duced by Messrs. Penn in the Hydra are represented in figs. 10 and 11; and these boilers have the advantage of a cylindrical shell, and may be worked with safety to a pressure of 40 lbs. on the square inch. But the furnaces are too small; and eylindrical furnaces are very inconvenient for enabling a proper slope to be given to the fire-bars, as the width of the BOILERS OF H.M.S. HYDRA, BY MESSRS. JOHN PENN AND SON. Transverse Section, showing Tubes. Transverse Section through Furnaces. bars at the back end is necessarily contracted; and the bars must either be made taper, or taper pieces must be cast to fill the vacuities at the front. is also too little room for the ashes. There The species of water-space boiler known as Lamb and Summers' boiler has now been widely introduced, and has this special feature of advantage, that it enables a rapid circulation of the water to take lined with fire- brick, and communicating with the furnaces by short necks, through which the heat ascended. In 1839 I introduced smokeless furnaces into different steamers, in which the arrangements had to vary to suit existing boilers; and in some cases I caused the smoke to pass over the incandescent fuel, in other cases to pass through heated fire-tile channels, and in other cases again two adjacent furnaces were fired alternately, and the smoke from the one passed through the glowing embers of the other. All these expedients, however, are imperfect; and no species of smokeless furnace has yet been contrived of such conspicuous eligibility, as to ensure its general adoption. I believe that a good smokeless furnace and a good self-feeding furnace will come together. GIFFARD'S INJECTOR FOR FEEDING BOILERS. This is an instrument for forcing water into boilers by means of a jet of steam proceeding from the boiler itself; and its action is somewhat paradoxical, as it is capable of sending water into a boiler which has a considerably greater pressure of steam than that which the steam comes from. The feed water must not be hotter than 120° Fahrenheit, to enable the injector to act; for one condition of its action is that the steam shall be condensed; and this circumstance appears likely to restrict the use of the instrumentunless suitably modified-as the feed water should certainly enter the boiler at the boiling point, to which it should be raised by heat otherwise going to water amongst them is greatly impeded; and it has in consequence been found, that the evaporative power of such boilers has been increased by removing some of the tubes altogether the loss of a part of the heating surface being more than compensated by the increased efficacy of the rest. An example of the form of tubular boiler commonly used in first-class steamers in 1867 is given in fig. 14, which is a cross section of the troop-ship |