With Mr. Witz we cheerfully admit all the advantages that he so clearly establishes with his perfect competency in such matters, but there still remain two points upon which we wish to be enlightened. Are not the starting up, the operation and the keeping in repair of a gas generator actually more complicated and more delicate than the same elements of a steam engine? Does not the poor gas manufactured in a gas generator present, from a hygienic point of view, danger sufficiently great to proscribe the use of such apparatus in many circumstances? Such are the points upon which we should like to be enlightened before unreservedly sharing Mr. Witz's enthusiasm, which, however, is justified, economically speaking, by the magnificent results of the experiments made by the learned engineer.--_La Nature_. [Illustration: FIG. 3.--GAS MOTOR OF 100 INDICATED HORSE POWER.] * * * * * IMPROVED PNEUMATIC HAMMER. We publish illustrations of a Thwaites suspension pneumatic power ½ cwt. hammer of a new design, for planishing pipes and plates, for which we are indebted to _Engineering_. As indicated in the perspective view (Fig. 1) the mechanism is supported at the center of a cross girder resting on two cast iron square pillars, box section, each bolted down to the foundations by four 1¼ in. diameter bolts. The measurements of these columns and girders are given in Figs. 2 and 3, the former an elevation of the hammer and the latter a plan, partly in section, of the cross girder, while Fig. 4 is a cross section showing the arrangements for operating the hammer. In the center is a cast iron guide for working the ram, the guide being extended on two sides to receive the disk crank journals, 2 in. in diameter by 3½ in. long. The disk cranks are connected to a hollow steel ram by a connecting rod. The ram is divided inside into two compartments, each having a phosphor bronze air piston. These are connected together by a steel piston rod, the top air piston forming a connection for the small end of the connecting rod. The outside diameter of the ram is 3¾ in., and the diameter of the air pistons 2¾ in. and 2-7/8 in. respectively. Cottered into the bottom of the ram is a steel pallet holder with a dovetail, so that the pallet can be renewed or exchanged for one of another shape when required. Keyed on to the crankshaft is a flanged pulley 10 in. in diameter by 3¼ in. between flanges. There is also an overhead countershaft with strap shifting arrangement. At the side of one of the columns a hand lever and quadrant are provided, as shown in the perspective view and in Fig. 2, for working an arrangement for tightening the belt when the machine is working. To this arrangement is connected a powerful brake which stops the machine in a few revolutions. It will be seen that the brake is applied as the belt is slackened for stopping the machine. For planishing pipes or tubes a long wrought iron mandrel is provided