As professor Capasso writes, one of the oldest needs to ensure the safe conduct of a vessel was the measurement of water depth. This was achieved with the oldest hydrographic tool, namely the sounding line.
In ancient egyptian paintings we see the man standing on the bow, with the instrument in hand, and Herodotus writes of it as an essential tool to know sea depth and bring samples back from the bottom. Even in the Acts of the Apostles Paul mentions its use: "And came the fourteenth night ... sailors hoped to discover land. And thrown the sounding line they found twenty paces, and a while after they found fifteen paces.
The sounding line for small depths consists of a weight of lead or iron, attached to the end of a light cable of hemp, graduated and with known length. It has been found in the remains of ancient Greek shipwrecks.
To go deeper the sounding line was subsequently made with greater weights and longer cables, reaching 600-700 fathoms. They had however several disadvantages, including the friction produced by the rough surface of the cable during descent: it could happen that the cable would stop at a certain depth without reaching the bottom, causing tangles that distorted the measure and made arduous the recovery.
"Weight loss" ounding lines were therefore invented, as the one developed by the American Brooke in 1854, where the weight was made up of a large ball of cast iron diametrically crossed by an iron rod fitted with two hooks, to which were hung two pieces of rope that held the ball: when the rod touched the bottom, the hooks were lowered, the rope released and the weight slipped from the rod, falling to the bottom.
Over centuries the hemp cable was replaced by braided steel wires, while the use of hands was replaced by a machine with a wheel, crank and dials indicators. The wheel was supported by a pedestal attached to the stern or the side of the ship, it was wrapped with a wire sufficient for depths of 200-300 meters, and carried a weight of about 10 kg, consisting of cylinders with grroves to collect samples from the bottom.
The sounding line designed by Magnaghi was widely used in Italy. In it the weight was given the shape of a fish to keep the wire almost vertical, when sailing at low speed, allowing to take subsequent measurements at short intervals.
Another sounding line with wheel and steel cable, popular with the British Navy, was the one built in 1878 by William Thomson. In this one the depth was obtained independently from the length and slope of the wire, deducing it from the pressure exerted on special "discolorating tubes"by the water above.
These tubes were thin glass tubes, 60 cm long by 3 mm in diameter, closed at one end, covered internally with a thin layer of silver chromate colored red-black. One of these tubes was introduced in a special metal case, connected to a rope about 10 feet long, and placed between the weight and the cable end.
During the immersion, the water penetrated into the tube compressing the air. The greater was the pressure of water above, the deeper water was entering in the tube changing the color of the silver chrome to reddish white.
The depth was calculated by a special graduated rule, reading at the point where the color changed.