The term 'slide' is interchangeable with 'platform'. In some documents both terms are used.
The real or imaginary point about which a gun is traversed is referred to as the Pivot.
By the 1880s they were classified as :-
A pivot The imaginary pivot is in front of the platform, in the embrasure.
B pivot Similar to A.
C pivot The pivot is central and the racer a circle.
D pivot The pivot is nearer to the rear of the slide than to the front.
E pivot Similar to D.
F pivot The pivot is behind both front and rear racers.
The A & B pivot usually give a field of fire of 70 degrees. The C pivot gives 360 degrees whilst the D gives 180 or 360 degrees. The E & F pivots give 180 degrees. A pivots are suitable for embrasures; D pivots for barbette emplacements with less than 140 degrees of lateral training and C pivots for barbettes with a larger amount. Casemate slides are used only with A pivot racers.
Dwarf slides are suited to all pivots. D pivots were not much used except for the colonial carriage. E & F pivots were specially designed for the tops of Martello towers and are rarely found elsewhere. In C and D pivots the actual pivot is usually an old gun, a 24 or 32 pr. S.B. set in concrete. An alternative is a cast iron pivot block.
The term Platform or Traversing platform was the official nomenclature for what was later referred to as a slide in official handbooks and documents. Platforms for medium and heavy guns were often described according to the type of pivot they required e.g. Slide, 7-Feet Parapet C Mark II.
The block about which the slide rotates and to which the slide is connected is known as the Pivot Block. All medium guns on C, D, E or F pivot racers require actual pivots. This consists of a cast iron block into which a steel plug three inches in diameter fits passing through a plate on the underside of the slide. The height of the pivot block depends on the type of slide used. It could be 12.25 inches or for the high block 18.375 inches. For some heavy RML emplacements old smooth bore guns were used as the pivot. These were one of three types, the 9pr. of 4.2 inches diameter, the 18pr. of 5.3-inches diameter and the 24pr. of 5.82 inches. These were approved for all calibres and you cannot therefore identify the type of gun and platform used by the nature of the pivot where it is an old smooth bore gun. The pivot performs some useful function in taking up part of the force of recoil. Where there is no pivot this has to be taken entirely by the racers. We found from the experience of firing a 32pr. S.B.B.L. gun in the caponier at Fort Nelson, where no pivot had been fitted, that the force of recoil can impose a strain on the trucks, which were cast-iron. In this case the relatively small force of recoil obtained from a small blank charge caused one of flanges of one of the front trucks of the slide to shear off. A pivot would have absorbed most of this force.
The curved track set into the floor of a gun emplacement which enabled guns to be traversed more quickly is the racer track. Racers for guns up to 10-inch are of wrought iron 27/8 inches wide. Racers of 10 inch R.M.L.s and above are of steel. 12-inch, 35 ton and 12.5 inch, 38 ton guns employ heavy racers 4 inches wide without flanges. 20 pr & 40 pr R.B.L.s required gun metal racers, ribbed and slightly coned towards the pivot. Racers are fixed on iron chairs or set in granite blocks.The configuration varies according the the platform and method of mounting and the position depends upon the pivot.
Section of racer for Medium Guns
Wrough Iron Flanged - - -Steel Flanged - - - - - - - - - Steel, Solid, without Flanges
Three heights of slide (platform) were employed for medium and heavy guns, the Casemate, Dwarf and Blocked-up. Curiously the casemate platform is lower than the dwarf. For heavy guns only casemate and dwarf platforms were employed with, in addition, many special platforms to suit particular needs, such as high angle, long range, turntable, small port, short recoil, sunken way etc. The casemate platform was only used inside a casemate where height was at a premium. In open emplacements dwarf and blocked up platforms were used. The blocked-up platform is basically a casemate platform with heightened rear and front blocks, possibly with a change of trucks to suit. Most can be fitted with pivot plates to suit B, C or D pivots.
Space for the 16ft. slide.
The space to be kept clear for a 16ft slide are as follows:
A Pivot emplacement
From the front racer to the front - 1ft. 6-in
From the pivot to the rear - 20ft.
C pivot emplacement
From the pivot to the front - 7ft. 6-in
From the pivot to the rear - 9ft. 6-in
D pivot emplacements
From the pivot to the front - 10ft.
