There is a very ingenious water-driven motor which is employed merely to record the amount of water flowing through it. This is the Thomson water meter which is illustrated in Figure 34. It consists of a circular chamber with inwardly dished or conical top and bottom walls. In the chamber is a flat disk with a ball and socket bearing. At one side there is a vertical diaphragm in the chamber which passes through a slot in the disk. This prevents the disk from revolving, but it is free to oscillate. It has a motion similar to the gyrations of a top when it is beginning to lose speed and die down, except that the disk does not revolve. When the disk is in contact with the bottom wall of the chamber on one side it contacts with the top wall on the other so that the chamber is virtually divided into two compartments by the disk, but by gyrating the disk these compartments are made to revolve. Water enters at one side and discharges at the other side of the vertical diaphragm. Now, if the disk is in the position shown in Figure 34, the water, on entering, bears upon the upper face of the inclined disk and wedges its way between the disk and the upper wall of the chamber, making the disk oscillate on its ball center. As the edge of the disk rises across the face of inlet port the water entering the chamber bears against the under side of the disk, continuing the gyratory motion. The water cut off on the upper side of the disk is carried around to the outlet and discharges, while a fresh supply flows in on the other side of the vertical partition and at the next half turn the water in the lower compartment discharges at the outlet side of the partition, while the compartment is filling on the other side of the partition. A measured amount of water flows through the chamber at each gyratory oscillation of the disk. A train of gearing is driven by the gyrating disk which operates a set of dial pointers and a measure of the amount of water passing through the meter is indicated. DIGGING WITH WATER JETS We have referred to the enormous velocity of the jets used to drive Pelton wheels. Where high heads of water are obtainable water jets are used very effectively for excavating purposes, particularly in mining plants for washing down gold-bearing gravel banks. If water is not found near such banks expensive canals, flumes, and pipe lines are constructed and even tunnels are bored to bring the water to the point where it can be utilized. Some of the giant nozzles spout streams from 2 to 8 inches in diameter with a pressure of from 50 to 200 pounds per square inch. The powerful streams tear into the gravel banks, washing them away into sluices in which riffle boxes are placed to catch the precious metal. The back pressure of these nozzles is very heavy and the larger ones have to be provided with strong anchorages. Water in motion resists any change of direction and long levers have to be provided to permit the miners to guide the nozzles.