officers' mess and get a bite toeat. As soon as you have those masks done, get your supper and thentelephone me at the club. If Carnes isn't back, I may have to ask youto drive me down toward Michaelville."
"I'll be very glad to, Doctor."
* * * * *
Carnes had not returned when Davis called Dr. Bird at the officers'club two hours later. Night had fallen and everyone on the provingground sat behind tightly closed windows with lights blazing on them,wondering whether the finger of death would reach in from the swamp totouch them. The fog had not yet made an appearance on the main postand Dr. Bird had no fear of it when he entered his car and drove downto pick up his assistant.
Davis came out to meet him with a curious hood made of vitiolene andrubber, pulled down well over his head. In his hand he carried asecond one. Dr. Bird adjusted the second mask and the two men loadedthe rear of the car with apparatus designed for collecting samples ofair. The outside of each sample cylinder was heavily coated with blackrubberine paint. At a word from the Doctor, Davis took the wheel anddrove off along the winding ribbon of concrete which led to the upperend of the Michaelville range.
For a mile they drove through a clear, calm night with no traces offog apparent. Dr. Bird's eyes continually searched the swamps on bothsides of the road.
"Stop!" he said suddenly, his voice coming muffled through theenveloping mask. The car stopped and the Doctor pointed to the west.Over the swamp a few stray fingers of fog were curling up from thewater.
* * * * *
Leaving Davis in charge of the car, Dr. Bird donned rubber hip bootsand with a gas cylinder in his hand, splashed through the watertoward the fog. He reached the place with no difficulty and spent tenminutes trying to collect a sample. Finally, with a mutteredexclamation, he removed his mask and inhaled deeply a dozen times.Carrying the mask in his hand, he made his way back to the car.
"False alarm," he said as he pulled on his mask. "It was so thin thatI couldn't get a sample so I tested it by breathing. There isn't atrace of cough in that fog. Drive on."
A half mile farther along the road, a curtain of fog swept in on them,momentarily hiding the road from view. They were through the belt offog in a few feet and the car came to a stop. Dr. Bird sprang out, gascylinder in hand. He returned to the car shortly.
"We may have what we are looking for, Davis," he said, "but I am notat all certain. It looked very much like ordinary fog. Let's go downto the range."
The car drew up between the two main buildings of the Michaelvillefront. The air was clear as far as they could see, but from under thenorth building, a tiny wisp of fog was coming. As it came under theglare of the three huge arc-lights which flooded the ground withlight, it grew more tenuous and gradually dissipated into nothingness.With an exclamation of satisfaction, Dr. Bird bent down and thrust theend of a cylinder under the building. He removed it in a moment as thefog began to stream from the upper end. Carefully he closed thepet-cocks of the tube and replaced it in the car. He filled a halfdozen tubes before he was satisfied.
"I'd like to go down to the water," he said through his mask. "Whatkind of a jigger do they run on that track?"
"It's a Ford scooter, I was told. It's probably in that shed."
* * * * *
Half an hour later the two men were running the scooter down the fourmiles of narrow gage track which separated Michaelville from the BushRiver. A few scattered patches of fog could be seen on either side ofthe track, but none were of sufficient thickness to warrant muchsuccess in sample taking. At the water front Dr. Bird looked acrossthe half mile wide river and grunted.
"The tide won't be in for another three hours," he said. "Right nowthere isn't over sixteen inches of water in there."
Carnes was waiting in the well lighted laboratory when they drove up.
"All right, Davis," said the doctor, "get busy on those samples. Ifyou can't make out the first two, don't crack the others but leavethem for me. Give Carnes your mask; he'll drive the rest of the night.
"What luck, Carnesy?" he asked, as the detective, wearing Davis' mask,drove toward the officers' club.
