Even as Hamerton and his companion were making this discovery an arc lamp above their heads was switched on, flooding the ground with its powerful light. Simultaneously the door of the subterranean store was thrown open and a row of trollies, propelled by an electric motor, emerged from a tunnel that had hitherto escaped their notice.
To stay where they were meant detection. Flight was equally dangerous, since they would have to cross the brilliantly lighted ground. By a common impulse the Sub and his comrade vaulted into two upturned barrels.
They were just in time. The trucks came to a standstill almost opposite their place of concealment, and a squad of men, dressed in engineers' uniform, began to fill the tilt-wagons. This they did by the aid of small electric cranes. Rapidly the heads of the requisite number of barrels were stove in, and the casks, raised by means of cliphooks attached to the wire ropes of the cranes, were emptied into the waiting trucks.
Hamerton could overhear the sergeant in charge of the party grumbling about the weather.
"Be careful, men," he cautioned; "if the moisture gets to the alum there will be a row. Now, sharp with the trucks, or Herr Captain will be in a rage. 'Z 21' must be filled before daylight. Hurry up, I say."
Away rumbled the trucks, the men either clambering upon them or returning to the subterranean room from which they had emerged. The arc lamp was switched off and all was dark once more.
Then the blaze of red light reappeared. Hamerton understood. This was the hydrogen factory where the gas for filling the Zeppelins was manufactured. He remembered that woollen or linen fabrics saturated in a solution of alum become practically non-inflammable. There were the men shovelling lumps of alum into one of the retorts.
"I have it," he muttered triumphantly. "They have discovered a process of making hydrogen practically non-explosive. By Jove! I wish I could wrench the secret from them. However, it's a step in the right direction."
Noiselessly the pair emerged from their place of concealment. Detroit was curious to know the nature of the conversation, but for the present it was extremely hazardous to speak. He had, however, made the discovery of the former contents of the barrel in which he was hidden, for happening to put his finger to his lips the unpleasant taste of alum asserted itself.
Stepping cautiously over the tram lines, since one of them might be a "live rail", Hamerton and Detroit resumed their down-grade journey till they had traversed nearly a quarter of a mile of the incline.