Hollow shafting, where large diameter is not. objectionable, has long been in use, made generally of cast iron, and frequently used as a drum or continuous pulley for the reception of belts. Such a shaft was used in the " pistol shop" of Colt's factory before the destruction of the building by fire about four years ago, and a similar line may now be in use in the reconstructed building. This shaft was five hundred feet long and fifteen inches diameter, made of hollow cast-iron cylinders, connected with each other by a solid shaft or bearing at each end, resting in a box as a journal, The result was an almost continuous drum, of five hundred feet in length, from which belts led to the counter shafts of the machines, the speed of each machine being regulated by the diameter of the pulleys on the counter shafts. We have heard also of wrought iron pipes of only two inches diameter being used as shafting successfully. Tredgold says that a round tube whose internal and external diameters are as seven to ten, respectively, has twice the lateral strength of a solid cylinder containing the same amount of material. A cylinder (solid) of cast iron, five inches diameter, has a transverse strength of 21,104 pounds, whilu one of eight inches diameter, containing the same cross sectional area of metal, has ti transverse strength of no less than 45,416 pounds. These facts would seem to show plainly the possibility of reducing the weight.matcrially, of shafting without a diminution of its strength. The weight of shafting is a mass the inertia of which must be overcome by the driving power, and in some cases the amount of power, otherwise useful, that in thus absorbed, is not less than twenty per cent. If by the use of lighter shafting this could be reduced only five per cent, the saving would be worth an effort. Shafting m st be of sufficient diameter to sustain the weight of pulleys and the strain of belts without springing, but if the requisite stiffness —resistance to torsion and springing—can be obtained by hollow shafts of much less weight, not only is money saved in the first cost (shafting being furnished by the pound), but the continual expense in the absorption of unnecessary power in driving the unnecessary weight would also be prevented. That hollow shafting of wrought iron can be made cheaply is sufficiently apparent when we examine specimens of pipotisecl for various purposes. And not only would the first cost bo less, but the ease of handling, owing to reduction in weight, would lessen the cost of turning, etc. Such shafting could also be easily oiled from the inside which would soem to be the proper method.