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POPULAR SCIENCE MONTHLY 



are used: 1. The intermittent dome kilns that resemble in their 

 operation common lime kilns. 2. Continuous kilns, of the Ditzseh or 

 Schoefer patterns. 3. The Hoffman ring-kiln. These kilns are 

 economical in fuel, but expensive in time and labor. The dome-kiln 

 was the first used, but has now disappeared from the United States, 

 and survives in Europe only in a few localities. The continuous kilns 

 require highly skilled labor, and are used only to a limited extent 

 either in the United States or Europe. The Hoffman ring-kiln is 

 widely used in Europe, but has found few patrons in the United States. 

 The second method of manufacture of Portland cement is by the 

 use of the rotary furnace, into which the mix is fed as a dry powder or 

 ad a wet mud. Although the rotary cement furnace was originally 

 patented in England, by Frederic Eansome, in 1885, it has been im- 

 proved in the United States to such an extent that it has become prac- 





Fk;. 5. Stone House wheiie Matekiai.s ake Sokted. 



tically an American invention. Eansome's patent required the use 

 of gas for fuel and the first rotary kilns installed in the United States 

 required gas, but gas was soon replaced by crude petroleum. This fuel 

 took the place of gas entirely, and was exclusively used until it was 

 replaced by pulverized coal, blown into the kilns and burned in a jet 

 like gas or sprayed petroleum. The current or blast of air which 

 carries the powdered coal into the kiln furnishes the oxygen for its 

 complete combustion. This is a convenient method for burning the 

 cheapest fuel on tlie market, and while it is not economical of fuel it 



