46 Dr. MacCulloch on a Method of Splitting Rocks by Fire. 



this part of the process, and now finds that the stone, on cool- 

 ing, is equally fissured, and equally admits the wedge. 



On examining the nature of the rocks submitted to this 

 process, I doubted its efficacy, and was only convinced by 

 witnessing the effects. They consisted of the roughest va- 

 riety of gneiss ; that kind which is composed, in a great de- 

 gree, of compact felspar, and of varieties equally tough and 

 refractory, of hornblende rock or hornblende schist. Nor 

 could any fissures be discovered in the blocks before the ac- 

 tion of the fire, by which, if not produced, they were at least 

 enlarged from a state previously invisible. As these are the 

 only rocks which this part of the country affords, I cannot say 

 whether the same effects would be found to take place in all, 

 more particularly in granite. It is probable, however, that 

 every rock is equally susceptible of being split by the same 

 cause ; as there are none in nature more compact and more 

 apparently free from flaws, than those in which the process 

 succeeded at Loch Ewe. 



It is easy to understand how the effect is produced, as it is 

 in glass, by the unequal expansion of the parts, even without 

 the assistance of water ; and it is equally easy to comprehend 

 how a fire of only three or four feet in diameter is thus capa- 

 ble of acting on a stone of many tons in weight. 



It is probable that the quantity of fire, as well as its dura- 

 tion, must be made to vary with the dimensions of the block 

 to be split ; but, in all the instances which he had attempted, 

 he had not at this time experienced a single failure, though a 

 very considerable number of rocks had been removed in con- 

 ducting the road for a space of about five miles. 



At the place in question, the peat was every where at 

 hand, and required scarcely any expense of carriage; none 

 other, in short, but that of being cut and cast in readiness 

 for use whenever it was wanted. Wherever it may require a 

 distant carriage in addition to that, it is evident that the 

 charge will be augmented. In Highland road-making, peat 

 is rarely far off ; and, indeed, when it is not at hand, the 

 same purpose may be served by the heath and turf, which is 

 every where present, and of which a great quantity is neces- 

 sarily removed in lining out the road. In clearing land in 



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