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Measuring the depths

Steam power and sounding machines

NMS T 1999 382 2 PF1076667

Wind and steam power, combined with human labour, ingenuity, and the materials of the ship, propelled Challenger on its voyage around the world while specialised instruments surveyed the ocean’s depths.

Challenger moved under steam and sail power, fusing new and old maritime technologies. Steam power had many advantages but consumed large amounts of coal, a bulky, solid fuel. For this reason, the crew operated the ship under sail when travelling long distances across the open oceans. Steam power was reserved for carrying out deep-sea investigations to keep the ship stationery against wind and currents and to power winches to bring material up from the ocean floor.

Dredging and sounding arrangements on board HMS Challenger

Creator Thomas Henry Tizard
Date 1885

The same resources that supported the British Empire to exert its power in different regions of the world made it possible for Challenger to conduct its oceanographic expedition. Portsmouth was part of a global network of naval bases and coaling stations in the 1870s that enabled the fleet to travel from England to distant points of the British Empire.

High-performance steam ships like Challenger demanded extensive maintenance: warships consumed vast amounts of coal; engines required skilled repairs and precision parts; large crews had to be fed, clothed and replenished; and the hulls of large steam ships could only be cleaned and repaired in dry docks, a key technology that naval stations provided. During the circumnavigation, the ship called upon many of the largest and most powerful British naval bases of the 19th century: Halifax, Bermuda, Gibraltar, Simon’s Bay, Sydney, and Hong Kong.

Title: Dredging and sounding arrangements on board HMS Challenger
Creator Thomas Henry Tizard
Date 1885
Creator Henry Nottidge Moseley
Creator John Young Buchanan
Creator John Murray
Object reference S.B. F. 5083 Cha.
From Narrative of the Cruise of H.M.S. Challenger with a General Account of the Scientific Results of the Expedition

Challenger carried out observations at 362 deep sea stations during its circumnavigation. At each station, the crew determined the exact depth of the ocean floor and direction and rate of the current; collected samples of seawater, sediment and marine life; measured temperatures at different depths; and observed and noted atmospheric and other meteorological conditions.

The Hydrographic Office of the Admiralty was responsible for sourcing the specialised instruments the crew used at these stations, working closely with manufacturers on experimental designs.

Chart showing the dredging and sounding stations

Creator Thomas Henry Tizard
Date 1885
Object reference S.B. F. 5083 Cha.

Knowledge of the ocean was integral to Britain’s maritime trade network and imperial expansion. As part of the effort to define ocean space for the safety of shipping, the British Admiralty contributed to the development of science at sea in several ways. The Hydrographic Office was established in 1795 to collect information on coastlines, winds, currents, and tides that served the trade and colonial interests of the British Empire.

While scientists were interested in the presence and evolution of marine life, the flow of ocean temperatures, and the formation of deep-sea sediments, the British government desired to extend the British-controlled telegraph cables that connected London with its colonies. Challenger routes were therefore not randomly chosen but plotted by the Hydrographic Office along the main trade routes of the British Empire, especially communication routes that connected London with America and the western coast of Africa.

Title: Chart showing the dredging and sounding stations
Creator Thomas Henry Tizard
Date 1885
Creator Henry Nottidge Moseley
Creator John Young Buchanan
Creator John Murray
Object reference S.B. F. 5083 Cha.
From Narrative of the Cruise of H.M.S. Challenger with a General Account of the Scientific Results of the Expedition,

One of the great difficulties of 19-century ocean science was that the deep sea could not be directly observed. Knowledge of the ocean depended on the movement of instruments to and from the ship, floating at the ocean surface, and the ocean depths below.

Deep-sea sounding is one of Challenger’s greatest contributions to oceanography. A process of measuring the depth of a body of water, sounding was carried out at each of the 362 stations, making visible the great depths, broad plains, and mountain ranges of the Atlantic Ocean, Pacific Ocean, and the Southern Ocean basins.

To collect ooze, mud, rock, and sand from the sea bottom to the surface required a modification of existing sounders which were primarily used to determine the depth of water beneath the ship’s hull. In 1868, HMS Hydra sounded across the Arabian Gulf in preparation for laying the Indian Cable. During this voyage a blacksmith and two sailors aboard the Hydra invented a sounder.

The Hydra, illustrated here, had a hollow brass tube (A) and a butterfly valve at its bottom (B). When it hit the bottom, the weight of the sounder drove the end of the cylinder into the ground; a spring (D) then released the weights (F). When the sounder was hauled up without its sinkers, the butterfly valves closed and trapped a sample of the seabed.

Hydra sounding machine

Creator Thomas Henry Tizard
Date 1885

Title: Hydra sounding machine
Creator Thomas Henry Tizard
Date 1885
Creator Henry Nottidge Moseley
Creator John Young Buchanan
Creator John Murray
Object reference S.B. F. 5083 Cha.
From Narrative of the Cruise of H.M.S. Challenger with a General Account of the Scientific Results of the Expedition

This Hydra sounder in the collection of National Museums Scotland may have been one of the six used in the expedition when it first set out in 1872. During the first leg, between Portsmouth and Cape Verde, they were the primary instruments used for deep-sea sounding.

Unfortunately, these instruments could only collect small samples of the seabed and were difficult to operate. The spring had to be delicately adjusted to release the sounder’s weights at the correct moment when striking the bottom of the ocean. A sudden change of tension on the line could prematurely release the weights before it reached the seabed.

