572 Correspondence. 



the places mentioned by Dr. Lord ; probably however the learned Geolo- 

 gical Committee of the Agricultural and Horticultural Society of Calcutta, 

 whose zealous exertions deserve every commendation, will be able to 

 settle the point, seeing that as they have manufactured a geological 

 map of the whole of India — they might as well daub with brown, blue, 

 grey, or with as many party colours as they like, a map of the country 

 west of the Indus. We hope and trust that the labours of the Committee 

 will not be thrown away, and that the Society will soon publish this 

 laudable undertaking. It matters not whether some of the members 

 of the said Committee, as has been reported, do not know the differ- 

 ence between clay ironstone, and coal, without an analysis has been 

 resorted to ! and even when that has been undertaken, the specimen re- 

 mains a nondescript! that is a trifle — so some individuals may think. 



We shall only notice one other paragraph of Dr. Lord's paper. 

 " Temperature of wells," says he, " as generally taken, without refer- 

 ence to the formation in which they occur, must needs be a most imper- 

 fect, indeed erroneous method of approximating to the mean tempera- 

 ture of the place, inasmuch as different rock formations, like different 

 metals, vary much in their powers of conduction. Thus a well at 

 Peshawur gave me a temperature of 64°> while one at Attock, almost 

 under the same parallel of latitude, and the same altitude above the sea, 

 was as high as 78°, the thermometer at sunrise of each standing at 

 about 80° ; but the well at Peshawur was in a loose clay mixed with 

 vegetable mould — a notoriously bad conductor of heat, while that at 

 Attock was in hard black slate ; which would thus appear to have a 

 very different quality ; again, a well at Akroford, in limestone, latitude 

 35o N. shewed a temperature of 54° F., while another in slate, a few 

 miles further north, stood at 48° — the altitude of both being nearly 

 equal, and the thermometer at sunrise below the freezing point ; so that 

 in this instance also the slate would appear to have had superior 

 powers of conduction." 



From the above observations it would appear that Dr. Lord is not 

 at all aware of the attention that has been paid to this subject for some 

 years past, in Europe. In Professor BischoflTs admirable work* on the 

 temperature of hot and thermal springs, "the unequal power of conducting 

 heat, possessed by different rocks, causes differences in the progression of the 

 increase of temperature towards the centre of the earth," is ably treated in 

 the 16th chapter, and of which the sentence in italics is the title. 



* First published in the Edinburgh New Philosophical Journal for 1837 and 38, and since 

 reprinted. 



