The native tombrobbers, well-sinkers, dredgers and the like, however, come upon valuable objects from time to time, which find their way into private collections.
Fishing nets were common in all historic times, and the lead sinkers (83) and stone sinkers (84) are often found under the XVIIIth-XXth Dyiiasties.
This method necessitates the use of very considerable pumping power during the sinking, as the water has to be kept down in order to allow the sinkers to reach a water - tight stratum upon which the foundation of the tubbing FIG.
In the United States and Scotland rectangular pits secured by timber framings are still common, but the tendency the pressure being reduced to that of the external atmosphere when it is desired to open the upper door, and increased to that of the working space below when it is intended to communicate with the sinkers, or to raise the stuff broken in the bottom.
Modern surveying ships no longer make use of hempen lines with enormously heavy sinkers, such as were employed on the " Challenger," but they sound instead with steel piano wire not more than 310 to 215 of an inch in diameter and a detachable lead seldom weighing more than 70 lb.