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We’ve all heard about leagues under the sea and how deep the Marianna’s Trench is; but have you ever stopped and thought how we developed our understanding of how to actually measure the depths?
Measuring depth is recorded by ‘sounding’. The word has no relation to noise or tone and instead derives from an old English word ‘sund’ meaning swimming water or sea. Another aspect of sounding is the data recorded in the study of bathymetry, which itself is derived from the Greek word bathos or ‘deep’.
In the modern age, ships and even the smallest boats have highly sensitive echo-sounding devices but in the past, more primitive means to gauge the depth of water under the ships’ keel to prevent accidental groundings were used. The earliest known form of measuring depth was a sounding line or lead line: essentially a lead weight on the end of a measured line of rope. When the line went slack; the number of feet of line was measured to ascertain the depth of water. This approach dates back to ancient times with archaeologists having discovered proof of this technique being used by the ancient Greeks and Romans. This approach remained the standard for thousands of years and even on the famed Bayeux Tapestry, there is a depiction of a leadsman casting a lead line during King Harold’s landing in Normandy. In shallower waters, the line was often replaced by the use of a sounding pole.
The lead line at sea was not as practical as it was in coastal or estuary environments yet it was still vital to measure the depth under the keel. Over time, mariners developed techniques for measuring the length of line paid out. In the pitch black of night it was important that the sailors could ‘feel’ the measure too, so they tied marks on the line made from different materials such as calico, serge, leather, and other items. During the day this made it easy to read depth and at night the different feels of the various fabrics allowed accurate depth soundings to be gauged. Usually, the marks were tied off at every second or third fathom, in a traditional order: 2, 3, 5, 7, 10, 13, 15, 17, and 20 fathoms.
The innovative Victorians often dazzled by the advances in technology and were the first to try and introduce mechanization to the process. In part, this was led by the Royal Navy who wanted more accuracy than was possible with lead lines. Inventors came up with an amazing collection of devices ranging from cumbersome brass machines to pulley systems that relied heavily on manpower.
One of the first successful sounding machines was developed in 1802 by Staffordshire clockmaker Edward Massey. His device used the existing technique of casting a line but combined it with a machine featuring a rotor that sank to the seabed. Once at the bottom, the rotor would lock and show the depth to the sailors on the ship once the line was drawn back in. Impressed by Massey’s invention, the Royal Navy would buy 1,750 of the machines and fit them on every warship then in commission.
The Royal Navy also purchased alternative devices including a buoy and nipper device devised by Peter Burt. This curiously named machine derived its name from the inflatable canvas bag (the buoy) and a spring-loaded wooden pulley block (the nipper). Burt’s device was towed behind the parent ship and the line fed through the pulley. The buoy meant that the line fell straight down to the seabed, whereupon making contact the spring-loaded pulley system would catch the rope at the same moment, ensuring an accurate sounding was made.
The main limitations of both these devices were the length of the line and both could not be used in depths of more than 150 fathoms. As the 19th century continued great strides were made in deep sea sounding with one of the greatest steps forward being the introduction of Kelvin’s Sounding Machine, designed by William Thomson (Lord Kelvin) in 1876. Instead of rope, the machine used a drum of piano wire together with a weightier lead to take the machine to the bottom. The first versions of Kelvin’s design proved heavy and awkward for sailors to use so a motorized version was soon introduced which greatly improved efficiency.
Throughout the 19th and 20th centuries, similar designs continued to be used but the development of echo sounding replaced the earliest concepts. Echo sounding is where the ship emits a sound wave into the water and a receiver onboard receives the echo once it has hit the seabed and bounced off. Computers today can accurately assess the distance by the time it takes for an echo to return to the ship. The first practical demonstration of this technology was shown by inventor Herbert Grove Dorsey in 1928 and was given the name Fathometer, which later became better known as the Echo Sounder.
The major limitations of this technique were purely human. The system needed highly skilled operators. The echo also could dissipate as the depths grew larger and really only offered accuracy in water less than 100 fathoms.
It wasn’t until 1933 that Dorsey further refined the echo sounder. That year he incorporated the transmitter and receiver in the same mounting— which was then could a transceiver. In this form, the system had an operating range of between one and 275 metres. Over the following six years, development work continued allowing the device to automatically record depths when tied in with a Hughes Veslekan’s graph recorder.
In 1940, the 808 fathometer was produced. Marine surveyors the world over would for the next quarter century rely on this technology until the 1960s, when digital devices began to replace them. The introduction of single and later dual-frequency scanning systems almost completely did away with the older systems. Today, the level of accuracy of soundings is astonishing with computer-enhanced representations of the seabed, highlighting dangers to shipping that would never have been seen with the lead line approach of the Greeks and Romans.