+-----------+---------+------------+--------------+--------------+ | | | | | Pressure | | | | | Pressure | at which | | | Length. | Thickness. | expected. | Tubbing is | | | | | | proved. | +-----------+---------+------------+--------------+--------------+ | | | | lbs. a | lbs. a | | | feet. | inches. | square inch. | square inch. | | | | | | | | Top | 130 | 1·17 | 30 | 45 | | | 60 | 1·31 | 60 | 90 | | | 60 | 1·57 | 90 | 135 | | | 60 | 1·76 | 120 | 180 | | | 60 | 1·96 | 150 | 225 | | | 60 | 2·16 | 180 | 270 | | | 60 | 2·35 | 210 | 315 | | Bottom | 60 | 2·55 | 240 | 360 | +-----------+---------+------------+--------------+--------------+ The joints between the rings of tubbing are made with sheet lead one-eighth of an inch thick, coated with red-lead. The lead is allowed to obtrude from the joint one-third of an inch, and is wedged up by a tool which has a face one-twelfth of an inch thick. The mode of suspending the tubbing to the rods will be understood by referring to Figs. 152 to 154. The rods are attached to a ring by the bolts connecting one ring of tubbing with another. The bottom ring of tubbing and the ring carrying the moss box have their top flange turned inwards, but their bottom flange outwards. A strong web of iron, forming the base of a tube 16-1/2 inches in diameter, is attached to the tubbing. The object of this tube is to cause the water in the shaft to ease the suspension rods, by bearing part of the weight of the tubbing. Cocks to admit water are placed at intervals up the tube, by which means the weight upon the rods can be easily regulated, so that not more than one-tenth to one-twentieth of the weight of the tubbing is suspended by the rods at one time. The ring holding the moss box is hung from the bottom joint in the tubbing by sliding rods. [Illustration: Mode of Suspending the Tubbing to the Rods. Figs. 152-154.]