MECHANICS AND USEFUL ARTS. 59 



furrowing. 2. The escape of free steam from the steam-chamber, 

 and the consequent removal of a considerable part of the pressure 

 upon the water, before its contained heat can overcome its iner- 

 tia and permit the disengagement of additional steam. 3. The 

 projection of steam combined with water with great velocity 

 upon the upper sides of the shell of the boiler, forcing it open 

 and perhaps breaking it in pieces. 4. The disengagement of a 

 large quantity of steam from the water now no longer confined 

 within the boiler, and the consequent projection of the separated 

 parts of the boiler to a greater or less distance. 



This theory agrees well with the circumstances of boiler explo- 

 sions, and receives support from the fact that they frequently take 

 place at the starting of the engine, when there is a sudden with- 

 drawal of pressure in the boiler and a violent disengagement of 

 steam and projection of water along with it. 



CAST-IRON STEAM BOILERS. 



In the early history of steam, cast-iron boilers were generally 

 used for high pressures, because the material possessed con- 

 structive advantages not to be found in wrought iron. It is well 

 known, that cast iron is better adapted to undergo the ordeal of 

 fire and water than wrought iron, and that by proper shape and 

 proportions it may be made as strong. If cast iron can be put 

 into such form as to be safe in case of rupture, it is doubtless the 

 best material for steam boilers, as one of its qualities is to break at 

 once without straining under improper treatment, unlike the 

 destructive fracture of wrought iron, consequent on its very 

 tenacity; the very brittleness of cast iron, at first sight a fatal 

 defect, is an element of safety, and for this reason this material is 

 preferable to wrought iron and to steel ; the more tenacious the 

 boiler, the greater are the destructive effects of an explosion. 



Harrison Boiler. The Harrison boiler, described in the " An- 

 nual of Scientific Discovery " for 1865, p. 36, is made up of 

 spheres of cast iron, 8 inches in diameter and three-eighths of an 

 inch thick, communicating with each other through curved necks 

 3 inches in diameter, and held together by bolts. These spheres 

 are arranged in rectangular slabs, set side by side, about two- 

 thirds of the number of spheres being filled with water, and the 

 remainder serving as steam space. The slabs being placed in an 

 inclined position, a free circulation of the water is effected. 



Though the tensile strength of cast iron is not so great as that 

 of wrought iron, the spherical form of each unit of this boiler 

 gives it an equivalent strength ; and to this form and the curva- 

 ture of the necks the inventor has ascribed the property which 

 this boiler has of casting its scale, as there is no abutment for the 

 arch of the crystallized scale to spring from. Wrought iron is 

 much more liable to corrosion than cast iron, and in proportion to 

 its purity, while the mixture of a small amount of carbon iu- 

 creases the ability to resist corrosion. In the cast-iron boiler 

 there is no weakening by rivets; and the units, if injured, can be 

 replaced by new ones without diminishing the strength. The 