Six-feet Parapet Slide. (6ft. Parapet Platform)
This was specially designed for use in coast batteries. It was of wrought iron and was mounted on live rollers to fire over a 6ft. parapet. It was 13ft. 2ins long and used the same racers as the 16ft. slide. The 64 pr of 64cwt. and the 80pr converted guns were to be mounted in this way. Later the 7-inch 6½ ton R.M.L. in the land service was given a slide of this nature. It was referred to as a blocked-up slide and consisted of firstly blocking up the slide to enable the parapet to be raised to a greater height above the racers. The mounting gave security to the detachment against all projectiles except those fired at high angles. The elevating gear was altered so that the crew could load the gun under cover at depression. It was an adapted dwarf platform raised by an additional 1 foot 10¾ inches enabling the gun to be fired over a parapet 5ft. 10½ high above the top of the racer and therefore 6ft. above the racer blocks. This improvement could be applied to all emplacements whatever the pivot letter type.
With a C pivot the radius of the emplacement should be 9ft. at top. A similar mounting was experimented with the 7-inch 7 ton gun. The pivot block is 18.375 inches high instead of 12¼ inches and it takes a pivot plug of four inches diameter instead of three inches, which the low mounting requires.
Shortened 13-Foot Slide.
This was a traversing platform for use with the converted 64 pr and 80pr guns and with the 7-inch R.B.L. It is the old 16ft. slide with three feet cut off and provided with a hydraulic buffer and the wooden slide replaced by a wrought iron one. New casemates for these guns had a sill height of 4ft. 3-ins or 3ft. 6-ins whilst the earlier sills were 2ft. 3-ins.
Shortened 11-Foot Slide.
This was a traversing platform intended to take the 7-inch R.B.L. for use in Great Britain only. Designed for mounting in the flanks of forts where it could be easily blinded, it fired over a sill 3ft. 6ins high. It was possible to mount a 64 pr. on it, although not recommended. A blocked up 11 ft. slide was produced for 6ft. parapets.
The type of armament employed depended on the location of the fort or battery. Land front defences did not employ anything larger than the 64pr. RML as the main armament on front faces (with 7-inch R.B.L., on flanks) whilstCoast Defence forts and batteries employed the 7-inch of 7-ton RML upwards. Recognising the various type of mounting for medium guns on land front forts is a matter of experience. Often it helps to look at the armament return for the fort concerned and cross reference this with the plan of the fort, where available, to identify each gun emplacement. A pivots rarely have a real pivot block. The exception appears to be those for the adapted naval broadside mount, examples of which can be found at Whitesand Bay Practice Battery and Pendennis Castle. Because the actual pivot point of an A pivot cannot be accurately determined it can be easier to find out the type of platform mounted on a land front A pivot by measuring the distance between the front and rear racers and the height of the sill of the embrasure. For example the No.13 medium platform had a front racer radius of 5ft and a rear racer radius of 14ft. This gives a distance of 9ft between racers. A height of 3ft. 6in for the parapet would suggest that the emplacement was for a Platform, Wood Traversing, Medium No.13 for 7-inch RBL or 64pr. RML
Racers for Medium Slides
Racers for Heavy RML Guns
Platforms employed in casemates can be treated in the same way. It is safe to assume that these will be casemate platforms on A pivots. No actual pivots will be found in casemates. Lewis gave the reason for this as being the necessity for putting the position of the pivot far forward, in order that the size of the shield opening may be at a minimum, and would make it impossible to secure an actual pivot against the chance of injury from a shot striking the shield; and as, if displaced, it would prevent the gun being traversed, it was therefore thought better to omit it altogether, and to take up the recoil entirely by the racers. This did mean that considerable inconvenience was caused in connection with position finding because of the lack of any means of accurately centering the slide when being traversed, it is apt to get slightly askew, and not always to lie on the bearing indicated on the training arc. On land front forts Haxo casemates were armed with the 7-inch RBL or the 64pr. RML. on an A pivot platform. Measuring the height of the sill and the racer radii will identify the platform.
In open embrasures on land front forts dwarf platforms were often used. For positions that were liable to enfilade such as the front faces of forts six foot parapet slides were often used. These took the form of a circular concrete plinth or drum with a circular racer and a central pivot block
Emplacements to take dwarf slides.
Muzzle loading guns on dwarf slides for C or D pivots are mounted in a pit over the edge of which they fire. If on an A pivot they are mounted in an emplacement similar in plan to a casemate and fire through a shallow embrasure, the sole of which is 4ft. 3-inches above the racer.
The dimensions for C and D pivot emplacements, which are called barbette emplacements, depend on the method of loading the gun. Older forms of barbette emplacements for heavy guns were copied from those for S.B. guns, the height of the parapet above the racer being the same and being still retained. The maximum radius possible for the emplacement was determined by the necessity of the the muzzle of the gun to project at least a foot over the parapet when run out to fire, the minimum by the the necessity of getting easily at the muzzle to load when the gun was run back. As the guns increased in size the height of the axis of the bore above the floor of the emplacement increased until it became too great for convenient working and so the Fixed Loading Stage was introduced.
