MUFFLE FURNACE FOR TEMPERING 173 vertical muffle for continuous service, constructed according to the designs of the author, was in use at the PoutilofI works (Petro- grad). In the arsenal the daily interruption of the work resulted in the rapid wearing out of the muffle, and after a run of two months it was decided to remove the cast-iron muffle and operate the furnace with a perforated brick hearth through which waste gases could pass off. The furnace was reconstructed in this manner, without a muffle, and upon being placed in operation worked perfectly, heating very uniformly and fulfilling perfectly all the demands for annealing and tempering which were made upon it. /•sv FIG. 135.—Cross-section through Firebox. FIG. 136.—Cross-section through Waste Gas Ports. The following shows the computations which were made: The volume of the heating chamber, according to the design, was 3 m3 30; the temperature of the gas was to be 1000°, the combustion being neutral, that is to say, without excess air. The instantaneous calorific intensity was computed as 2000°, giving a drop in temperature of the gases of 200° per second, the" time which the gases remain in the heating chamber being 2000-1000 200 - = 5 seconds.