Chap. XI.

MECHANICS.

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of judging of the progress of machinery ; after the balance clock was used in France , about1370, the most learned men applied themselves to its improvement.

The nature of all the wheels and pinions which are contained between the two brassplates of a clock are highly curious as a machine, the first or great wheel of which in aneight-day clock movement has concentric with it a cylindrical barrel with a spiral groovecut on it: to this cylinder is attached one end of a cord, wrapped round in the groovefor any determined number of turns, and to the other end of the cord is hung a weightwhich constitutes a power or force to set the wheels in motion.

Fig. 1401. represents a machine used in porcelain manufactories for grinding the materials.The power communicates motion to fouror six lanterns whose axles descend verti-cally into a circular tub A A A A at whosebottom is a stone C, with another, D, laidthereupon whose diameter is greater thanthe radius of C: the stone D performs theoffice of a muller, and is fixed to the lowerextremity of the axle ac by a cramp edhc.

The two stones C and D are often advan-tageously replaced by porcelain plates, butin this case the plate D is loaded with aweight: the materials are put into the tub,which is filled with water; after some timeit is found that the stones C and D do notwear equally, the parts farthest from thecentre of rotation c experiencing the greatestloss, and the two surfaces do not touch at alltheir points, fn gm h remaining hollow. Torender these inconveniences less sensible, alittle liberty is allowed between the stone I)and the cramp edba, so that as the edgeswear it falls, but this only palliates the evil.

The stone D can only bemadeto wear equallyby causing each of its points to pass overequal spaces in equal times : to obtain thisthe following method is employed; the ver-tical axle passing through the cross pieceB B, is supported by a rabbet ?m, and is ter-minated by a branch cb which forms a rightangle with ah ; from the centre d of themuller Da rod tie rises, passing through acircular hole in the branch c b, and carry-ing the wheel e ; the wheel h turns freelyround the rod f h, which rises from thebranch c b, and is supported by the rabbetsin the same rod fh : the wheel p is fixed tothe traverse B B, the axle a b passes freely through its centre : the diameters of the threewheels e, h , and g are equal.

The upper section in our figure represents a very simple method of obtaining the requiredparallelism. From the centre d of the muller D a cylindrical rod de rises, passing freelythrough a circular opening made in the horizontal branch r b of the vertical axle a b .- fromanother point/of the muller, at a distance d f greater than c d, another rod fq rises, passingfreely through a circular opening in the horizontal branch k i of the vertical ‘axle hi, whichis sustained by a cross piece B B; the two branches i k and r b should be equal. When theaxle a b turns, the point d will trace a circle round the centrec, whose radius will be equal to dc ; the point f will haveanother circle, whose radius f h will be equal to erf; these tworadii will be continually parallel.

Fig. 1403. is a machine for polishing looking-glasses, con-structed by M. Sureda at the imperial manufactory ofSt Petersburgh. The polishers are worked by mechanismprecisely similar to that just described : abed is an iron barsupported by five bent bars of equal length, and disposed soas to be always parallel to each other, while the centre oneturns round its axle by the action of the mover. By reasonof this motion each point of the bar traces a circle of a radiusequal to the length of the elbows, a property which allowsof its weight being diminished by sustaining it with cords,n,s, attached to the ceiling. '4 ‘S-

Fig. 1401.