation sandstone of Craigleith Quarry. The weekly ob- 

 servations at all these stations have been projected into curves, 

 which present the most remarkable concordance of general features 

 for the four years now complete, and give a just confidence in the 

 comparability of the results obtained. The thermometric read- 

 ings have all been rigorously corrected for the expansion of the 

 alcohol in the tubes ; and, starting from these data, the results in 

 the following tables have been obtained, partly by graphical me- 

 thods, partly by calculation. The quantity marked A is the log. 

 range at the surface. The quantity B (which is the most inter- 

 esting result) marks the rate of diminution of the range as we 

 descend, and is proportional to the square root of the specific heat 

 of the soil divided by its conducting power. M. Regnault of Paris 

 has kindly undertaken to determine the specific heats by direct 

 experiment, whence the conductivity will become known ; and the 

 comparison of the results for four years proves that we have al- 

 ready obtained a near approximation to its value, which is well 

 defined for the different formations, but especially for the Sand- 

 stone, when contrasted with the other two. The results of Tables 

 III. IV. and V. are deduced from the numbers in Table I. ; and 

 for the sake of comparability with the foreign observations, the 

 French foot and centigrade degree have been employed as units. 

 So well do the observations of the different thermometers for any 

 one station accord together, that, taking any two of the ther- 

 mometers and combining their results by pairs, we should obtain 

 nearly the same conclusions. These conclusions are also in ac- 

 cordance with those which the Epochs in Table II., and the rate 

 of Progress of Heat downwards in Table VI., present ; the best 

 conducting soil (that for which B has the smallest numerical 

 value) transmitting the heat fastest. 