Hydra sounding machine

Date 1868-1878
Place of creation England
Object reference NMS T.1977.111
Credit / Usage information Image courtesy of National Museums Scotland (CC-BY-NC-SA)

Title: Hydra sounding machine
Date 1868-1878
Place of creation England
Object reference NMS T.1977.111
Credit / Usage information Image courtesy of National Museums Scotland (CC-BY-NC-SA)

The Baillie was invented on HMS Sylvia in 1871 by Navigating Lieutenant Charles William Baillie (1844-1899), addressing some of the shortcomings of the Hydra. In 1872, after Sylvia was decommissioned, Baillie sent a model and design of his new sounder to the Hydrographic Office. There, the improved sounder could be built and replicated, and then sent to survey ships for testing and use.

The Baillie replaced the Hydra’s complicated spring with a freely moving iron bar, indicated as (c) in this illustration. When the Baillie sounder was held upright by the sounding line, the wire that held the sounding weights rested on two metal ‘shoulders’ or small hooks. When the sounding tube hit the bottom, the sounding line slackened and the ‘shoulders’ retracted inside the sounding tube and the weights were released.

Baillie sounding machine

Creator Thomas Henry Tizard
Date 1885

Constructed from iron instead of brass, the Baillie was much heavier and larger than the Hydra. Although both sounders were fitted with butterfly valves, with its extra length and weight, the Baillie rod could go further into the ocean bed. Although it lost its weights on reaching the seabed, the sounding tube was heavy enough to penetrate soft sediments to take a long core sample.

In this way, the Baillie’s ability to make accurate measurements of depth and retrieve significant samples of the ocean floor were mutually useful for surveying the sea floor for navigation, the laying of telegraphic cables, and the developing field of ocean science.

Title: Baillie sounding machine
Creator Thomas Henry Tizard
Date 1885
Creator Henry Nottidge Moseley
Creator John Young Buchanan
Creator John Murray
Object reference S.B. F. 5083 Cha.
From Narrative of the Cruise of H.M.S. Challenger with a General Account of the Scientific Results of the Expedition

Baillie sounders like this one from the collection of National Museums Scotland were deployed on Challenger in late August 1873 as the primary instrument for conducting deep-sea soundings throughout the southern Atlantic, an extensive tour of the Pacific, and part of the Southern Ocean.

Most notably the Baillie was instrumental in the discovery of the Challenger Deep—now known as the Mariana Trench—the deepest point on the surface of the Earth. To measure this great depth, the sounder, consisting of a 450lb lead weight attached to a line, was deployed on 23 March 1875 to the ocean bottom and was retrieved from a depth of 4,475 fathoms (or seven miles).

Baillie sounding machine and weights

Date 1871-1878
Place of creation England
Object reference NMS T.1977.112
Credit / Usage information Image courtesy of National Museums Scotland (CC-BY-NC-SA)

Title: Baillie sounding machine and weights
Date 1871-1878
Place of creation England
Object reference NMS T.1977.112
Credit / Usage information Image courtesy of National Museums Scotland (CC-BY-NC-SA)

The Baillie was used with a hemp sounding line, a material that had been tried and tested and on which key measurements and other instruments could be attached including a collecting water bottle, pressure gauge and thermometers. In addition to the measurement of depth, temperature measuring was one of the chief objectives of the expedition and used to formulate theories about the global circulation of the oceans and the distribution of marine life.

Casella-Miller thermometers, like these below from National Museums Scotland, were used extensively on Challenger. Their U-shaped mercury tube recorded maximum and minimum temperature as the thermometer was lowered and raised into the ocean.

  • Title: Casella-Miller Deep Sea Thermometer
    Creator Dr William Miller
    Date 1870-1880
    Creator Louis Casella (instrument maker)
    Date 1870-1880
    Place of creation London, England
    Object reference NMS T.1999.382.1
    Credit / Usage information Image courtesy of National Museums Scotland (CC-BY-NC-SA)

  • Title: Casella-Miller Deep Sea Thermometer
    Creator Dr William Miller
    Date 1870-1880
    Creator Louis Casella (instrument maker)
    Date 1870-1880
    Place of creation London, England
    Object reference NMS T.1999.382.2
    Credit / Usage information Image courtesy of National Museums Scotland (CC-BY-NC-SA)

  • Title: Casella-Miller Deep Sea Thermometer
    Creator Dr William Miller
    Date 1870-1880
    Creator Louis Casella (instrument maker)
    Date 1870-1880
    Place of creation London, England
    Object reference NMS T.1999.382.5
    Credit / Usage information Image courtesy of National Museums Scotland (CC-BY-NC-SA)

Sounding on a large warship like Challenger was more complicated than on smaller survey vessels so further modifications to the ship and this process had to be made. The ship’s height above the water made it necessary to carry out sounding from a raised platform above the main deck, with the line attached to the main yardarm which held the sails. After touching the bottom, the sounder was hauled to the surface with the help of a small steam engine.

The ocean depths put strain on both the yardarm and the sounding line. The hemp sounding line had a breaking strain of 700kg, enough to support the sounder and its weights, but the motion of the ship and the movement of the ocean added additional tension. Forty accumulators consisting of rubber bands, shown as (B) in this diagram, were added to the line to absorb an increase in pressure and prevent the line from breaking.

Diagram to illustrate the method of sounding

Creator Thomas Henry Tizard
Date 1885

Title: Diagram to illustrate the method of sounding
Creator Thomas Henry Tizard
Date 1885
Creator Henry Nottidge Moseley
Creator John Young Buchanan
Creator John Murray
Object reference S.B. F. 5083 Cha.
From Narrative of the Cruise of H.M.S. Challenger with a General Account of the Scientific Results of the Expedition