"No stray plane landed or even flew over here last night so far as Icould learn. Most of the boats on the bay were either known or lentthemselves to ready identification. There were four that I couldn'texactly place, but I think we can safely discard all but one. Somefishermen were pulling nets on the bay about half a mile outside themouth of the Bush River last night. About eleven, a boat runningwithout lights passed them. They said that they could not hear anengine running, but just a dull hum and the gurgle of a propeller.They hailed it, but got no answer. It faded away into the darkness andthey think it was headed toward the mouth of the Bush River. They hadtheir nets up and reset in another hour but the boat didn'treappear."
"Hmm. High tide was at ten minutes after midnight. There was plenty ofwater in the river at that hour. It sounds promising."
"I thought of telephoning Washington and getting a Coast Guard cutterput on patrol in the bay but I didn't like to do it without yoursanction."
"It might have been a good idea, but on the whole it's probably betterthat you didn't. Carnes, we'll go down to the water front and seewhether anything shows up to-night. High tide will be abouteleven-thirty. It's about half-past nine now. We'd better get going."
* * * * *
On the second drive to Michaelville, the fog patches were quitenoticeably denser than they had been earlier in the evening. Threetimes the car had to pass through bands of fog which covered the road.As they passed the second one Carnes suddenly began to cough.
"What's the matter, old man?" cried Dr. Bird, a note of anxiety in hisvoice. For a few moments Carnes could not answer for coughing. Heseized the mask to tear it from his head but Dr. Bird restrained him.In a few minutes his voice became intelligible.
"It seemed like that fog bit right into my lungs, Doctor," he gasped."I felt as if I were choking. It's better now."
"Are you sure your mask isn't leaking, Carnes? It'll be all up withyou if it does. Test it."
The detective closed the intake valve of the mask and expelling all ofthe air from his lungs, took a deep breath. The air whistled noisilyin through the outlet valve.
"The devil!" cried the doctor. "Take that mask off and let me look atit."
A few moments were enough to make the needed repairs and they droveon. Carnes still coughed from time to time. At Michaelville, theystarted the scooter and ran down the track to the river. They secretedthe scooter under the parapet on the water pent-house and walked tothe river's edge.
"There's no telling just where they may land, Carnes," said the doctorreflectively, "but this looks like the most likely place. I'll tellyou what we'll do. The river narrows a good deal about half a mileeast of here. You go up to the narrows and keep watch while I stayhere. If any craft passes you, follow it upstream until you find me.If they land, handle the situation as well as you can alone. If youhear any shooting, come as fast as you can leg it. I'll do the same."
* * * * *
The detective stole away into the darkness and Dr. Bird settledhimself for a long vigil. For an hour nothing broke the stillness ofthe night. Suddenly the doctor was on his feet, peering downstream. Afaint purring murmur came over the water, so faint that no one withless sensitive ears than the doctor's could have detected it. Assuredafter a few minutes of listening that some kind of a craft was comingup the river, the doctor sank back into his hiding place, an automaticpistol firmly grasped in his long tapering fingers.
The purr came nearer, but it was not appreciably louder. The gurgle ofwater past the prow of the boat could be heard and Dr. Bird could seea long ribbon of white on the water where the craft was passing. Hestepped from his cover and leaned forward, straining his eyes to seethe boat. It passed beyond him and continued up the river. He steppedquickly along the river bank, trying to kee
p it in sight. Suddenly hepaused. The boat had turned and was coming back. Hurriedly hereturned to his hiding place.
The boat came down the river until it was opposite the point where hecrouched, and then it turned and came in toward the shore. Dr. Birdgripped his pistol and waited. When the craft was less than twentyfeet from shore it stopped and a guttural voice spoke. Dr. Birdstarted. He had expected the language to be Russian, but it came as ashock to him, nevertheless. He strained his ears and cursed hisinability to make out the words. Dr. Bird had been assiduouslystudying Russian under the tutelage of his new secretary for somemonths, but he had not yet progressed to the stage where he couldreadily understand it. The gift of languages was one