"They are tying a string across the sidewalk on Lanham's side of the alley, I believe," whispered Jack, "so as to throw Mr. Williams head foremost into that mud-hole at the mouth of the alley." By this time, the three boys had finished their arrangements and retreated through the gate into the porch of the Riley house, whence they might keep a lookout for the catastrophe. "I'm going to cut that string where it goes around the tree," said Bob, and he crouched low on the ground, got the trunk of the tree between him and the Riley house, and crept slowly across the street. "I'll capture the string," said Jack, walking off to the next cross-street, then running around the block until he came to the back gate of Lanham's yard, which he entered, running up the walk to the back door. His knock was answered by Mrs. Lanham. "Why, Jack, what's the matter?" she asked, seeing him at the kitchen door, breathless. "I want to see Susan, please," he said, "and tell Mr. Williams not to go yet a minute." "Here's a mystery," said Mrs. Lanham, returning to the sitting-room, where the teacher was just rising to say good-night. "Here's Jack Dudley, at the back door, out of breath, asking for Susan, and wishing Mr. Williams not to leave the house yet." Susan ran to the back door. "Susan," said Jack, "the triplets have tied a string from the corner of your fence to the locust-tree, and they're watching from Riley's porch to see Mr. Williams fall into the mud-hole. Bob is cutting the string at the tree, and I want you to go down along the fence and untie it and bring it in. They will not suspect you if they see you." "I don't care if they do," said Susan, and she glided out to the cross-fence which ran along the alley, followed it to the front and untied the string, fetching it back with her. When she got back to the kitchen door she heard Jack closing the alley gate. He had run off to join Bob, leaving the string in Susan's hands. Dr. Lanham and the master had a good laugh over the captured string, which was made of Pewee's and Riley's top-strings, tied together. The triplets did not see Susan go to the fence. They were too intent on what was to happen to Mr. Williams. When, at length, he came along safely through the darkness, they were bewildered. "You didn't tie that string well in the middle," growled Pewee at Riley. "Yes, I did," said Riley. "He must have stepped over." "Step over a string a foot high, when he didn't know it was there?" said Pewee. "Let's go and get the string," said Ben Berry. So out of the gate they sallied, and quickly reached the place where the string ought to have been. "I can't find this end," whispered Pewee by the fence. "The string's gone!" broke out Riley, after feeling up and down the tree for some half a minute. What could have become of it? They had been so near the sidewalk all the time that no one could have passed without their seeing him.