Of the many secret files of World War Two that have been declassified, only one contains the word ‘mystery’ in its title. The event that confounded the British War Cabinet and the high-command of the RAF was termed the Slough Bomb Mystery and it took place on 13 July 1940.
National Archives file reference AIR 8/356. The Slough Bomb Mystery.
Over the ten months since the war began there had been little activity over mainland Britain from the Luftwaffe. This was so contrary to public expectations reinforced by extensive air-raid preparations that it had become known as the Phoney War. Over the preceding six weeks, however, the fact that Britain had entered a desperate struggle for survival had been brought firmly home with the fall of France, the Dunkirk evacuation and the grim undertones of Churchill’s speeches to the nation.
Only a small few factories in Britain had been bombed and Slough Trading Estate hadn’t yet suffered an attack. If the Luftwaffe had decided at this time to strike on one particular factory on the Trading Estate then it couldn’t have selected better than High Duty Alloys Ltd. Situated at 80 Buckingham Avenue the factory produced a range of specialised alloys named Hiduminium which had been developed by Rolls-Royce for use in aero-engines. All RAF planes had pistons made from forged Hiduminium and supply was considered so vital that a shadow factory was being set up in a remote part of Cumbria. The Slough factory was working 24/7 to meet the needs of the aircraft manufacturers.
Advertisement for High Duty Alloys from 1939
Returning to 13 July, at 11:10 pm a single large explosion rocked High Duty Alloys, killing (at least) three workers and injuring 30 others. Although there was no doubt that a bomb was responsible for the explosion, the peculiar thing was that no aircraft activity had been detected over Slough. If the bomb had been dropped from an aeroplane then it must have been flying at very high altitude not to have been heard by the aircraft spotters situated all around the Trading Estate. For an aircraft flying at such an altitude (and in darkness) just managing to hit the Trading Estate with a bomb would have a been largely a feat of luck. To have caused a direct hit on High Duty Alloys seemed implausible. The dropping of just a single bomb was also unusual.
The War Cabinet request an explanation
The government’s Chief Inspector of Explosives, Colonel Thomas, was dispatched to the site the next day to determine whether the bomb, instead of being dropped, could have been planted as an act of sabotage. Thomas examined the deep crater going through the concrete floor of the factory. Embedded in the soil at the bottom were part of a metal grid previously set into the floor and also parts of the overhead crane. From this, he concluded that a bomb falling from above had carried these parts down with it and sabotage could therfore be ruled out.
Sir Hugh Dowding, the Head of Fighter Command received a memo on 19 July informing him that the Chief of Air Staff had been requested by the War Cabinet to investigate and report to them on the circumstances of the Slough bomb. The memo went on: “It is obviously unsatisfactory that bombs should drop in this mysterious way and I write to ask you if you could let me have some explanation of how this might happen, for the information of the War Cabinet”. The expectation of Britain’s elaborate air intelligence system was that it should be impossible for enemy aircraft to operate undetected. If the Slough bomb had been dropped by an aircraft, then a serious failure of the system had occurred.
Incident is forgotten
Dowding reported back on 23rd July. After outlining the known details he concluded that either the bomb had not been dropped by an aircraft, or that if it had, the aircraft had been flying so high as to be beyond the range of detection by sound. He rounded off with ‘Personally, I am inclined to suspect the IRA’. This might have seemed an odd statement, but given the great unlikelihood of a bomb having dropped so fortuitously, perhaps it seemed less improbable that Col. Thomas’s conclusion was incorrect. The IRA had commenced a vigorous campaign of attacks on British soil in 1939 named the s-plan and High Duty Alloys would certainly have been eligible as a target. What might have appeared to be a temporary hiatus of the s-plan during the previous weeks, however, proved to have been a complete fizzle out, much decreasing the likelihood that the IRA might have been to blame.
In consideration that the bomb might have come from an aircraft flying at very high altitude, Dowding had mentioned two X-raids over the sea which would have had time to reach Slough at 23:05 (X-raid was the term used for unidentified aircraft detected by radar). Shortly after Dowding’s report, further corroboration of aircraft activity emerged in the form of a former Observer Corps officer, Colonel G.C.K. Clewes who was staying near Slough that night. While outside, he heard an aeroplane pass overhead twice at what he thought was great height. He had no difficulty in identifying from its sound that it was German.
Matters effectively ended there. The War Cabinet never pressed for any further explanation. The Battle of Britain had begun and the RAF would soon be stretched to breaking point. The Assistant Chief of the Air Staff concluded “The bomb which fell on the works at Slough bids fair to become a mystery of the air comparable only to that of the famous Marie Celeste at sea!”. The file remained classified and only became available to the public in 1971, by which time the incident would probably have been far from the minds of those who originally knew of it.
Some new light
Aerial image of Slough, created by Luftwaffe, 15 August 1940.
In its researches, we at ‘Old Curiosities’ recently came across a very interesting artifact which may relate to The Slough Bomb Mystery. Displayed opposite is a reconnaissance photograph of Slough Trading Estate taken by the Luftwaffe on 15th August 1940. The title is obviously curious as it explicitly names High Duty Alloys Ltd. (HDA) rather than the Trading Estate. It appears to confirm that HDA was considered the target of highest importance on the Trading Estate. The date is interesting, taken on 15 August 1940 – just over a month after the bomb. Was the purpose of this photograph to assist a future raid, or to assess the damage from the previous bomb? It seems strange that HDA is not specifically marked in any way on the image. It is interesting to note how well the document was produced as if it were for distribution in volume.
Perhaps this document was one of many similar target images that were distributed to bombing groups. If a group was assigned a particular target to raid, mission-planners would already have appropriate photographs to hand. These photographs would then be annotated by hand according to the particular details of the attack. This seems to fit with a similar Luftwaffe reconnaissance image that can be found online. In this case, the photograph is of Glasgow-Govan. Although the image is of the Clyde Shipyard and Docks, in a similar manner to the Slough image, it is subtitled Harland & Wolff Ltd. There are various annotations added by hand in red. This includes a single building marked with a solid outline which is, interestingly enough, the Harland and Wolff foundry. It would therefore seem likely that foundries in general were prime targets for the Luftwaffe.
It could have been a guided weapon
We were led to wonder if it was possible that the Luftwaffe may have employed some form of targeting using infra-red light. As foundries produce a great deal of heat, they would surely stand out very clearly if they could be viewed in the infra-red part of the spectrum. A so-called ‘glide bomb’ equipped with sensors that reacted to infra-red light would be able to self-steer towards a heat-producing target. The Germans certainly did develop and use various forms of glide-bomb, but they only came into service later in the war. The Fritz X for instance was an anti-shipping bomb which entered service in 1943 and was guided to its target by radio control from the plane which dropped it. A self-steering bomb using infra-red sensors might have been easier to develop. Germany had certainly recognised the great potential of passive infra-red homing for weapon delivery. It is possible that by July 1940, the technology existed to create a specialised ‘homing bomb’ for deployment against targets which generate substantial heat such as foundries and power stations.
If any evidence could be located to confirm that this is what happened the Slough bomb mystery would finally be solved. If such a weapon had been used against High Duty Alloys it would be mildly ironic. The existence of the infra-red form of light was discovered in Slough by its most famous resident, the English astronomer (of German origin), Sir William Herschel.
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