So one day Clayton counted the revolutions of a wagon wheel and did the calculations based on the diameter of the wheel. He found 360 revolutions equaled one mile. He counted all day. At the end of the day, he gathered everyone's estimates and, Howell said, "sure enough, he'd been the one who was right."

Clayton pestered Young for a machine that would take accurate measurements, and Young turned to Pratt, who designed the odometer based on Clayton's needs. The group resembled a modern engineering team, with Young as manager, Pratt as engineer, Clayton as customer and Harmon as craftsman.

"You quickly realize this was an absolutely brilliant man," Howell said of Pratt, "that people don't realize what a genius he was."

A wagon wheel was outfitted with a piece that, at the end of each revolution of the wheel, turned a tooth on the odometer attached to the wagon. Every six turns equaled one mile. With 60 teeth on the gear, the device was extremely accurate

A second gear counted every 10 miles, but Clayton and Harmon didn't follow Pratt's specifications for that part and rain caused it to swell and break.

Still, Clayton's job became a lot easier. Since he kept track of the exact distance and time on the trek, the odometer acted as a crude speedometer, too. The pioneers made roughly two miles per hour, which means Clayton had to check the odometer only every 30 minutes or so.

"That's better than a red bandanna," said Joey Jacobsen, the BYU student Howell enlisted to manufacture the replica of the odometer.

Jacobsen made no attempt to use old tools to make the replica, which is 18 inches long, 15 inches high, 3 inches thick and is made up of four gears turning on three shafts.

"No way," Jacobsen said, "we used any modern tool we could find."

The exercise still gave him a new appreciation for the inventiveness of the pioneers.

"I'm impressed they made it, especially on the trail," Jacobsen said. "It's one thing if they had a shop in Nauvoo, but to make it nights and mornings out of the back of a wagon?"

Clayton returned the 1,032 miles he'd measured from Salt Lake City to Winter Quarters, Iowa, in 1848. Young sent him to St. Louis to publish "The Latter-day Saints' Emigrants' Guide." The booklet was so popular there was at least one unauthorized version. Three other guides either relied on Clayton, borrowed heavily or plagiarized.

The odometer, Clayton and his guide provided incredible detail to pioneers who used the trail over the next several years and led much of the Gold Rush right through Salt Lake City on its way to California, a financial boon to the fledgling LDS community.

"The pioneers were cash-strapped and now you have these 49ers with cash coming through who need repairs and supplies," Howell said. "

Howell will present his research at a "History of Mechanisms Symposium" in Philadelphia in September. His work has been peer-reviewed and will be published in the proceedings of the symposium by the American Society of Mechanical Engineers.

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E-mail: twalch@desnews.com