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Description
This is part of a series of nine drawings made for the non-fiction book "The Fortified Region Focsani-Namoloasa-Galati" published by the Romanian Military Museum about the eponymous Fortified Line (also known in English as the "Sereth Line" and by it's abbreviation "FNG") built by Romania for defence against a presumptive Russian invasion during 1888-90.

Sixth drawing, which shows the Md.1887 Gruson disappearing cupola for a quick firing 5.3cm gun.

Whereas the 3.7cm and 5.3cm Fahrpanzer mobile gun cupolas were used in first line batteries or flanking roles, the brunt of the close-in fighting would have been taken by the second line batteries, each equipped with six Gruson disappearing cupolas, also armed with the same 5.3cm gun as the Fahrpanzers, but more durable and with a larger supply of ready ammunition.

The catch, of course, is that these cupolas are more complex, harder to manufacture and a whole lot more expensive than the Fahrpanzers, while still having the same drawbacks as the latter - while the gun has a range of roughly 3000m, the view from inside the cupola was abysmal, and, coupled with the lack of any searchlight facilities it meant there was a significant chance a surprise night attack would have simply overrun the position - after all, any gun, no matter how "quick firing" can't equal a machine gun for close defence.

This is why, ultimately, the whole Schumann system would have been a failure despite introducing the idea of "defence in depth" which would prove its validity on the blood soaked battlefields of World War One.

More amusing is the fact that, by the time the forts of the FNG Line were constructed, the modern machine-gun was already here - in fact Hiram Maxim had unveiled his first prototype in 1884 and it's first operational deployment was during the Emin Pasha Relief Expedition in 1886...

But back to the subject of the drawing, the 5.3 disappearing cupola - as the name suggests, the turret and gun are set on a counterweight mechanism that allows them to raised or lowered at will. The standard position would have been with the turret down and the gun retracted inside it, which offered them protection as well as concealment.
When retracted, the lip of the cupola fits snugly into a recess of the cast iron "voussoir", the strengthening ring that protects the upper part of the barbette from plunging fire. To keep rainwater out when turret was in it's closed position, the top of the cupola was also provided with a "skirt" made out of thin metal plate which extended 1cm over the voussoir, sealing the gap.

As the enemy approached, the turret would have been brought up "into battery", or in firing position. The system was so finely balanced that a single person pulling on a lever were enough to raise or lower the whole cupola and gun which together weighed 1,530kg, or one and a half metric tons.

Once in action the gun was operated by a single person which served both as the cupola commander and gunner and which sat on a wooden chair attached to the central spindle. Turret traverse was achieved by the means of a geared pedal wheel, while the -5° and +10° elevation for the gun by the same handwheel system used in the Fahrpanzer. Since the gun could spew up to 30 rounds per minute in the direction of the enemy, the commander/gunner was provided with a shelf holding 10 rounds for "quick reaction shooting" and had to rely on an assistant (who also raised or lowered the cupola) to hand him additional rounds from the shelves build on the sides of the gun barbette, which held a total of 590 rounds as well as a locker for parts and accessories (visible on the lower left in the picture).

Construction wise, the gun pit was built mainly out of concrete but with 50cm thick brick vaulted ceilings since unreinforced concrete, while strong in compression, fares spectacularly poorly under tension. The drawing shows the base design as applied on the batteries of Focsani, since subsequent builds altered it to include two layers of brick on the facade of the emplacement as well as a concrete ledge on top.

The barbette itself is made out of 3mm thick steel plate with an added ring of rolled steel 3cm thick just below the cast iron voussoir, which in turn was secured into the pit by a pouring of "special" "extra strength" concrete (with a mix ratio of 1:1:2, which means 1 measure cement, 1 of sand and 2 of aggregate), which in all actuality was little better than the regular sort (mix ratio 1:2:4 by volume) and really poor by modern standards, hence it's depiction in the drawing by a "cracked" texture, to differentiate it from the "regular" concrete which has a "layered" texture due to the uneven pouring and differences between concrete batches.

Another representational licence was to present the cast iron pieces of the turret, the counterweight and voussoir in a "natural" cast iron colour, when, in reality, they probably would have been painted white like the rest of the inside of the turret. This was done simply to make the different pieces of the turret more immediately discernible.

Also, the inside of the voussoir might have been painted with a panorama of the surrounding countryside to aid the quick acquisition of targets and prevent the disorientation of the gunner like was the norm in French or Belgian forts (here are some good examples from the fort of Haudainville, near Verdun and at La Falouse , part of the Marginot Line) but, so far, I have found no evidence that such paintings ever existed in the Romanian cupolas.

The other drawing in this series can be found here:

Cover, showing cutaway through 5.3cm mobile gun turret
Plate I, the 3.7cm mobile gun turret and transport carriage.
Plate II, the 3.7cm gun and ammunition.

Plate III, the 5.3cm mobile gun turret in its concrete emplacement .
Plate IV, the 5.3cm mobile gun turret in transport configuration.

Plate V, the 5.3cm disappearing cupola.

Plate VI, the 5.3cm gun and ammunition.

Plate VII, the 12cm gun and mortar armoured battery.

Plate VIII, the 12cm gun and mortar and their ammunition.

