TRANSACTIONS OF THE SECTIONS. 



la 



Trap Rock of Calton Hill. 



Notwithstanding the cause of uncertainty which has been alluded to, these 

 results make it highly probable that the augmentation of mean temperature 

 from 3 feet to 24 feet below the surface, apparently l°-38 Fahr. in the first 

 period and "84° in the second period, must be really more than half a degree, 

 or more than the greatest elevation of temperature that had been observed, for 

 a depth of 21 feet, in any other part of the earth. The author was struck 

 with this, and reflecting that probably the Edinburgh observations are the only 

 ones that have been made on the interior temperature of other igneous rocks 

 than granite, supposed it to indicate the comparatively modern time at which the 

 trap rock of Calton Hill has burst up in an incandescent fluid state. This conjec- 

 ture, shortly after it occurred to him, was confirmed by the intelligence he received at 

 Kreuznach, in Rhenish Prussia, that the temperature in the porphyry of that locality 

 increases at the rate of from 2° to 3° Reaumur in 100 feet downwards, being more 

 than double or triple the rate of augmentation which had been observed in numerous 

 localities in England, France, and other parts of Europe, in granitic rocks and sedi- 

 mentary strata, and found to be about 1° Fahr. of elevation of temperature in fifteen 

 yards at the least or in twenty yards at the greatest, as Professor Phillips has shown 

 in his Treatise on Geology, in Lardner's Cyclopaedia, from careful observations made 

 by himself and others. The author pointed out, that the mathematical theory of 

 heat, — with data as to absolute conductivities of rocks, such as those supplied by 

 Professor Forbes, £ind with the assistance of observation on the actual cooling of 

 historic lava streams, such as the great outbreak from Etna which overthrew Ca- 

 tania in 1669, or of those of Vesuvius which may be seen in the incandescent state, 

 and observed for temperature a few weeks or months after the commencement of 

 solidification, — may be applied to give estimates, within determined limits of ac- 

 curacy, of the absolute dates of eruption of actual volcanic rocks of prehistoric 

 periods of geology, from observations of temperature in bores made into the vol- 

 canic rocks themselves and the surrounding strata. 



On the £!lectric Qualities of Magnetized Iron. 

 By Professor W. Thomson, M.A., F.R.S. 



The well-known ordinary phaenomena of magnetism prove that there is a wonderful 

 difference between the mutual physical relations of the particles of a mass of iron 

 according as it is magnetized or in an unmagnetic condition. Joule's important 

 discovery, that a bar of iron, when longitudinally magnetized, experiences an 

 increase of length, accompanied with such a diminution of its lateral dimensions as 

 to leave its bulk unaltered, is the first of a series by which it may be expected we 

 shall learn that all the physical properties of iron become altered when the metal 

 is magnetized, and that in general those qualities which have relation to definite 

 directions in the substance are differently altered at different inclinations to the 

 direction of magnetization. In the present communication, the author described 

 experiments he had made — with assistance in defraying the expenses from the Royal 

 Society, out of the Government grant for scientific investigations — to determine the 

 effects of magnetization on the thermo-electric qualities, and on the electric conduc- 

 tivity, of iron. 



The first result obtained was, that longitudinally magnetized iron wire, in an 

 electric circuit, differs thermo-electrically in the same direction as antimony from 

 unmagnetized iron. This any one may verify with the greatest ease by applying a 

 spirit-lamp to heat the middle of an iron wire or thin rod of iron a couple of feet 

 long, with a little magnetizing coil of copper wire (excited by a cell or two of any 

 ordinary galvanic battery) adapted to slide freely on it, and so bring a magnetizing 

 force to act on two or three inches in any part of the length of the iron ; and, when 



