“The experience of American manufacturers, which has been more extensive than that of others, has proved the value of direct compression as distinguished from indirect. By direct compression is meant the application of power to resistance through a single straight rod. The steam and air cylinders are placed tandem. Such machines naturally show a low friction loss because of the direct application of power to resistance. This friction loss has been recorded as low as 5%, while the best practice is about 10% with the type which conveys the power through the angle of a crank shaft to a cylinder connected to the shaft through an additional rod.” =Receivers.=--Compressed air is stored in receivers which are simply iron tanks capable of withstanding a high internal pressure. The purpose of these tanks is to provide a reservoir of compressed air, and also to allow the air to deposit its moisture. From the receivers the air is conveyed to the workings through iron pipes, which decrease gradually in diameter from the receivers to the front. =Rock Drills.=--The various forms of rock drills used in tunneling have been described in Chapter III., and need not be considered in detail here except to say that American engineers usually employ percussion drills, while European engineers also use rotary drills extensively. A comparison between these two types of drills was made in excavating the Aarlberg tunnel in Austria, where the Brandt hydraulic rotary drill was used at one end, and the Ferroux percussion drill was used at the other end. The rock was a mica-schist. The average monthly progress was 412 ft., with a maximum of 646 ft., with the rotary drills, and an average of 454 ft. with the percussion drill.