The bows are placed between the two boards and are of the same length as the upper board. They are both made of four pieces of linden wood three digits thick, of which the two long ones are seven digits wide at the back and two and a half at the front; the third piece, which is at the back, is two palms wide. The ends of the bows are a little more than a digit thick, and are mortised to the long pieces, and both having been bored through, wooden pegs covered with glue are fixed in the holes; they are thus joined and glued to the long pieces. Each of the ends is bowed (arcuatur) to meet the end of the long part of the bow, whence its name "bow" originated. The fourth piece keeps the ends of the bow distended, and is placed a cubit distant from the head of the bellows; the ends of this piece are mortised into the ends of the bow and are joined and glued to them; its length without the tenons is a foot, and its width a palm and two digits. There are, besides, two other very small pieces glued to the head of the bellows and to the lower board, and fastened to them by wooden pegs covered with glue, and they are three palms and two digits long, one palm high, and a digit thick, one half being slightly cut away. These pieces keep the ends of the bow away from the hole in the bellows-head, for if they were not there, the ends, forced inward by the great and frequent movement, would be broken.
The leather is of ox-hide or horse-hide, but that of the ox is far preferable to that of the horse. Each of these hides, for there are two, is three and a half feet wide where they are joined at the back part of the bellows. A long leathern thong is laid along each of the bellows-boards and each of the bows, and fastened by T-shaped iron nails five digits long; each of the horns of the nails is two and a half digits long and half a digit wide. The hide is attached to the bellows-boards by means of these nails, so that a horn of one nail almost touches the horn of the next; but it is different with the bows, for the hide is fastened to the back piece of the bow by only two nails, and to the two long pieces by four nails. In this practical manner they put ten nails in one bow and the same number in the other. Sometimes when the smelter is afraid that the vigorous motion of the bellows may pull or tear the hide from the bows, he also fastens it with little strips of pine by means of another kind of nail, but these strips cannot be fastened to the back pieces of the bow, because these are somewhat bent. Some people do not fix the hide to the bellows-boards and bows by iron nails, but by iron screws, screwed at the same time through strips laid over the hide. This method of fastening the hide is less used than the other, although there is no doubt that it surpasses it in excellence.
Lastly, the head of the bellows, like the rest of the body, consists of two boards, and of a nozzle besides. The upper board is one cubit long, one and a half palms thick. The lower board is part of the whole of the lower bellows-board; it is of the same length as the upper piece, but a palm and a digit thick. From these two glued together is made the head, into which, when it has been perforated, the nozzle is fixed. The back part of the head, where it is attached to the rest of the bellows-body, is a cubit wide, but three palms forward it becomes two digits narrower. Afterward it is somewhat cut away so that the front end may be rounded, until it is two palms and as many digits in diameter, at which point it is bound with an iron ring three digits wide.
The nozzle is a pipe made of a thin plate of iron; the diameter in front is three digits, while at the back, where it is encased in the head of the bellows, it is a palm high and two palms wide. It thus gradually widens out, especially at the back, in order that a copious wind can penetrate into it; the whole nozzle is three feet long.
The hide is common to the head as to all the other parts of the body; the plates are covered with it, as well as the front part of the upper bellows-board, and both the bows and the back of the head of the bellows, so that the wind may not escape from that part of the bellows. It is three palms and as many digits wide, and long enough to extend from one of the sides of the lower board over the back of the upper; it is fastened by many T-headed nails on one side to the upper board, and on the other side to the head of the bellows, and both ends are fastened to the lower bellows-board.
In the above manner the bellows is made. As two are required for each furnace, it is necessary to have twelve bellows, if there are to be six furnaces in one works.