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before it begins to take the rapid, precipitous descent which lies between the butterfly dam and the city of Joliet. The water lapping the top of the butterfly dam will stand at the same level as the water which lies at the edge of Grant Park at the mouth of the Chicago River.

Between the butterfly dam, near Lockport, and the City of Joliet there is a sudden drop in surface altitude. Because of this drop the Drainage Channel is able to turn a large number of turbines and, in consequence, to produce a large number of kilowatts of electric energy. Also, because of this drop, the Drainage Channel, between Lockport and Joliet, ceases to run in a cañon, cut into the solid earth, and begins to run between enormous concrete walls, resting on the surface of the earth and confining the flow of its waters to a kind of aërial aqueduct, standing prominently above the surrounding country.

If any part of these walls should ever give way-which is unlikely-the farms of the neighborhood would be flooded.

The danger is hypothetical, which, translated from the language of the engineers, means you-don't-need-to-worry-about-it. But in order to guard against even a hypothetical danger the butterfly dam has been called into existence.

It is the first time that the butterfly principle has been applied to an engineering undertaking of any magnitude. When the butterfly dam at Lockport has been swung across the stream the farms of Illinois may go on producing corn in peace. The southwestern boundary of Lake Michigan will hold back all the water that has ever been discharged into that inland sea by the streams of Michigan, Indiana, Illinois, and Wisconsin. And Chicago will have given the world. the first butterfly dam which has ever been erected on a large scale.

But-and this is a "but" which explains the adoption of the butterfly principle-the dam at Lockport will not usually be swung across the stream which it is constructed to intercept. It will usually lie parallel with the stream. The

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LOCKS, TO BE LARGEST IN THE WORLD, UNDER CONSTRUCTION.

The water will have a drop of fifty-eight feet.

ordinary attitude of this extraordinary dam will be that of a log lying lengthwise to the current.

There will be two metal pins in the butterfly dam. One of them, weighing six tons, will traverse the central girder from the top. The other, weighing ten tons, will traverse it from the bottom. The central girder, with its seven rooms, is so constructed that the walls of these rooms are pierced with the holes. through which these pins will be thrust. Turning,

slowly

turning, on the sixteen. tons of cold steel which form its spinal column, the butterfly dam swings from its occasional position athwart the stream, where it holds back Lake Michigan, and gradually approaches its normal, ordinary position, lying finally in the same direction with the current which it controls and allowing the severed portions of that current to flow by unI checked on either side of it. One wing of the dam now lies upstream. The other wing lies downstream. The appearance of the dam is now that of a barge floating endwise down to Joliet. Now, just

Slowly, irresistibly, it swings back on its big metal pins. The rushing waters of Lake Michigan are driven back before it. It turns below water like any swing-bridge that turns in the air above.

In the space of a few minutes the channel can be closed, the waters of the

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END VIEW OF CENTER GIRDER OF BUTTERFLY DAM.

suppose that some one little stretch of one of the concrete walls of the channel below the dam has given way. Suppose-it is almost impossible, but-suppose that the waters of the Drainage Channel, with the whole pressure of Lake Michigan behind them, are surging out over the crops of Will County. This dam has not broken. It has not given way. It has not yet been used. It is still to be called into service. It is the reserve force of the engineering army of the Sanitary District.

lake hurled back, and disaster is averted. The top of the dam is as high as the top of Lake Michigan. And water cannot flow above its own level. The butterfly dam swings shut and the fields of Will County are safe. This is the use, this is the adaptability, of the butterfly dam. In times of watery peace it lets the whole body of the current go by. In times of watery war it intercepts every drop. It spreads its steely wings, or contracts them, in obedience to the welfare of the state of Illinois.

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HE oily face of the campboy was turned up with a gaping smile to Carter and his round blue eyes were big with the immensity of his interest. "Gold!" he whispered. "It's full of gold! Jim told me to tell you to take care of it for him."

"The deuce he did," muttered Carter. He eyed the absurdly small leathern grip which, on Bud's information, he had just pulled out from under Jim Bell's bunk in their miner's shack, and swore softly under his breath.

"Yes, he said they was gold in it," insisted the boy, "and he told me to tell you to take care of it for him. He said it would be six weeks before he could get back and he wouldn't trust nobody but you."

Carter swore again, not so softly. Then he got to his feet, lifted the weighty bag to the bench by the door and sat down to stare at it. It was heavy-very heavy. He hadn't supposed Jim had so much dust as that meant. But Jim was always such a close-mouthed, saving fellow, you couldn't tell what he might have.

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"Why, there must be there must be pounds of the darn stuff, sure as my name's Dan Carter," he said.

It certainly looked like it and felt like it. It was a small, worn, brown leather satchel, smaller than any of the kinds. usually used for carrying clothing, bound up tightly with a scarred strap that had once been black, with a lock at the top and fasteners at the sides, from which the nickel-plate had been worn by long usage. It was not a pretty thing. In fact, it was a sufficiently disreputable looking object to serve admirably as an entirely safe deposit receptacle for its fill of gold dust.

