176 LECTURE XIX. 



cumference of wheels, the frame supporting the timber is made to turn 

 round a centre. A circular saw is used in the construction of blocks and 

 pullies ; and in order to make the motion more secure from the effect of 

 accidental irregularities, the wheels are made to turn each other by contact 

 only, without teeth. The machinery for making blocks, in the 'Royal 

 dock yard at Portsmouth, has been lately much improved 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 revolves continually 

 appears to be very considerable, since a .much greater velocity 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, the 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 processes of this kind might be performed 

 with advantage under water ; it is well known that glass may be 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, instru- 

 ments 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 are 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 ; and it has been conjectured that 

 the cold continually occasioned 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 polishing steel, 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 horizontally on a circular sur- 

 face, and in order that the action may be equally divided throughout the 

 surface, it is allowed to revolve on an axis by means of the friction ; its 

 motion being confined to one direction by the action of a catch. 



Various substances, chiefly of mineral origin, are also used, on account 

 of their hardness, as intermediate materials for grinding and polishing 

 others. These are diamond dust, corundum, emery, tripoli, putty, glass, 

 sand, flint, red oxid of iron, or crocus martis, and prepared chalk ; they 

 are sometimes applied in loose powder, and sometimes fixed on leather, 



* See Herschel's Discourse on the Study of Natural Philosophy, p. 48. 



