There are several ways of carrying out the process of doubling, which in its simplest sense consists in laying a given number of folds of yarn together and putting a twist into them. But beyond this we come to spindle banding, which is cord, or rope in miniature, and it is made of three or more ends of the doubled yarn just mentioned, such doubled yarn becoming, in fact, the strand of a small rope. To lay these strands properly into a cord they should not only be twisted together, but each should be twisted separately in the opposite direction to the twist of the cord. A banding machine, therefore, has to impart a double twist, and to perform the work perfectly each twist should be capable of easy regulation; and the drag upon the bobbins should admit of being adjusted to requirements. These conditions are met in the James machine, as evidenced by the samples of work produced by it. As shown in our engraving, the apparatus consists of three heads, of four spindles each, being capable therefore of doubling a four-strand cord. The heads work independently of each other, and by throwing one or two of the spindles out of action a three-strand or a two-strand cord will be produced. The cord is twisted regularly, and may be made continuously to any length. The uniformity of the twist depends upon the fact that the cord is taken up at a regular rate, by a simple and neat motion, consisting merely of a pair of pulleys, one grooved and the other with a roughened surface. After leaving these, the cord is coiled upon the reels seen on the top of the framework. The twist given to the cord depends upon the rate at which it is taken up, the speed of the center spindle remaining constant. The twist in the strands is governed by the speed at which the bobbin spindles revolve. This may be adjusted as required, by a series of change wheels. An effective stop motion is also applied to automatically stop the head in which a breakage takes place, whether of the cord itself or of a single strand. Either head is started by depressing the handle or knob in front of it. A feature for which particular merit is claimed is that a heavy drag is put upon all the strands separately for the purpose of taking out all the stretch before twisting, which is an important desideratum for the production of good banding. This is accomplished by hanging weights on the spindles, which cause the strands to be twisted under tension. The tension is altered as required by the size and nature of the yarn, by removing or adding to the weights. The heads being independent of each other enables the machine to be employed on three cords of different material and thickness. The production is about 1,300 yards per head for ten hours, and it is stated that a girl can mind as many as 80 or 90 heads.--_Iron._ BOILER TUBES. The following table gives the draught area and heating surface of the various sized boiler tubes and flues: