So, how did they conduct hydrographic surveys before sonar was invented?
Until sonar echo sounders came along in the mid 1930s mariners known as "leadsmen" used a lead line to measure water depth. That's lead, the metal, used as a sinker. Made of perhaps 25 to 30 fathoms of line, known to landlubbers as rope, a lead line was tagged at intervals of every two or three fathoms up to 20 fathoms of depth.
The tags of leather and differing types of cloth were such that they could be "read" by feel at night when not visible. Below is a chart of the traditional lead line markers. Sometimes the 13 fathom tag was made of blue cloth rather than leather.
The tags of leather and differing types of cloth were such that they could be "read" by feel at night when not visible. Below is a chart of the traditional lead line markers. Sometimes the 13 fathom tag was made of blue cloth rather than leather.
After casting the line, the leadsman would call out the indicated depth. The great American novelist Samuel Clemens, author of the Huckleberry Finn and Tom Sawyer stories, took his pen name from the callout for two fathoms of depth: "by the mark, twain".
Here are some examples of the leadsman's callouts:
Here is a description of how the leadsman cast his line and probed the briny deep. This excellent article was liberated from the NOAA website noaa.gov/transformations/hydrography/.
Determining depth by lead line is clearly limited to shallow areas, 20 fathoms (120 feet) or less. This suggests that this method was used more to avoid running aground than conducting surveys. Other than tying two leadlines together end-to-end, other methods were needed.Chain soundings were one variation, with tags attached to the chain at certain intervals. The first illustration shows markers at 100 centimeter spacing. Alternately, chains could be tagged in feet, fathoms or even surveyor's chains, a unit of length.
It became clear to installers of the first undersea telegraph cables that ocean surveys would be required prior to laying cable. Some interesting machines were developed in the US and UK to probe deeper into the oceans. Common to them was the idea of measuring the amount of wire or line paid out instead of marking it with cloth, leather strips or tags. Wheels or pulleys were machined such that their circumference equaled one unit of length, whatever that unit might have been. The wire ran once around the calibrated wheel. Gears were attached to pointers that indicated the number of wheel revolutions, metering the length paid out into the sea.
Naval officer Charles Sigsbee developed his sounding machine in the late 1870s (second picture). For some reason the globular lead weight was supposed to be left behind on the ocean bottom after each cast, drawing the cylindrical part back up to the surface. Sigsbee later was commander of the USS Maine when in 1898 it exploded in Havana harbor during the Spanish American war.
The Passerelle sounding machine is shown in the third picture. The metering apparatus is marked "A" in the drawing.
During the Napoleonic wars every ship in the British navy had one of Edward Massey's sounding machines (fourth picture). While intended more for shallow waters, he sold 1,750 of them to the British government after it was recommended by the Board of Longitude, which was supposed to be inspecting nautical clocks.
Aboard Michelson a device much like the Massey machine was used to measure wire line paid out in Nansen bottle casts.
A bit of research reveals that all sorts of "sounding machines" were invented by just about everyone. Even William Thomson, better known as Lord Kelvin, got into the act with his 1907 Motor Sounding Machine, later installed on RMS Titanic and her sister vessels. Kelvin was renown for his work in thermodynamics, magnetism and electricity. However, he was a failure at aeronautics, declaring in 1895 that "heavier-than-