Description
This is part of a series of nine drawings made for the non-fiction book "The Fortified Region Focsani-Namoloasa-Galati" published by the Romanian Military Museum about the eponymous Fortified Line (also known in English as the "Sereth Line" and by it's abbreviation "FNG") built by Romania for defence against a presumptive Russian invasion during 1888-90.

Sixth drawing, which shows the Md.1887 Gruson disappearing cupola for a quick firing 5.3cm gun.

Whereas the 3.7cm and 5.3cm Fahrpanzer mobile gun cupolas were used in first line batteries or flanking roles, the brunt of the close-in fighting would have been taken by the second line batteries, each equipped with six Gruson disappearing cupolas, also armed with the same 5.3cm gun as the Fahrpanzers, but more durable and with a larger supply of ready ammunition.

The catch, of course, is that these cupolas are more complex, harder to manufacture and a whole lot more expensive than the Fahrpanzers, while still having the same drawbacks as the latter - while the gun has a range of roughly 3000m, the view from inside the cupola was abysmal, and, coupled with the lack of any searchlight facilities it meant there was a significant chance a surprise night attack would have simply overrun the position - after all, any gun, no matter how "quick firing" can't equal a machine gun for close defence.

This is why, ultimately, the whole Schumann system would have been a failure despite introducing the idea of "defence in depth" which would prove its validity on the blood soaked battlefields of World War One.

More amusing is the fact that, by the time the forts of the FNG Line were constructed, the modern machine-gun was already here - in fact Hiram Maxim had unveiled his first prototype in 1884 and it's first operational deployment was during the Emin Pasha Relief Expedition in 1886...

But back to the subject of the drawing, the 5.3 disappearing cupola - as the name suggests, the turret and gun are set on a counterweight mechanism that allows them to raised or lowered at will. The standard position would have been with the turret down and the gun retracted inside it, which offered them protection as well as concealment.
When retracted, the lip of the cupola fits snugly into a recess of the cast iron "voussoir", the strengthening ring that protects the upper part of the barbette from plunging fire. To keep rainwater out when turret was in it's closed position, the top of the cupola was also provided with a "skirt" made out of thin metal plate which extended 1cm over the voussoir, sealing the gap.

As the enemy approached, the turret would have been brought up "into battery", or in firing position. The system was so finely balanced that a single person pulling on a lever were enough to raise or lower the whole cupola and gun which together weighed 1,530kg, or one and a half metric tons.

Once in action the gun was operated by a single person which served both as the cupola commander and gunner and which sat on a wooden chair attached to the central spindle. Turret traverse was achieved by the means of a geared pedal wheel, while the -5° and +10° elevation for the gun by the same handwheel system used in the Fahrpanzer. Since the gun could spew up to 30 rounds per minute in the direction of the enemy, the commander/gunner was provided with a shelf holding 10 rounds for "quick reaction shooting" and had to rely on an assistant (who also raised or lowered the cupola) to hand him additional rounds from the shelves build on the sides of the gun barbette, which held a total of 590 rounds as well as a locker for parts and accessories (visible on the lower left in the picture).

Construction wise, the gun pit was built mainly out of concrete but with 50cm thick brick vaulted ceilings since unreinforced concrete, while strong in compression, fares spectacularly poorly under tension. The drawing shows the base design as applied on the batteries of Focsani, since subsequent builds altered it to include two layers of brick on the facade of the emplacement as well as a concrete ledge on top.

The barbette itself is made out of 3mm thick steel plate with an added ring of rolled steel 3cm thick just below the cast iron voussoir, which in turn was secured into the pit by a pouring of "special" "extra strength" concrete (with a mix ratio of 1:1:2, which means 1 measure cement, 1 of sand and 2 of aggregate), which in all actuality was little better than the regular sort (mix ratio 1:2:4 by volume) and really poor by modern standards, hence it's depiction in the drawing by a "cracked" texture, to differentiate it from the "regular" concrete which has a "layered" texture due to the uneven pouring and differences between concrete batches.

Another representational licence was to present the cast iron pieces of the turret, the counterweight and voussoir in a "natural" cast iron colour, when, in reality, they probably would have been painted white like the rest of the inside of the turret. This was done simply to make the different pieces of the turret more immediately discernible.

Also, the inside of the voussoir might have been painted with a panorama of the surrounding countryside to aid the quick acquisition of targets and prevent the disorientation of the gunner like was the norm in French or Belgian forts (here are some good examples from the fort of Haudainville, near Verdun and at La Falouse , part of the Marginot Line) but, so far, I have found no evidence that such paintings ever existed in the Romanian cupolas.

The other drawing in this series can be found here:

Cover, showing cutaway through 5.3cm mobile gun turret
Plate I, the 3.7cm mobile gun turret and transport carriage.
Plate II, the 3.7cm gun and ammunition.

Plate III, the 5.3cm mobile gun turret in its concrete emplacement .
Plate IV, the 5.3cm mobile gun turret in transport configuration.

Plate V, the 5.3cm disappearing cupola.

Plate VI, the 5.3cm gun and ammunition.

Plate VII, the 12cm gun and mortar armoured battery.

Plate VIII, the 12cm gun and mortar and their ammunition.