A Treatise On The Principles And Practice Of Harbour Engineering

46 HARBOUR ENGINEERING. The Sounding Machine consists of a wheel wîth a grooved rim, on which is wound the sounding line, a fine steel wire having the lead attached to its free end. The wheel is mounted in a frame which is arranged for damping to the gunwale of a boat at its starboard quarter. The sounding-boat is also equipped with a sprit and a leading block fitted over the starboard bow. On the back of the wheel is a spiral reel, on which a second line, called the “ Preventer Line,” is wound. The free end of this line, after being passed forward and through the sprit block, is taken back to the lead to which it is attached, the line thus forming a right-angled triangle. The function of the line is to prevent the trailing of the lead astern of the boat when the latter is moving. It not only ensures the verticality of each dip, but it also enables the lead to be maintained within a few feet of the bottom, so that dips may be taken frequently and regularly. Any casting forward of the lead is entirely obviated. The wheel and the reel are so proportioned relatively to each other and to the horizontal line from the wheel to the sprit block, that they each pay out or take in the requisite amount of their respective lengths to maintain the lead-line vertical at all deptbs of its range. The wheel measures 10 feet in circumference at the bottom of its rim groove; consequently, the length of sounding line paid out per revolution is 10 feet, with fractions of a revolution in proportion. At the front of the wheel frame there are two scales, along which a pointer is caused to travel at a rate proportional to the vertical travel of the lead. These scales are adjustable; one may be set so that the pointer indi- cates upon it the absolute depth below the surface level, while upon the other is indicated the depth relative to any assigned datum line. Indications are also afforded by the rim of the wheel, which is graduated and provided with a fixed pointer. In taking soundings with this machine, the operator first sets the lead at the surface of the water with zero on the wheel at the fixed pointer, and the scales adjusted to the movable pointer. He then grasps the rim of the wheel with his left hand, releases a catch with his right hand, and allows the wheel to revolve until the lead strikes the bottom; then, reversing the motion of the wheel, he reads from the pointers the depth indicated at the instant he feels the sounding line become taut. Continuing the movement a little further, he raises the lead clear of the bottom in readiness for the next dip. The operator is assisted in the picking up of the slack line by the reaction of a coil spring on the wheel axle. The Horizontal Distance Measurer consists of a drum on which is wound a length of fine steel wire. The drum is mounted, with its axis vertical, in a bracket fixed at the stern of the sounding-boat. As the boat moves away from its starting-point, to which one end of the wire is attached, the drum rotates and pays out the wire, the unwinding being regulated by a hand- wheel and screw acting upon a band brake. The revolutions of the drum