1014 Transactions of the American Institute. 



Mr. Fisher thought that this might be accounted for by the fiict that 

 in the single screw propeller the water from the propeller is driven 

 against the rudder, and does not have clear way. 



Mr. C. E. Emery thought that the reason why twin screws placed 

 at the sides of a vessel are more effective than a single screw at the 

 stern, is that the twin screws work in more solid water than the 

 other. ■ 



Mr. J. Johnson presented his method of forming a tunnel under- 

 neath the East river. The subject has been fully discussed at former 

 meetings. 



Self-acting Alakm foe Boileks. 

 Mr. Edwin Ferguson exhibited a model of his self-acting alarm for 

 low water in boilers. The apparatus seemed to promise efficiency 

 and durability. 



March 18th, 1869. 



Professor S. D. Tillman in the chair; Mr. C. E. Emery, Secretary. 



In relation to the first item of scientific news on "zinc in boilers 

 to prevent scale," read by the chairman, Dr. L. Feuchtwanger 

 remarked that he tried this method in 1835 ; he cast cylinders of zinc 

 for this purpose. They were some six inches long, and suspended in 

 the water. 



Dr. P. H. Vanderweyde said that, if the zinc is in contact with the 

 iron, it should have the opposite eftect. Copper vessels have been 

 plated with zinc to prevent this. So when zinc is in contact with 

 tlie iron it has the opposite effect. Zinc is the most electro-positive 

 of the common metals. The iron will be protected at the expense of 

 the zinc. There was a patent granted some three years ago, for put- 

 ting a plate of copper in the boiler, which keeps the boiler very clean ; 

 but, at the same time, the boiler is covered with the oxyd of iron, 

 Avhich slowly dissolves the iron. Zinc is a very active agent in pro- 

 ducing galvanic currents! The zinc will prevent the iron from oxy- 

 dation, but copper will have the opposite effect. 



Mr. C. E. Emery stated that, if a piece of zinc is suspended in the 

 water in a boiler, the pitting of iron of the boiler is prevented, and it 

 stops galvanic action also. 



