"Get them by all means, if it is possible to do so. Don't worry about paying for them: secret service funds are not subject to the same audit that army funds get. If you can locate them, drive a couple of cattle or half a dozen sheep well into the cave and tether them there. If you don't get them, have your sentries posted well away from the cave mouth, and if any disturbance occurs during the night, tell them to break and run. I hope it won't come out, but I can't tell."
A herd of cattle was soon located and two of the beasts driven into the cave. Two hours later a series of horrible screams and bellowings were heard in the cave. Following their orders the sentries abandoned their posts and scattered, but the noise came no nearer the mouth, and in a few minutes silence again reigned.
"I hope that will be all that will be needed for a couple of days," said the doctor to the commanding officer, "but you had better have a couple more cattle driven in in the morning. We want to keep the brute well fed. Is there a tank stationed at Fort Thomas?"
"No, there isn't."
"Then radio Washington that I want the fastest three-man tank that the army has sent here at once. Don't bother with military channels, radio direct to the Adjutant General, quoting the Secretary of the Treasury as authority. Tell him that it's a rush matter, and sign the message 'Bird' if you are afraid of getting your tail twisted."
Twice more before the apparatus which the doctor had ordered from Washington arrived cattle were driven into the depths of the cave, and twice were the screams and bellowings from the cave repeated. Each time searching parties found the cattle gone in the morning. A week after the doctor's arrival, a special train came up, carrying four mechanics from the Bureau of Standards, together with a dozen huge packing cases. Under the direction of the doctor the cases were unpacked and the apparatus put together. Before the assembly had been completed the tank which had been requested arrived from Camp Meade, and the Bureau mechanics began to install some of the assembled units in it.
The first apparatus which was installed in the tank consisted of an electric generator of peculiar design which was geared to the tank motor. The electromotive force thus generated was led across a spark gap with points of a metallic substance. The light produced was concentrated by a series of parabolic reflectors, directed against a large quartz prism, and thence through a lens which was designed to throw a slightly divergent beam.
"This apparatus," Dr. Bird explained to the Signal Corps officer, who was an interested observer, "is one which was designed at the Bureau for the large scale production of ultra-violet light. There is nothing special about the generator except that it is highly efficient and gives an almost constant electromotive force. The current thus produced is led across these points, which are composed of magnalloy, a development of the Bureau. We found on investigation that a spark gave out a light which was peculiarly rich in ultra-violet rays when it was passed between magnesium points. However, such points could not be used for the handling of a steady current because of lack of durability and ease of fusion, so a mixture of graphite, alundum and metallic magnesium was pressed together with a binder which will stand the heat. Thus we get the triple advantages of ultra-violet light production, durability, and high resistance.