90 REPORT 1877. 



greenstone, and north end of the same district would furnish the grits, sand- 

 stone, and limestone. Nearly half are granites, others millstone-grit, with 

 limestone, chert, Triassic sandstone, and coal-measure sandstones. About 

 280 feet above sca-levcl. 



The group extends over an area of about \k mile by i a mile wide. At 

 depths of from 1 foot up to 20 feet in " drift, -0 ' these boulders arc found in 

 heaps; the "drift" has been penetrated (for a deep sewer) 30 feet, arid 

 bottom not reached. 



Group No. 2.— At Thurnby, 5 miles south-east of Leicester. Size of 

 boulders from 2 feetx l|xl| down to cubic blocks about 9 inches on each 

 face. All edges sharp and angular. Rocks of the same nature occur at the 

 south end of Charnwood Forest. All seen are granites, syenites, greenstones. 

 Height above the sea is about 500 feet mean tide Liverpool. 



The area occupied is a mile square, but they are scattered in groups and 

 patches. The boulders occur at depths of 1 to 2 feet in " drift.'' Great 

 numbers of them have been collected and idilized for roads, others arc now 

 seen for many miles along the turnpike, supporting the footpath at intervals 

 about lii feet apart : this is a very common way of utilizing these boulders 

 in modern times; formerly they were all used up in foundations of houses, 

 churches, abbeys, walls, barns, &c. 



Fourth Report of a Committee, consisting of Prof. A. »S. Herschel, 

 M.A., F.R.A.S., and G. A. Lebour, F.G.S., on Experiments to 

 determine the Thermal Conductivities of certain Rocks, showing 

 especially the Geological Aspects of the Investigation. 



Having been led during the past year, by a renewal of their appointment 

 (with the provision of a grant amply sufficient to enable them to recommence 

 it), to pursue, and if possible to complete the experimental investigation in 

 which they have now for the fourth year been engaged, of the Thermal Con- 

 ductivities of certain Bocks, the Committee have attempted to complete this 

 research as far as the different kinds of rocks within their reach appeared to 

 offer geologically the most practical inducements to fix their places exactly 

 in a thermal-resistance scale, and to verify as certainly as possible the re- 

 sults of the observations which they have obtained in former years. 



The same form and size of rock-plates, the same steam-heater and cooler, 

 and exactly the same form of thermopile as that described in the account of 

 the experiments presented in last year's lleport were resorted to in order to 

 extend, and in part also to repeat, some of the former experiments in this 

 year's series of similar determinations. The apparatus was, however, modi- 

 fied in some essential points, in order, by changing entirely the circumstances 

 of the experiments, to leave no doubt of the reality of the observed thermal 

 conductivities, and of the extent to which their values can be trusted as repre- 

 senting correctly the true conductivities of the rock-specimens examined. For 

 this purpose a new thermopile was constructed, having as one of its elements 

 iridio-platinum instead of iron wire (German silver being, as before, the 



