36 ANNUAL OF SCIENTIFIC DISCOVERY. 



it nearly indestructible. The experience even with kitchen uten- 

 sils demonstrates this ; but it is more satisfactory to observe the 



* 



fact in engineering operations on the great scale. When Nelson 

 first introduced the hot-blast, he employed wrought-iron heating 

 stoves, and although only 300 Fahr. was at first fixed upon for the 

 temperature of the blast, the stoves were rapidly destroyed. It need 

 hardly be mentioned that a wrought-iron gas retort would be worth- 

 less, where cast iron answers well, being inferior only to fire clay. 

 It is the same with forge tuyeres, cast iron lasting indefinitely. 

 Since superheated steam began to be generally employed, much dif- 

 ficulty has been experienced from the rapid corrosion of the superheat- 

 ers. The Peninsular and Oriental Company's engineers have been 

 compelled to adopt copper, instead of plate iron, heating surfaces for 

 this purpose. Messrs. Richardson & Sons, of Hartlepool, have, on 

 the other hand, adopted cast iron, and their superheaters of this ma- 

 terial show no corrosion whatever, after four years' use. The sul- 

 phurous fumes from locomotive engines rapidly corrode the plate iron 

 station roofs, while the cast-iron girders and cornices remain unaf- 

 fected. Cast-iron bridges are indestructible by rusting, while large 

 quantities of iron scales are being removed from wrought-iron 

 bridges, including the Conway and Britannia tubes. From abundant 

 experience with cast-iron steam boilers and the tubes of cast-iron 

 heating apparatus, it may be taken as settled, that where the thick- 

 ness is moderate, cast iron may be thus employed without the possi- 

 bility of corrosion. 



Strength of Cast Iron. The tensile strength of cast iron varies 

 between 5 tuns and 15 tuns per square inch. Considered as a ma- 

 terial for boilers only, the minimum strength should be regarded ex- 

 actly, as from 16 tuns to 18 tuns has been taken for wrought-iron 

 plates. Cast-iron boilers eight feet in diameter and of great length, 

 were at one time made, but these were manifestly objectionable. 

 The spherical form of moderate diameter, is preferable ; and what- 

 ever is the bursting strength of a riveted wrought-iron cylinder, hat 

 of a cast-iron sphere of the same diameter and the same thickness of 

 metal, will be the same. The plate iron, of a strength of 18 tuns per 

 square inch, is virtually weakened to 10 tuns by the loss in riveting, 

 and, as the hollow sphere is twice stronger than the hollow cylinder 

 of the same diameter and thickness, the cast iron, having no joints, 

 becomes equal in this comparison to the wrought plate. If we could 

 always count upon the maximum strength of iron, to wit, 27 tuns per 

 square inch for wrought, and l.~> tuns for cast, a 14 feet cast-iron 

 sphere would have the same strength to resist bursting as the seven 

 feet cylinder of the Lancashire 40-horse boiler, supposing the same 

 thickness of metal in each case. 



No Scale in Harrison Boilers. But there is no occasion to make 

 a boiler as a single large sphere, for it is now ascertained from exten- 

 sive experience that hollow cast-iron spheres of small diameter, do 

 not retain the solid matter deposited by the water. Small water 

 tubes, and indeed all small water spaces in ordinary boilers, always 

 choke with deposit when the feed water contains lime ; but cast-iron 

 boiler spheres, although they may be temporarily coated internally 

 with scale, are found to part with this whenever they arc emptied of 



