Returning to our subject, we come next to Pranayarna, controlling the breathing. What has that to do with concentrating the powers of the mind? Breath is like the fly-wheel of this machine, the body. In a big engine you find the fly-wheel first moving, and that motion is conveyed to finer and finer machinery until the most delicate and finest mechanism in the machine is in motion. The breath is that fly-wheel, supplying and regulating the motive power to everything in this body.