To meet the increase in size of the gun the emplacement was increased in size and a a fixed step or loading stage, was carried around the front of it. This step is 7-inches high for 10-inch and 11-inch RML guns. The 7-inch and 9-inch guns required none. This enabled the loading numbers to stand high enough to enter the the charge and ram home and allowed the projectile to be raised vertically to the muzzle without striking the parapet. Many emplacements like this for guns under 35-tons were used.
Platforms used for coast defence in open barbette positions were often those designated as C for sunken way. These tended to be adapted from ordinary dwarf platforms. They were fitted to an emplacement that afforded greater protection to the crew, who were unnecessarily exposed to fire whilst loading in the old type loading-stage pits.
Designated as 'Emplacements with Sunken Loading Way and Movable Loading Stage' they had a trench, or loading way, cut round the front of the emplacement where the old step had been, to a depth of 7ft. below the crest, thereby protecting the crew standing in it. In this trench a wooden stage was arranged to run on rails so that the men standing on it could reach the muzzle of the gun to enter the charge. the ground behind the gun was lowered so that it was standing on a drum or irregular shape approached in rear by a ramp or two or three steps.
A further refinement was made to these emplacements by fitting the slide to a circular drum carrying the racers. the traversing gear was altered to work from the 7ft. level below the crest and a convenient crane for raising the projectiles was supplied instead of the old muzzle derrick fixed on the gun. The stage was fixed to the slide instead of running on rails. These emplacements can be identified by their diameter, height, the width of the emplacement, the height of the parapet and other features such as the steps cast in the drum for the crew to mount. In all emplacements of this type for 10-inch and 11-inch RML guns the drums are circular with a radius of 8ft. 3-inches, the top, to a depth of 6-inches being formed to a radius of 8ft. 1 inch to give a clearance to part of the traversing gear. The drums for the 9-inch guns were made to a radius of 6ft. 3-inches in front and 8ft. in rear to provide additional support to the racer.
12.5-inch gun - 6ft. 9-ins
12 -inch 11-inch 10-inch - 7ft. 9-ins
9-inch - 5ft. 10-ins
The sea defence casemated batteries were armed with the 7-inch of 7-tons upwards. The large casemated works such as Gilkicker, Hurst, Bovisand, Garrison Point etc. often had a mixture of 9-inch and 10-inch in the casemates with larger 11-inch and 12 or 12.5-inch on the upper roof battery. Only guns of 10-inch and upwards were fitted with mantlets and the presence of mantlet bars is therefore a clue that the gun occupying the position was at least a 10-inch R.M.L.
These rings, two or three fitted in the overhead vaulting of heavy gun casemates used for mounting the gun on its carriage, can be an aid to identifying the gun employed. The loops are placed in the centre-line of the casemate, facing to the side, their positions varying according to the nature and weight of the gun employed. The distances from the pivot of the gun are given by Lewis as follows;-
gun -----front ------middle ------rear
12-ton-- 6‘ 6• ---------------------17‘
18-ton --9‘ 0• ---------------------19‘ 6•
25-ton --6‘ 6• -------13‘ 6• -----19‘
38-ton --6‘ 6• -------16‘ 6• -----22‘
Lewis does add that the positions of loops for other guns would probably be different, depending on the mode of mounting adopted, and the positions of the centres of gravity of the guns.
I have yet to find a caponier that has racers fitted for anything other than a 32pr. S.B.B.L. gun. I can safely state that this was the standard armament of caponiers for guns on traversing platforms. The only alternative to this was the use of carronades and these were not mounted on traversing platforms therefore no racers will be found. This does not mean that the absence of racers is evidence for carronades being employed. The only real proof is the armament return for the fort concerned.
One example of a emplacement that adds confusion is Fort Staddon, at Plymouth. Here the armament return states quite plainly that the caponiers were fitted with 32pr. S.B.B.L. guns, but measurements made of the extant racers showed the following :-
Centre Caponier at Fort Staddon
Height of pivot - 8ins (2 in thick)
Height of sill - 2ft. above racer
Racers Front 1ft. 8in
Rear 4ft. 11in
This gives a difference between the racers of 39ins. The 32pr. should have racers with a front radius of 1ft. 6in and rear of 6ft. 10in giving a difference of 64 in. The armament return shows 32prs. mounted so the platform must have been a special adaptation with the rear trucks moved further in. This is the only example of such a modification found so far and may be due to the fact that Plymouth received the first allocation of 32prs. for flank defence.
A Handbook of Military Terms used in Connection with Fortifications of the Victorian Period by David Moore. Published by The Palmerston Forts Society. ISBN 0 9523634 2 9
Permanent Fortification for English Engineers - Lewis