When apparatus for opening tubes are used, care must be taken to begin with the external tubes, and to open these gradually. The same operation is afterward performed upon the neighboring ones, in approaching the center, and then the first ones are taken up. The tubes should never be opened at one operation, but each one should be subjected to several passes. At the second pass, the rollers should be placed a little more deeply, and should then be half within the tube-plate. The tube thus opens behind the plate and forms a bearing against it, and this not only renders it tighter, but also increases its adhesion to the plate. Finally, the operation is finished by beating down the edge of the tube that has been raised a little by the preceding pass. If this edge is already somewhat deteriorated, or if it is not very thick, tightness may be had by means of rings. The use of rings should be avoided as much as possible, because they diminish the section of the tubes, and render the cleaning of their interior more difficult. They should only be employed as an exception, and should be considered as an unavoidable evil. Even in old boilers, in which the holes have become oval, they should be considered only as a means of rendering a small number of tubes tight. 2. REMOVAL OF WORN-OUT TUBES. The tubes are taken out independently of one another through the front tube-plate, after an incision has been made with a chisel through the part of the tube that is fixed into the back plate. When the holes in the front tube-plate are not greater in diameter than the external diameter of the tube, and the latter is incrusted, this process becomes very difficult, and the use of it often completely spoils the tube. In fact, we can only remove the tube by live force, and for this purpose we either use the shock of a heavy body or mechanical apparatus upon whose arrangement I shall not dwell. In all cases the holes of the tube-plate are injured. The edge of these must, in fact, detach the scale from the tube before the latter can be removed from the boiler, and, when a little of this scale remains adherent, it produces grooves in the hole, which render it very difficult later on to make the new tube tight. It is consequently preferable to cut the tubes immediately back of the plates by means of a special apparatus consisting of a cone provided with a small circular steel saw. This operation should be begun at the bottom of the boiler near the blow-off plug, and be continued in advancing toward the top. The cut tubes fall to the bottom of the boiler, and are removed through the blow-off hole of the front tube-plate. The pieces of tube that remain in the plate are afterward easily removed by cutting them with a chisel.