detonators. It relied on crushing an internal vial of acid which then corroded a retaining wire, which sometimes made it inaccurate in cold or hot conditions. Later the L-Delay, which instead allowed a lead retaining wire to "creep" until it broke and was less affected by the temperature, was introduced. SOE pioneered the use of plastic explosive. (The term "plastique" comes from plastic explosive packaged by SOE and originally destined for France but taken to the United States instead.) Plastic explosive could be shaped and cut to perform almost any demolition task. It was also inert and required a powerful detonator to cause it to explode, and was therefore safe to transport and store. It was used in everything from car bombs, to exploding rats designed to destroy coal-fired boilers. Other, more subtle sabotage methods included lubricants laced with grinding materials, intended for introduction into vehicle oil systems, railway wagon axle boxes, etc., incendiaries disguised as innocuous objects, explosive material concealed in coal piles to destroy locomotives, and land mines disguised as cow or elephant dung. On the other hand, some sabotage methods were extremely simple but effective, such as using sledgehammers to crack cast-iron mountings for machinery. Submarines Station IX developed several miniature submersible craft. The Welman submarine and Sleeping Beauty were offensive weapons, intended to place explosive charges on or adjacent to enemy vessels at anchor. The Welman was used once or twice in action, but without success. The Welfreighter was intended to deliver stores to beaches or inlets, but it too was unsuccessful. A sea trials unit was set up in West Wales at Goodwick, by Fishguard (station IXa) where these craft were tested. In late 1944 craft were dispatched to Australia to the Allied Intelligence Bureau (SRD), for tropical testing. Other SOE also revived some medieval devices, such as the caltrop, which could be used to burst the tyres of vehicles or injure foot soldiers and crossbows powered by multiple rubber bands to shoot incendiary bolts. There were two types, known as "Big Joe" and "Lil Joe" respectively. They had tubular alloy skeleton stocks and were designed to be collapsible for ease of concealment. An important section of SOE was Operational Research, which worked mostly from Station IX but also called on the facilities of Station XII and HQ. It operated through the User Trials Section and later the Air Supply Research Section and was formally