Proceedings of the Polytechxic Association. 1001 



stroiic;. Fairbairn in his experiments sliows tliat a sheet of iron has 

 only fifty-six per cent of its original strengtli after riveting. There 

 have been varions devices to make boilers safe, but the best ]>lan is to 

 make them so that when there is a rupture but a small part of the 

 steam can be scattered around. The Harrison boiler fully answers 

 this requirement, and during the six years they have been in gen- 

 eral use, they have proved very efficient and safe. The Harrison 

 sectional boiler is made of cast iron, which is theoretically but half 

 the strength of wrought iron*. But we argue that an inferior mate- 

 rial made in a superior shape is as strong as a superior in a bad 

 sliape. It is known that cast iron is very brittle, and that it gives 

 way suddenly; this is an important fact and a fortunate one, as it 

 gives warning of a rupture, and but a small part is injured ; but the 

 wrought iron boiler gives no warning, and when it does explode it 

 does so with terrific fury. The circulation in the Harrison boiler is 

 free, but a little circuitous, it is as perfect as in an ordinary kettle. 

 It is said that if water is rushed over heated iron very rapidly it will 

 make steam quickly ; but there is just one way to make water boil 

 just as it was in the days of Adam. The experiment of using a fan 

 wheel in a boiler to scatter the water about, was a perfect failure. 

 In the Harrison boiler steam can be raised in forty minutes. Mr. 

 Harrison has spent many tliousand dollars simply in experimenting, 

 and his experiments on boilers have lasted for twenty years. The 

 series of globes which constitute his boiler are eight inches in diame- 

 ter, and are very strong. They have been in use for two years with- 

 .out forming any scale, while others scale in six months, using the 

 same water. Out of 56,000 globes in use, we have had but fifteen 

 that have cracked legitimately, and these have all cracked in the 

 neck. This is a fraction of one pes* cent. So this side of the account 

 does not look bad. 



Mr. John B. Root remarked tliat a l)oiler deposits its scale invaria- 

 bly at the lieated surface ; theve is a little bubble of steam exploded 

 there, and a small sediment is left. This takes place on a stove the 

 same as in a steam boiler ; but when it is done in the latter, the sed- 

 iment is kept some time in solution, and can be carried off with the 

 steam. But if tlie bubble of steam can be carried up toward .the 

 surface of the water, where the pressure is less, no sediment will be 

 directly deposited. Circulation is of mucli benefit in relieving the 

 boiler from scale. Steam is a non-conductor, and these bubbles form 

 on the surface of the iron, and obstruct the passage of the heat to the 



