If there are but few tubes to be removed, a passage is made for them toward the blow-off plug by removing a few of the tubes beneath. When the tubes to be removed are not too far from the plug, this method is very satisfactory. Even though there were a few more tubes removed, the cost of such removal would be more than compensated for, because this method is cheaper, and preserves the tubes and plates, and because the boiler, by receiving a larger number of clean tubes, will afterward utilize the fuel better. 3. REPAIR OF TUBES IN SERVICE, AND PUTTING THEM IN PLACE AGAIN. Either when the removed tubes are to be employed anew, or when they are to be classed as old material, it is equally necessary to free them from the incrustation that covers them. The methods employed vary according to the shop. The cleaning of tubes by beating or scraping the incrustation is very difficult, and requires much time. In some shops the tubes are dipped into an acid bath. In this way only the incrustation composed of carbonate of lime is dissolved, that into the composition of which sulphuric acid enters not being attacked. In some large shops there are iron drums in which the tubes are placed. When these drums are revolved the incrustation becomes partially detached, but very rarely completely, and it is always necessary to finish the work by hand. It also happens that the bits of scale that become detached and that remain between the tubes produce grooves therein; besides, the cost of installing these drums is quite high. Per contra, the writer has seen a, as yet, little known method employed in the shops of the Berlin-Hamburg Railway, one that he has used himself, that he has introduced into several shops, and that he can recommend as the best. The tube to be cleaned is submitted to a rotary motion around its longitudinal axis. The workman grasps it with a sort of wooden pincers whose jaws are provided with coarsely toothed steel plates, and, pressing the legs of this more or less tightly, slides it slowly along the tube. The incrustation is thus reduced to dust, and the tube, after the operation, is absolutely clean. The apparatus used for revolving the tubes is shown in Figs. 1 to 3. It consists of quite a short shaft, which revolves in two pillow-blocks and receives its motion through pulleys. Outside of the bearing to the right, this shaft terminates in a cone provided with channels whose diameter is proportioned to that of the tubes. The tube to be cleaned is firmly fixed upon the cone, and provided at its other extremity with a plug that serves to center it. As the cleaning is accompanied with much dust, it must be done in open air or in a special shop. At the same time, a classification is made of the damaged tubes that can no longer be employed, except as ends of the tubes that may be employed in shorter boilers, and of those that are entirely unserviceable.