This peculiar symbol that looks like some sort of secret code, is in fact the symbol painted on the hull of a ship to ensure the ship is not overloaded. The line through the middle of the circle is known as the International Load Line, Plimsoll line or water line, and that line is not to disappear under water when the ship is loaded up. When Samuell Plimsoll (1824-1898) came up with the scheme, the circle and the horizontal line where all that was required, but over time the additional symbol on the right was added to allow for different water conditions and hence different water densities.
Plimsoll was not the first to come up with the idea; there were already loading regulations in Crete 2,500 BC and in the Venetian Republic, and the city of Genoa and the Hanseatic League required ships to show a load line. In the case of Venice this was a cross marked on the side of the ship, and for Genoa three horizontal lines. In the 1860s, losses of ships through overloading increased dramatically and Samuel Plimsoll set out to find a solution. In 1867, he tried to get a bill passed through Parliament dealing with the load line question, but in vain as there were too many shipowning MPs who feared for their profits. Subsequently, Plimsoll published Our Seamen. An Appeal in which he set out his concerns and solution. Following the publication, a Royal Commission on unseaworthy ships was set up. In 1876, the United Kingdom Merchant Shipping Act made the load line mark compulsory.
On 14 February 1928, almost 30 years after Plimsoll’s death, a notice appeared in The Times, announcing plans to erect a statue to honour Plimsoll.
Permission is being sought from the Office of Works to set up, at the Westminster end of the Embankment, a statue of Samuel Plimsoll, known as “the Sailors’ Friend”, and the originator of the load line for British shipping. The statue which weighs three tons, is of bronze on a granite base, and is the work of Mr. P.V. Blundstone, of Kensington. The Seamen’s Union decided, instead of endeavouring to raise money for the memorial by public subscription, to defray the cost themselves. Mr. J. Havelock Wilson, the president of the National Union of Seamen, said that the idea of a memorial to Plimsoll had been in the minds of those associated with the seamen’s movement for some time. Seafarers from all parts of the world would attend the ceremony.
It took a year and a half to sort out the permission and logistics, but the Daily Mail of 21 August, 1929, could announce in their ‘To-day’s events’ column that “Sir Walter Runciman unveils memorial to Samuel Plimsoll, Victoria Embankment Gardens”. A very short announcement, but one with a lasting result as we can still walk past the gardens (on the outside, at the Embankment side) to see the statue.
Although plimsoll or plimsole shoes, that is shoes with canvas uppers and a rubber sole, had been known since the 1830s, they only had a name change from ‘sand shoe’ to ‘plimsole’ in the 1870s, because the horizontal line on the rubber resembled the plimsoll line on ships. And as with ships, if the water came above the line, you got wet.
More information on the Plimsoll Line and on Samuel Plimsoll himself on the Wikipedia-pages here and here.