18 REPORT 1855. 



this direction ; while for all cases that have been examined, the residual thermo- 

 electric effect of a permanent strain is the reverse of the temporary thermo-electric 

 effect which subsists as long as the constraining force is kept applied. Those of the 

 other metals which have been as yet examined, namely. Copper, Lead, Cadmium, Tin, 

 Zinc, Brass, Steel, and Platinum (specimens supplied as chemically pure by Messrs. 

 Matthey and Johnson being in general used), showed uniformly the reverse effect 

 to that of iron when similarly treated. The effects of permanent lateral compression 

 by hammering were those which were chiefly tested for this list of metals, and were 

 in almost every case of a very marked and unmistakeable kind. Curious results 

 were also obtained by carefully annealing portions of wires which had been suddenly 

 cooled, and leaving the remaining parts unannealed. Tin and Cadmium thus 

 treated have, as yet, given only doubtful results ; Platinum has not been tried ; Iron, 

 Steel, Copper, and Brass have given decided indications, in which the unannealed 

 portions showed the same kind of thermo-electric effect as had been found to be 

 produced by permanent lateral compression. 



Oti the Use of Observations of Terrestrial Temperature for the investigation 

 of Absolute Dates in Geology. By Professor W. Thomson, M.A,, F.R.S. 



The relative thermal conductivities of different substances have been investigated 

 by many experimenters ; but the only absolute determinations yet made in this 

 most important subject are due to Professor James Forbes *, who has deduced the 

 absolute thermal conductivity of the trap rock of Calton Hill, of the sandstone of 

 Craigleith Quarry, and of the sand below the soil of the Experimental Gardens, 

 from observations on terrestrial temperature, which were carried on for five years 

 in these three localities (all in the immediate neighbourhood of Edinburgh), by 

 means of thermometers constructed and laid, under his care, by the British Asso- 

 ciation. The author of the present communication explained briefly a method of 

 reduction depending on elementary formulae of the theory of the conduction of heat 

 given by the great French mathematician Fourier, which proved to be more complete 

 and satisfactory than the method indicated by Poisson, which had been adopted by 

 Professor Forbes. He applied it both to the series of observations used by Professor 

 Forbes, and to a continuation of the observations on the trap rock of Calton Hill, 

 which has been carried on up to the present time at the Royal Observatory of Edin- 

 burgh, and of which eleven years complete have been supplied to the author in 

 manuscript, through the kindness of Professor Piazzi Smyth. The results, as re- 

 gards thermal conductivities, show that the determinations originally given by 

 Professor Forbes do not require very considerable corrections ; and are satisfactory, 

 inasmuch as values derived from the diminution of the extent of variation of the 

 temperature for the deep thermometers agree very closely with those derived from 

 the retardation of the periods of summer heat and winter cold at the different 

 depths. They show very decidedly a somewhat greater conductivity of the trap rock 

 at the greater depths (from twelve to twenty-four feet) than between the three feet 

 deep and the six feet, or between the six feet and the twelve feet thermometers, but 

 do not establish any such variation in the properties of the sandstone, and of the 

 sand of the two other localities. A comparison of the mean temperatures of the 

 four thermometers, for the whole sixteen years' observation, shows an increase of 

 indicated temperature in going downwards in Calton Hill, which apparently is much 

 more rapid between the upper than between the lower thermometers ; so much so, 

 as not to be referable to the greater conductivity of the rock in the lower position. 

 The author remarked, that, to make the observations available for giving with accu- 

 racy the mean absolute temperatures at the different depths, it would be necessary 

 to have the thermometers taken up and re-compared with a standard thermometer. 

 It is most probable that the zero-points of all the thermometers have risen consider- 

 ably since they were first laid, because the apparent mean temperatures, as shown 

 by the thermometers, are much higher of late than they were at first. Thus, for 

 the period of five years examined by Professor Forbes, and for the succeeding period 

 of eleven years, the means at the different depths are as follows : — 



* Account of some Experiments on the Temperature of the Earth near Edinburgh, 

 Trans. Roy. Soc. Edinb. vol. xvi, part 2. 



