Encyclopedia Brown and the Case of the Jumping Frogs

The Case of the 
Rhyming Robber

Police across the nation wondered: How did Idaville do it?
The town had sparkling white beaches, a Little League team, and a computer museum. It had churches, a synagogue, two delicatessens, and four banks. In short, Idaville looked like many other seaside towns.
But it wasn't. 
Every person who broke the law in Idaville was caught.
How was this possible?
What was the secret?
Only Mr. and Mrs. Brown knew.
The mastermind behind Idaville's war on crime was their only child. They called him Leroy, and so did his teachers. Everyone else in Idaville called him Encyclopedia.
An encyclopedia is a book or set of books full of facts from A to Z, just like Encyclopedia's head. His friends thought of him as a whole library that could whistle Beethoven.
Mr. Brown was chief of the Idaville police. He was smart and brave. His officers were well trained, honest, and loyal. But sometimes they came up against a crime they could not solve. Then Chief Brown knew where to go--home to dinner.
After saying grace, he went over the case.
Ten-year-old Encyclopedia listened carefully. When he had heard the facts, he asked one question.
One question was all he needed to solve a mystery.
Encyclopedia never spoke about the help he gave his father.
For his part, Chief Brown would have liked to announce to the world, "A bust of my son belongs in the Crimebusters' Hall of Fame."
But who would believe him? Who could believe that the mastermind behind Idaville's spotless police record was a fifth grader?
At dinner Tuesday, Chief Brown toyed with his soupspoon. Encyclopedia and his mother knew what that meant.
The police had come up against a case they couldn't solve. 
"Do you want to talk about it, dear?" Mrs. Brown asked gently.
Chief Brown nodded. "A fortune in jewelry belonging to Mrs. Hubert Cushman was stolen from her home last week."
"Give Leroy the facts," Mrs. Brown said. "I'm sure he can help. He's never failed you yet." 
Chief Brown laid down his spoon. "The thief who stole Mrs. Cushman's jewelry calls himself The Poet."
"I've heard of him," said Mrs. Brown.
"He steals jewelry and then sends a poem with a riddle in it to his victim," said Chief Brown. "The riddle tells where he buried the jewelry. Mrs. Cushman received her poem yesterday."
"How do his victims know if the riddle really tells where their jewelry is?" Mrs. Brown asked.
"He got careless twice," Chief Brown said. "He made the riddles too easy. The stolen jewelry was found."
"So he really does bury the jewelry," said Mrs. Brown. "What happens when the riddle isn't solved?"
"It's believed that he comes back sometime later, digs up the jewelry, and keeps it." 
"My, is he ever something!" exclaimed Mrs. Brown.
"He's what is called a gentleman thief," explained Chief Brown. "Gentlemen thieves commit crimes mainly for the thrill. Outsmarting the police means more than the loot. It's all a sport with them."
Chief Brown took a piece of paper from his breast pocket. He unfolded it and handed it to Mrs. Brown. "This is the riddle Mrs. Cushman received."
Mrs. Brown read it, frowning.
"It doesn't make sense," she said. She passed the sheet to Encyclopedia. "Here, Leroy. What do you make of it?"
Encyclopedia read The Poet's riddle:

Take the Landsmill Highway north,
And look along the border.
The second clue is marked in reverse,
But the first clue is in order.
The Poet

Encyclopedia had never been on the Landsmill Highway. Nevertheless, he closed his eyes. He always closed his eyes when he did his deepest thinking.
Mr. and Mrs. Brown waited anxiously.
A minute went by, and then another. Had the famous jewel thief, The Poet, outsmarted the boy detective?
Encyclopedia opened his eyes. He asked his one question. "Are there mile markers along the Landsmill Highway, Dad?" 
Chief Brown was surprised by the question. "Why, yes, there are."
"Then," said the boy detective, "Mrs. Cushman's jewelry won't be hard to find."
Where was it buried?