Page:Popular Science Monthly Volume 3.djvu/274

262 principle being the detachment of a weight when the bottom is struck. The weight is a 64-pound shot (E, Fig. 1), cast with a hole through it. An iron rod (A B) passes through this hole, with an opening or chamber at the lower end "armed" with tallow. When the instrument strikes, the end of the rod is driven into the material of the bottom, which fills the chamber. At the same time a pair of hinged arms (D) at the top, which were upright in the descent, fall down and release the cord (C), which sustains the ball by a leather collar below. As the loops of the sling are relieved from the teeth of the arms, the rod slips through the hole in the shot, and comes up alone with its enclosed sample of sediment. The difficulty with this machine was the washing out of the material in the ascent. This was remedied by Commander Dayman, by adapting a valve, opening inward, to the terminal chamber of the rod.

In 1860 the assistant engineer of H.M.S. Bulldog contrived a dredging-lead that combined the principle of Ross's clamm with the disengaging weight of Brooke. It is an ingenious and well-known machine, though hardly as simple as could be desired. Prof. Thomson thus describes it:

"A pair of scoops (A) close upon one another scissors-wise on a hinge, and have two pairs of appendages (B), which stand to the opening and closing of the scoops in the relation of scissor-handles. This apparatus is permanently attached to the sounding-line by the rope (F), which in the figure is represented as hanging loose, and which is fixed to the spindle on which the cups turn. Attached to the same spindle is the rope (D), which ends above an iron ring. E represents a pair of tumbler-hooks, fastened likewise to the end of the sounding-line; C a heavy leaden or iron weight, with a hole through it wide enough to allow the rope (D) with its loop and ring to pass freely; and B a strong India-rubber band, which passes round the handles of the scoops. In the figure the instrument is represented as it is sent down and before it reaches the bottom. The weight (C) and the scoops (A) are now suspended by the rope (D), whose ring is caught by the tumbler-hooks (E). The elastic ring (B) is in a state of tension, ready to draw together the scoop-handles and close the scoops, but it is antagonized by the weight (C), which, pressing down into a space between the handles, keeps them asunder. The moment the scoops are driven into the ground by