284
MODERN STEAM PRACTICE.
engines of this dass have a trunk passing through both ends in asimilar manner, by which means more bearing surface is obtainedfor carrying up the piston, it being supported as it were with atubulär beam,—thus reducing the wear and tear in the cylinder.The air valves are arranged in the covers, and consist of round
discs, working on suitable grat-ings, with guards to limit the lift.The valves for the entrance ofthe air are placed centrally roundthe stuffing box for the pistonrod, that part of the cover beingstrongly ribbed in the casting;and an annular chamber is castround the circumference of thecover, which is fitted with valvesof the same description for theexit of the air,—a large openingbeing left at the bottom, in Con-nection with the main pipes, &c.,to the furnaces. Small coversare fitted for the convenience ofinspecting the exit valves.
This engine is used to blow airto two furnaces; the area of theblowing piston is Ö3'C>72 squarefeet, and it discharges 24,976cubic feet per minute. As amplerubbing surface in all the work-ing parts has been well consi-dered, this engine has been foundto be very economical in thematter of repairs. The diameterof the fly wheel is about 21 feet;and as the whole of the ma-chinery is built up on one frame,from end to end, the foundation is laid in brickwork, with about2 feet of stonework on the top. This dass of engine is certainlythe cheapest that can be supplied for heavy work; and the enginehouse need not be so large as for the overhead-beam arrange-ments.