I spent like an hour looking for it. Below is a google translated french text about this floating lighthouse. They definitely were not looking for easy solution. Wonder where this lighthouse is today.
We have represented above a new floating lighthouse arrangement which was installed in England in 1869. This new lighthouse, invented by Mr. A. Freyer, was placed at the entrance to Liverpool Harbor.
The impossibility of constructing an ordinary lighthouse at this point, and the necessity of pointing out the dangers of the entrance to the port of Liverpool, decided the British Admiralty to make use of this particular disposition. Here the boat is deleted; or rather, he is submerged. A simple floating vertical column, supports the lantern at its top. The void which is in the base of the column, makes it float.
The fire is about 40 meters above the sea, and the center of gravity of the whole system about 10 meters below sea level, so as to avoid any sway.
Regardless of the lights, the lighthouse is equipped with a bell, which is set in motion to guide the ships towards the entrance of the harbor, when the fogs are too thick.
France has only five floating lights on its coast, for up to now it has not been recognized that it is necessary to increase the number; these are those of Ruytingen, off Gravelines; - Minquiers, in the Channel; - Rochebonne, off the Bay of Biscay; - Talais, near the mouth of the Gironde; - from Mapon, inside the same river. These floating lighthouses cost to establish, the first 110 000 francs, the second 155 000 francs, the third 265 000, the fourth 84 000 and the last 30 400 francs, not including the lighting fixtures. The smallest, that of Mapon, is only 70 tons; the largest, that of Rochebonne, is 350 tons.
The dimensions of the French flagship boats vary according to the place of anchorage and the height of the fireplace. They are all the more considerable as the sea is deeper and more furious during heavy weather, and the focus of illumination is higher above the waterline.
The shape of pontoons with floating lights is not that of ordinary ships. They are very narrow at their lower part; in order to offer little grip to the blade when it comes up; but they are flared at the top, so as to repel the water that tries to invade the bridge. Finally they are equipped with false keels, which have the effect of making the roll less sensitive.
They must be exceptionally strong because they are designed to withstand the shock of the most violent storms. Whatever the force of the sea, they must remain standing, to make the indicator light shine in the eyes of navigators in danger. Compared to iron and wood, it has been recognized that wooden hulls are more advantageous than iron hulls.
The lightships are held in place, or by a single anchor, or by two anchors, that is to say two anchors with one leg, joined by a heavy chain, whose length reaches, up to 2 or 300 meters. Buoys and a floating chain complete the anchoring system.
The floating lights are diversified in the same way as the headlights themselves. They are fixed or eclipsed, white or colored. It is still possible to vary the number of them, and to put one here, two there, even three, but rarely.