Proceedings of the Polytechnic Association. 997 



The cylinder-boiler, however, requires so much space in order to pro- 

 vide heating surface in so simple a form, and is so expensive and 

 dangerous, that we must look for some other construction of steam 

 generators for general use. In order to reduce the size of tlie boiler 

 with the same heating surface, M'e, so to speak, double it up by put- 

 ting in flues. This introduces the drawback of unequal expansion 

 and contraction ; a most fruitful source of mischief. To make the 

 boiler still smaller without reducing the heating surface, we double 

 it up again bj changing the flues to tubes, which increases the objec- 

 tion just mentioned ; and in practice, foot for foot, the heating surface 

 is not as efiicient as that of the old plain cylinder. A perfect boiler 

 would combine the heating surface of a cylinder-boiler with water held 

 in small quantities in extremely strong receptacles. Cylinder-boilers are 

 not commonly made in the best manner. Any rupture at high pres- 

 sure allows the great volume of water to flash into steam with a 

 corresponding force of explosion. In most safety boilers, made up of 

 tubes and with thin volumes of water, there is t®o much heat for the 

 quantity of water when the boiler is forced to its work in actual ser- 

 vice, and the rapid evolution of steam drives the water out of contact 

 with the heating surface. This is obviated in the Harrison sectional 

 boiler, by using spheres, which have twice the strength of cylinders, 

 which contain water enough to keep it in contact with the heating 

 surface, and which allow circulation crosswise as well as longitudin- 

 ally of the ranges of spheres. These are so united that before a pres- 

 sure is reached sufficient to burst the spheres, the connecting-ruds will 

 stretch, so that each joint constitutes a safety-valve, and thus avoids 

 the results of a sudden escape of a large quantity of highly heated 

 water. The plain cylinder boiler explosions are the most terrific 

 known in this or any other country. The difterent parts of the boiler 

 are heated unequally, so that it has been said that if a boiler of this 

 kind was made of silk, it would be found to be puckered up in many 

 places. Another defect in many boilers is the rivets are improperly 

 fitted ; if the holes in the plates are not in line, a toper punch is 

 driven in the hole and made to crowd in line. Large boilers are 

 unsafe. The best method of circulation in boilers is to hold small 

 quantities of water in small receptacles, and should any of these be 

 shattered, no serious results can be apprehended. Mr. Harrison claims 

 that as far as a boiler can be made perfect he has done bo ; and, in 

 regard to the matter of circulation, liis l^oiler has as much as is 

 required. A good engineer and fireman are all important requisites 



