96 Mr. G. Gore on the Rotation of 



hemispherical chuck of either hard wood or iron is turned in the 



lathe to fit the pattern, and several sheets of the best quality of 



German silver are formed into cups upon it in the lathe with the 



greatest possible degree of exactness by the mechanical process 



termed " spinning." The thickness of the metal sheet employed 



will not aflfect the future operations ; but that best suited for the 



spinning process is termed " No. 10." In this mode of forming 



the hemispheres there is some difficulty in continuing the proper 



curvature completely up to the edge of the 



cup, in consequence of the persistent elasti- Fig. 3. 



city of the metal ; but this may be overcome 



by spinning the sheet of metal to some 



distance beyond the edge of the chuck in 



the form of a cylindrical prolongation and 



cutting this superfluous elastic portion off, 



or by increasing the curvature of the chuck 



near its edge, as in fig. 3*. 



Having obtainedanumber of hemispherical cups, carefully ex- 

 amine their forms and dimensions by means of a template, a rule, 

 and a pair of callipers, and see that their height when placed upon 

 their edges is exactly equal to half their diameters ; if they are 

 more, they must be reduced in a lathe. It will be found that in 

 the process of spinning, the metal, being weaker in cohesion across 

 its fibres than along them, will have stretched slightly more in 

 that direction than in the other, and the hemispheres will in con- 

 sequence be slightly oval across their axes, probably about ^^2^d 

 of an inch in a 3i-inch ball ; a mark should therefore be made 

 upon the outside of the cup to indicate the direction of the 

 fibres, so that when two hemispheres are joined together their 

 ovals may be placed at right angles to each other, and thus cor- 

 rect the ellipticity. The cups should now be selected in suitable 

 pairs, and the edge of each one drawn slightly inwards by gentle 

 blows of a very small wooden mallet uniformly all round the 

 edge, the edge of the cup being held upon a suitably-shaped 

 piece of bright steel or iron held firmly in a vice. To ascertain 

 that the two cups forming a pair are of exactly the same circum- 

 ference, they should be coated with a film of solder composed of 

 5 parts of tin and 1 part of zinc all round the edges, and to g'oth 

 of an inch down on the inner side, and slightly soldered at inter- 

 vals of about 1 inch all round with the aid of a strong solution of 

 hydrochlorate of ammonia. If the cups are found to be unequal 

 in circumference, they should be separated, and corrected by 

 means of a mallet in the manner described. The object of using 

 the above particular solder is to prevent the junction of the ball 



* Suitable hemispheres may be obtained of Messrs. Griffiths and Co., 

 Bradford Street, or Mr. Revill, Constitution Hill, Birmingham. 



