66 ANNUAL OF SCIENTIFIC DISCOVERY. 



Karmarsch. Average specific gravity of materials used for patterns : Pine- 

 wood, 0-500; oak, 0'785; beech, 0*721; pear-tree, 0'689; birch, 0-664; alder, 

 0-551; mahogany, 0-600; brass, 8-300; zinc, 7-000; tin (3 to 4 tin 1 lead), 

 7'900; lead, 11-000; cast-iron, 7-250. Compositions, red metal (10 to 15 per 

 cent, zinc), 8'600 ; bronze (copper, tin, and zinc, zinc and tin together 15 to 20 

 per cent), 8-450 ; bell-metal (zinc and tin 20 to 25 per cent.), 8'900 ; cannon- 

 metal (tin 5 to 10 per cent), 8'760. 



The shrinking or contracting in cooling, is ; 



For brass 1 from 21 



For bronze 1 from 26 



For zinc 1 from 27 



For cast-iron 1 from 32 



For cannon metal 1 from 40. 



This means that 21 cubis inches of melted fluid brass will, after cooling, oc- 

 cupy only 20 cubic inches. 



If s is the specific gravity of the pattern, S specific gravity of the casting, 

 a the ratio of shrinking, P weight of the pattern, and C the weight of the 

 casting, the rule is : 



P S (q-l) 

 s a 



The following table gives the numbers with which the weight of the pat- 

 tern is to be multiplied to obtain the weight of the casting, nearly : 



THE CASTING MADE OF 



If you wish to know the weight of a casting in brass from a pine-wood 

 pattern, weigh the pattern say 3 ounces and multiply by 15 -8X3=47 '4 

 ounces; if cast in iron, 14'OX2=42 ounces. 



MAKING STEEL TYRES. 



At the works of F. Crupp, of Eisen, in Westphalia, tyres are formed of 

 cast-steel in a very ingenious manner. A flat bar of steel is taken, two holes 

 bored in the ends of the bar, and by powerful machinery is cut through from 



