The SIMPLE SOUNDING consists of a heavy weight tied at end of a line, graduated on its entire length at every 10 meters, or 50, or 100.
The weight is lowered as quickly as possible, and the number or color of the last tick mark on the rope, which remains above water when the weight touches the bottom, gives the approximate depth.
Ordinarily the sounding weight of a prismatic mass of lead, about 60 cm long, whose weight between 3 to 50 kg.
The weight has a ring at the top, to tie the line, and the bottom is cup-shaped and is coated with a layer of tallow, that collects a sample from the seabed to determine its nature, if muddy, sandy, gravelly or rocky.
Around the middle of nineteenth century, M. Brooke, U.S. naval officer, invented a device consisting of a cannonball, which came off at the contact with the bottom, and simultaneously took a sample from the seabed.
The ball, weighing about 28 kg, has a hole through which passes an iron rod, having - at its lower end - a small container, whose purpose is to retain a certain amount of seabed.
The upper end of the rod is provided with two movable arms that serve both to tie the line and to free another small rope that, during the descent, retains the ball and, when there is contact with the bottom, leaves it free.
The sounding known as "Bull-Dog" - quite cumbersome to handle - associates the principle of weight separation to two large spoons, arranged as a pair of scissors. These spoons are attached to the line that in its lower end is fixed to the pivot on which articulate the spoons themselves. Attached to the same pin is a rope tied to an iron ring attached to a kind of rocker arm in turn connected to the main line.
The spoons are connected to the weight by an elastic rubber band, ready to close the spoons as soon as its action is no longer blocked by the weight that keeps them open.
Once the spoons are in contact with the bottom, the tension of the rope is loosened, the two rocker arms fall and release the iron ring that frees the weight. Then the rubber closes the spoons, that are tied to the line by the rope.
During the expedition of the Lightning in 1868, the sounding known as "Fitzgerald's probe" was employed by naturalists, with excellent performance even in adverse weather conditions. The line passes in the eye at half length of an horizontal iron bar. This connects to a metal chain and, at the opposite end, is introduced into a hole at the top of a metal rod.
The rod ends with a box at the bottom having a sharp lip and a lid kept by two small articulated arms (to the left in the figure). The bar, at mid-length, has two metal teeth that serve to support a heavy weight.
As soon as the lip of the box hits the bottom, the horizontal upper bar loses its horizontal position. The weight causes the tip of the bar to disengage from the rod hole.
Simultaneously, the lid of the box opens and the box fills with matters from the seabed. When the unit is lowered to the surface, the lid of the box remains closed due to the tension in the line.
The Hydra-sounding probe is designed to collect both a sample from seabed and a water sample. It consists of a large brass tube that forms the axis of the instrument and includes four separate cavities: the lower three are closed by conical valves that open towards the top and let the water pass along the tube, while the fourth upper cavity contains a plunger inserted into the tube and connected to the line.
The upper cavity has a large holes on each side at about half its length, while the body of the plunger has a small through hole. The pplunger has a metal tooth, to which is connected a spring. The weights on the brass tube consist of three or four iron cylinders, ranging in weight between 90 and 140 kg, depending on the depths to explore.
The weight is suspended by a wire that retains the tooth, to which the spring is attached, and is kept in position by the tension exerted by the weight. When the tool is lowered water flows through the hollow brass tube passing through the holes in its walls. As soon as the instrument touches the bottom, the plunger moves in the upper cavity pulled by the weight.
Plunger's movement closes the valves and water remains inside the tube together with a sample the seabed in the bottom cavity.
When the weight touches the bottom, the tension in the wire that supported it is loosened, the tooth moves under the action of the spring, and the weight is dropped into the sea, while only the brass tube comes to the surface, together with a sample of sediment and a water sample collected at maximum depth.