He filled the little tube with mercury also. "The point of this," he observed, "is fine enough to prevent the mercury running through of its own weight—about as fine as a hair."
He dipped the point and held it in the sulphuric acid and blew through the capillary tube. When the mercury bubbled through the point in minute drops, he stopped blowing. It drew back for a short distance by capillary attraction and the acid followed it up.
"You can see that connections are made to the mercury in the arm and the tube by short pieces of platinum wire," he continued. "It isn't necessary to go into the theory of the instrument. But the most minute difference of potential between the two masses of mercury will cause the fine point at the junction of the liquids to move up and down.
"Connected to the aerial and the earth, with a crystal detector in series, it is only a matter of applying an ordinary photo-recording drum, and the machine is made."
He had been setting up a light-tight box, inside of which was a little electric lamp. Opposite was a drum covered with bromide paper. He started the clockwork going and after a few moments' careful observation, we went away, and left the thing, trusting that no one was the wiser.
Nothing further occurred that day, except for frequent reports from Burke, who told us how his men were getting on in their shadowing of Forsythe & Co. Apparently, the death of Leon had put a stop to revolutionary plots, or at least had caused the plotters to change their methods radically.
The time was shortening, too, during which Burke could keep the passengers of the Haytien under such close surveillance, and it was finally decided that on the next morning they should be released, while all those suspected were to be shadowed separately by Secret Service agents, in the hope that once free they would commit some overt act that might lead to a clew.