1002 Transactions of tub American Institute. 



water. When steam begins to form on the heated surface of the 

 iron, the etficiencj of the boiler immediately begins to diminish. The 

 point of circulation in boilers has been much overlooked. Unless 

 steam bubbles can be prevented, circulation is not of much value. 

 The Harrison boiler, in a meclianical and philosophical point of view, 

 is a failure. To use wrouglit iron and cast iron together is not philo- 

 sophical, the globes of cast iron in the Harrison boiler, are fastened 

 together with rods of wrought iron. Tlie cast iron must crack or the 

 wrought iron must stretch. Cast iron put togetlier with bolts cannot 

 be durable, as the wrought iron would stretch when heated, and 

 when the rods cooled they would have to be tightened. As to the 

 Harrison boiler taking less room, a boiler made of pipes can be made 

 to occupy half the space of one with globes. 



Mr. Coleman stated that wrought iron could not expand, practi- 

 cally, more than the cast iron, when it is in the water, and cannot 

 receive more heat than is in tlie water ; and the cast iron is in the 

 steam, and so the two metals are about at the same temperature. At 

 the Cliarlestown sugar house, in Massachusetts, one of the Harrison 

 boilers lias been in constant use for three years, and subject to an 

 intense heat, but he.had nev^er seen the least sign of a leak in it ; and 

 this was the case with several of these boilers in other places. A test 

 of an instrument is in its working, and no theor}' will answer in its 

 stead. 



After some further discussion of the subject, the Association 

 adjourned. 



March 24, 1870. 



Prof. Samuel D. Tillman, in the chair; Robert Weir, Esq., Secretary. 



The Chairman opened the proceedings by reading the following 

 notes on science : 



The Graphotype. 



This ingenius substitute for wood engraving, now used extensively 

 in Europe, is the invention of Mr. De Witt Clinton Hitchcock, an 

 Anierican. The process consists in coating sheet zinc with a surface 

 which can be easily removed, excepting that portion covered by the 

 drawing. Over a smooth zinc plate is thinly spread very fine pure 

 chalk powder, and on tliisis placed a highly polished steel plate, when 

 the whole is subjected to a pressure of 150 tons. After removing the 



