CONDUCTIVITY OF MOIST AIR.
Many physical treatises still assert that moist air conducts electricity, though Silberman and others have proved the contrary. An interesting experiment bearing on this has been described lately by Prof. Marangoni. Over a flame is heated some water in a glass jar, through the stopper of which passes a bent tube to bell-jar (held obliquely), which thus gets filled with aqueous vapor. The upper half of a thin Leyden jar charged is brought into the bell-jar, and held there four or five seconds; it is then found entirely discharged. That the real cause of this, however, is condensation of the vapor on the part of the glass that is not coated with tin foil (the liquid layer acting by conduction) can be proved; for if that part of the jar be passed several times rapidly through the flame, so as to heat it to near 100° C., before inserting in the bell-jar, a different effect will be had; the Leyden jar will give out long sparks after withdrawal. This is because the glass being heated no longer condenses the vapor on its surface, and there is no superficial conduction, as in the previous case.
FLOATING PONTOON DOCK.
Considerable attention has been given for some years past to the subject of floating pontoon docks by Mr. Robert Turnbull, naval architect, of South Shields, Eng., who has devised the ingenious arrangement which forms the subject of the annexed illustration. The end aimed at and now achieved by Mr. Turnbull was so to construct floating docks or pontoons that they may rise and fall in a berth, and be swung round at one end upon a center post or cylinder--nautically known as a dolphin--projecting from the ground at a slight distance from the berth. The cylinder is in deep water, and, when the pontoon is swung and sunk to the desired depth by letting in the necessary amount of water, a vessel can be floated in and then secured. The pontoon, with the vessel on it, is then raised by pumping out the contained water until she is a little above the level of the berth. The whole is then swung round over the berth, the vessel then being high and dry to enable repairs or other operations to be conducted. For this purpose, one end of the pontoon is so formed as to enable it to fit around the cylinder, and to be held to it as to a center or fulcrum, about which the pontoon can be swung. The pontoon is of special construction, and has air-chambers at the sides placed near the center, so as to balance it. It also has chambers at the ends, which are divided horizontally in order that the operation of submerging within a berth or in shallow water may be conducted without risk, the upper chambers being afterwards supplied with water to sink the pontoon to the full depth before a vessel is hauled in. When the ship is in place, the pontoon with her is then lifted above the level of the berth in which it has to be placed, and then swung round into the berth. In some cases, the pontoon is provided with a cradle, so that, when in berth, the vessel on the cradle can be hauled up a slip with rails arranged as a continuation of the cradle-rails of the pontoon, which can be then furnished with another cradle, and another vessel lifted.
It is this latter arrangement which forms the subject of our illustration, the vessel represented being of the following dimensions: Length between perpendiculars, 350 feet; breadth, moulded, 40 feet; depth, moulded, 32 feet; tons, B. M., 2,600; tons net, 2,000. At A, in fig. 1, is shown in dotted lines a portion of the vessel and pontoon, the ship having just been hauled in and centered over the keel blocks. At B, is shown the pontoon with the ship raised and swung round on to a low level quay. Going a step further in the operation, we see at C, the vessel hauled on to the slipways on the high-level quay. In this case the cylinder is arranged so that the vessel may be delivered on to the rails or slips, which are arranged radially, taking the cylinder as the center. There may be any number of slips so arranged, and one pontoon may be made available for several cylinders at the deep water parts of neighboring repairing or building yards, in which case the recessed portion of the pontoon, when arranged around the cylinder, has stays or retaining bars fitted to prevent it leaving the cylinder when the swinging is taking place, such as might happen in a tideway.
Fig. 1. IMPROVED FLOATING PONTOON DRY DOCK.