MECHANICS AND USEFUL ARTS. 37 



water. This fact is the most striking discovery that has been made 

 in boiler engineering. It removes the fatal defect of small subdivid- 

 ed water spaces, which can now be employed with the certainty of 

 their remaining constantly clear of deposit. This discovery has been 

 made in the use of a cast-iron boiler invented by Mr. Hamson of Phil- 

 adelphia, and which is constructed as follows : Any number of cast- 

 iron hollow sphere's are employed, whose dimensions are eight inches 

 in external diameter, and three-eighths of an inch thick, communicat- 

 ing with each other through open necks, and being held together by 

 internal tie bolts. A number of these spheres are arranged in the form 

 of a rectangular slab ; and several of these slabs, set side by side, and 

 connected together, form the boiler, about two-thirds of the whole 

 number of spheres being lllled with water, while the remainder serve 

 as steam room. The bursting strength of these spheres corresponds 

 to a pressure of upwards of 1,600 pounds per square inch, as veriiied 

 by repeated experiment, being, therefore, from six to seven times 

 greater than that of the ordinary Lancashire boilers of large size. The 

 evaporative power, as in all other boilers, depends upon the extent 

 and ratio of the grate area and heating surface ; but in practice Irom 

 seven and a half to eight pounds of water are evaporated, per pound 

 of coal in a cast-iron boiler, which, for each tun of its own weight, 

 supplies steam equal to ten indicated horse-power. The joints be- 

 tween the spheres are made by special machinery, securing the utmost 

 accuracy of fitting, and there is no leakage, either of water or steam. 

 The spheres occupied as steam space, are screened by lire bricks from 

 the direct action of the heat ; but enough is allowed to reach them to 

 secure complete drying, and if desired, any degree of superheating of 

 the steam. The slabs into which each series of spheres is assembled, 

 are placed in an inclined position, which secures the thorough calcula- 

 tion of the water. The whole quantity of water carried in a 40-horse 

 boiler, is three tuns, the boiler weighing 13 tuns, and presenting 1,000 

 square feet of water-heating, and 500 square feet of steam-drying sur- 

 face. In Manchester, with the feed-water taken from the Irwell, or 

 from the canal, a hard scale is soon formed in the ordinary boilers ; 

 but in the cast-iron boiler a succession of thin scales of extreme hard- 

 ness are found to form upon and to be ome detached of themselves 

 from the inner surfaces of the water spheres. These scales are blown 

 out with the water at the end of the week, and only small quantities 

 can be discovered when purposely sought for. 



A pint of loose scales and dirt is the most that has yet been found 

 in a careful internal examination, after nine months daily work. JSTone 

 of the iron is removed with the scale, the weight of the spheres after 

 three years 1 service, being the same as when new. In America, Mr. 

 Harrison's cast-iron boiler has been worked for six years. 



The self-scaling action, which has been found to be the same in all 

 cases where the boiler has been worked, can only be explained by con- 

 jectures. 



STEEL BOILERS. 



Important experiments hive recently been made in Prussia with ste^el 

 boilers. Two boilers, e:;ch 30 feet lung and 4 feet in diameter, without 

 flues, were placed side by side. One was made of steel plates one-fourth 

 4 