But Carter was puzzled. How in the world Jim Bell has accumulated any such amount of dust as must be in that little grip without his, Dan Carter's, knowledge, was a thing hard to fathom. Jim was his chum and they had been unsuccessful beyond a very ordinary strike now and then which had produced just enough, as Dan said, "to keep 'em rooting after more." And there had been no opportunity for Jim to store away a hidden hoard, even with his partner's knowledge. Dan had seen the little satchel, as part of Jim's kit, scores of times, but

had never asked a question, for Dan was not given to asking questions of a personal nature. But now that Jim had voluntarily sent him the information that the bag contained a large quantity of the precious metal, two things troubled him at once. Why had Jim never before told his friend and chum of this hoard; and why, having kept the matter secret for so long a time, should he tell him now?

On the day before, Jim had started for the settlement, twenty miles down the stream on which they were working, after supplies. He had taken their two Mexican helpers and Bud, the boy, with him. Bud had just returned, bringing the news that Jim had broken his leg in a fall in the gulch, fifteen miles away, and had gone on, in care of the Mexicans, to find a surgeon, sending the boy back to apprise Dan of the occurrence. And

he had added this strange verbal message about the gold to his bare statement of the fact of his accident, as if he never expected to return at all.

Dan ruminated deeply. "By Jing!" he said at last, repeating his former statement, "there must be pounds of the stuff there."

"They is," assented the boy.

not a nice secret to have up here in the hills, under no safer keeping than Bud's, for Bud was sure to talk.

Carter moved uneasily in his chair. There was an uncomfortable little stir at his heart as he remembered that there was a gang down on the lower creek that would be just as well pleased to have gold ready washed out to hand as to dig after it themselves, and who wouldn't care who knew it-least of all a lonely, unknown miner on the ridge, whose partner was twenty miles away from home with a broken leg.

And then, Bud himself was a thief, just a pilfering nuisance, to be sure, who couldn't keep his hands off anything that struck his fancy, no matter who it belonged to, from revolver cartridges to whiskey. It had been a wonder to Dan that even Jim, good-natured, easy-going, big-hearted Jim, had tolerated him so long. Fifty times, since the boy had come to them, Carter himself had been ready to kick him down to the creek bottom-almost.

"It's a wonder Jim couldn't have thought out some way to send me word without confidin' the whole darn tale to the kid," he thought.

He reached over and tried again the

Carter glanced quickly at him and the weight of the little grip. "Gee, Jim's a crease between his brows deepened.

"Well, I don't know how he ever come to tell you, Bud," he remarked slowly. "Course, I s'pose he had to tell somebody and a feller can't always choose who he will tell."

.

He did not expect Bud to answer this statement, and his thoughts ran on swiftly. Now that he considered it again, of course it wasn't remarkable that Jim had told Bud. What else could he do, lying hurt down there in the gulch, with nobody but a couple of greasers to take him out to the settlement and to a doctor? Besides Jim had always felt kindly toward the poor, three-quarterswitted waif, whom he had certainly no reason to love except that he had saved the forlorn youngster from starving to death among the camps. A man sometimes felt kindly toward a person for whom he had done a good turn. Carter himself knew that, and probably Jim felt that way about Bud. But that was no reason for trusting him, and this was

close-mouthed duck!" he muttered and frowned again, for, somehow, in the light of the hard times they had been through together the discovery of Jim's secrecy hurt him.

Bud's eyes were bulging with interest in the grip and the strange look that the miner had always attributed to the boy's mental lacks was strong in them.

"Dan," he whispered suddenly, leaning forward with his hand on the miner's arm, "what'd you do if Chinny Mike Dolley should come up here after that gold?"

"Chinny" Mike was the notorious leader of the down-creek gang, who was known for a bad man, but who had earned his title by an odd habit of talkativeness. Carter's teeth shut tight.

"I'd shoot him," said the miner coldly. "I'd shoot him so full of holes he'd look like a sieve, 'sonny." And then he added with sudden vehemence, "And you, too!" "Me?" The boy cowered.

"Yes, you-fer Chinny Mike ain't

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said it, that he should call Jim poor when all that gold was his, and he thought about it as he put the bag carefully back under the bunk. By the time he had pushed it well out of sight next to the wall, however, he had concluded that it was some other feeling for Jim than pity that had brought out the words.

"Darn him!" he said aloud and smiled. The bag was more of a real worry to him than the fact that Jim was hurt. As he sat at the table that night facing Bud and dividing the rough supper with him, it lay heavy in his thoughts as the gold had been in his hand. He was anxious about Jim also, though Bud had reported that Jim had received no worse

wouldn't have been no trouble," he thought. "There won't be, anyway, I guess, but I got to keep Bud scairt."

He glanced across at the boy's dirty little face. The youngster certainly seemed to have been frightened by his threat. He had not looked the man in the eyes since. He carefully avoided Dan's gaze now. He even seemed tired. and sleepy, as well he might after riding nearly all day on his trip home.

"I guess he's scairt enough," repeated Dan to himself, "an' it's all right.'

He rose from the table to go about the housework of the little cabin. Of course he would take care of Jim's gold for him. Jim didn't have to ask that of him, even

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