INTRODUCTION
square inch steam pressure and 27 inches of vacuum, gave an output of 20 horsepower at 5,000 revolutions per minute. Steam consumption was only 40 pounds per brake horsepower per hour, which was indeed a remarkably good result for that time. Screw Type. Still another kind of turbine, of only historical interest, should be mentioned. A large number of inventors have worked on the development of a screw type like Fig. 6. Hewitt worked for a long time on a turbine of this kind, and finally con- cluded the results were not satisfactory. Steam was admitted to this turbine through the chamber A, and passed through holes in the plates P, P into the helical grooves on the shaft. In these grooves the steam was expanded and then escaped to the exhaust pipes E, E at the two ends. Effective action of the steam was probably obtained only in the first part of the grooves; and after being deflected into a helical course, it rushed through to the exhaust without much additional effect in moving the shaft. Excessive leakage of steam between the helical threads and the casing was another serious difficulty. Recently a somewhat similar arrangement having two " screw wheels " meshing together not unlike spiral or helical gears has been successfully developed by the Buffalo Forge Company (see Figs. 157 and 158, page 217a).
