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230 LECTURE XIX. 



ing blocks, in the Royal dock yard at Portsmouth, has been lately much im- 

 proved and enlarged; it is worked by a steam engine, the action of which is 

 applied to a great variety of purposes. The advantage of a saw which re- 

 volves continually, appears to be very considerable, since a much greater velo- 

 city may be given to it than can be obtained when the motion is alternate. 

 Such a saw has also sometimes been applied to cutting off piles under 

 water. 



In mills for sawing marble into slabs, tlie saws are drawn backwards and 

 forwards horizontally: they are made of soft iron, without teeth; and sand 

 being applied to them, with water, during the operation, the sand is partly 

 imbedded in the iron, and grinds away the marble. 



'Granite is worked by driving a number of thin wedges very gradually into 

 it, at various parts of the section desired ; and sometimes wedges of wood are 

 employed, which being moistened by water, their expansion separates the parts 

 from each other. It is also said that many stones may be divided by drawing- 

 lines on them with oil, and then exposing them to heat. Perhaps some pro- 

 cesses of this kind might be performed Avith advantage under water; it is well 

 known that glass maybe cut in a rough manner under water, without much 

 difficulty, by a common pair of scissors. 



For reducing the magnitude of a substance in a particular part, instruments 

 of attrition are used; rasps, files, grindstones, and hones; and of all these 

 the immediate actions appear to resemble those of chisels and saws. The 

 hatches of files are cut with a hard chisel while the steel is soft, and the files 

 arc afterwards hardened. In using the grindstone,- water is applied, in order to 

 avoid the inconvenience produced by too much heat; and sometimes tallow is 

 substituted for water with equal advantage: but oil is not found to answer the 

 same purpose; audit has been conjectured that the cold continually occasion- 

 ed by the melting of the tallow at the point of friction, serves as a substitute 

 for the cooling effect of the evaporation of the water. For grinding and po- 

 lishingsteel, the grindstones are made to revolve, either vertically or horizontally, 

 with a velocity so great as to describe sometimes as much as 60 feet in a 

 second. The steel is also in some cases drawn backwards and forwards hori- 

 zontally on a circular surface, and in order that the action may be equally di- 



