Commence hollowing the semicircular blower by working round the circular part with a series of regular blows from a bullet-faced hammer, and holding the edge of the metal over a shallow hole in the hollowing block. This will curve the metal to a slight depth round the edge. Then bend the metal about lin. from the edge along the straight part, so that it makes a sharp angle; this will keep the bottom stiff while the remainder of the hollowing is done. Now commence on the circular part again, and work round from the edge in towards the centre, in a series of concentric circles, working it in a deeper hole if necessary than that used for commencing in. When the blower is hollowed to the depth necessary, go over the hollowed part again with a series of light regular blows until it is rendered smooth. Knock out smooth the break along the bottom, and then bend the ends round to the same curve as the hollowed part. A few blows from a flat-faced hammer, delivered upon the centre or flat part of the blower, may be necessary to set it so that it will be free from twist.