MECHANICS AND USEFUL ARTS. 17 



produced in that manner direct from the iron ore. The furnace is 

 constructed somewhat similar in form to the Rachette furnace, 

 namely, with two parallel sides sloping downward so as to form a 

 kind of trough between them. The ore is charged at both sides 

 on the top of the furnace, and slides down the inclined planes of 

 tiie two sloping sides. At the bottom of the furnace the gases 

 from the producer and the necessary supply of air are admitted, 

 and produce an intense flame, the products of combustion rising 

 upward through the masses of ore, which are acted upon in a sin> 

 ilar manner to that in the blast furnace. With very pure manga- 

 nese ores it is possible to manage the process so as to decarburize 

 the newly produced iron immediately after it is made, or rather 

 the heat can be made sufficient for melting a metal which con- 

 tains less carbon than common cast iron as made in the blast fur- 

 nace, and at a lower temperature. This metal is natural steel, 

 or "raw" steel, and, made from ores of sufficient purity, may 

 have all the qualities of the best cast steel. Mr. Siemens' new 

 process, if successful and economical, would do away with blast 

 furnaces, and all other processes for making and refining iron 

 now in use ; but it is too little advanced at this moment to allow 

 of a judgment of the probability of its practical success, to say 

 nothing about relative economies. Its practicability remains to be 

 established ; but if we consider how much the same inventors have 

 already established, how difficult it was to believe in the success 

 of the Siemens furnace itself, when first brought out, and how 

 completely they have succeeded in this respect, we may be justi- 

 fied in entertaining some hope that this new invention will ulti- 

 mately prove equally successful, although at present it may ap- 

 pear very revolutionary, and contrary to adopted notions. 

 Engineering. 



Every year this country becomes less indebted to the artisans 

 of the Old World for mechanical products which the skill of our 

 own workmen cannot supply. Our locomotive builders have 

 hitherto relied upon English manufacturers for their steel wheel- 

 tires, subjecting them to heavy expense and vexatious delays. 

 Now, however, we shall be independent of the Old World in this 

 respect, as the Nashua Iron Company have erected Siemens 1 re- 

 generative gas-furnaces, for the manufacture of steel weldless 

 tires. Editor. 



HOT-AIR ENGINE. 



The hot-air engine is based upon the fact that the most econom- 

 ical mode of obtaining power from heat is by its direct applica- 

 tion to the expansion of air, or other permanent gases, rather than 

 by that of steam or any other vapor. The hot-air engine now 

 described differs from the so-called "caloric engines" in several 

 essential particulars as to its construction, so that it is free from 

 those defects which have hitherto prevented the practical carrying 

 out of the. caloric theory. In this engine the motive power, in- 

 stead of being derivec] from the expansion of air heated in a sep- 

 arate generator, as in former engines, is produced by the expan- 

 sion of air heated by contact with the fuel itself, and, in addition 



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