Fort Siloso: Battery Observation Post
Trigonometry was used with a Depression Range Finder to calculate range. The angle of the telescope and height above sea level were used in calculations.
In 1896, there was only an Observation Post (OP) on Mount Siloso, but in 1896, with plans for the 6-Inch QF Battery for Mount Siloso, a Battery Command Post (BCP) was required.
The First Battery Command Post
The BCP would be built on the site of a store room which was located at the top of the stairs leading up to Mount Siloso. The new Command Post would be equipped with a Depression Range Finder (DRF). The DRF was invented in the 1880s and greatly improved range finding for coast artillery batteries.
The DRF used Trigonometry to calculate the range of a target. Using the diagram to the left and simply put, the height of the DRF above sea level was known. This is side ‘h’ on the triangle.
The DRF was sighted on the waterline of the target ‘t’ and the angle of depression at ‘x’ was used to obtain the angle at ‘t’ (angles of a triangle add up to 180°). Trigonometry then calculated the distance ‘d’. In order to be accurate, other data needs
to be taken into account. This includes the the use of Tide Tables to ensure an accurate height above the sea is always used. The distance and bearing of the Gun Battery from the DRF had also to be taken into account when ranging a target.
Another instrument, a Position Finder (PF) was used in conjunction with the DRF. This device, invented by a Colonel Watkin of the Royal Artillery. This was similar to a DRF, but working in a horizontal plane, plotted the course of the target vessel. PF Cells were small buildings constructed near coast batteries to house the equipment and provide information to the BCP. At Fort Siloso, a PF Cell was built beside the BCP before WWII. It is not known if there was one at the Fort before then, but Fire Control until the 1930s was provided from Mount Imbiah where there was a PF Cell. This was some 770 yards (705 metres) from the BCP, and slightly south of it.
By 1912, there was the need for improved Battery Command facilities for Fort Siloso and a major rebuild of the Command Post was undertaken. This added a new top floor and a Post for the Direction of the
Fort’s Electric Lights (Searchlights).
The result is a building which is very similar to that which is seen today.
The Battery Observation Post (BOP) from outside the Casemates. A 1993 photograph.
The BBOP and the PF (Position Finding) Cell to its right.The steel shutters for the windows are open in this 1993 photograph. The windows are all unglazed in their original condition.
The front of the BOP in 1993. The Depression Range Finder was in the upper floor.
The Restored Battery Observation Post after restoration. The passageway bottom right leads to the lower floor. The windows have been glazed and new windows have been inserted in the front of the Range Finder position. All this gives a very false impression of what the Observation Post once looked like. Windows would never have been glazed as incoming fire would send shards of glass flying around the inside of the post.
The rear of the BOP showing upper and lower entrances and the staircase to the Casemates.
Wall brackets on the right-hand side of the BCP. The left-hand one would have had a bell or a shell casing hanging from it to be used as an alarm signal. The right-hand pair are supports for the steel shutters when they are lowered.
When the Fort was modernised during the 1930s, further modifications were made to the BCP. These included a room constructed behind the 1912 Command Post.
It would seem that the BCP was then renamed Battery Observation Post. Probably due to it coming under the command of the new Faber Fire Command on Mount Faber.