"What's the matter, Tom?" asked Ned, who had come out for a breath of air. "Hurt your foot?"
"Hurt my foot! No! What makes you ask that?"
"You aren't dancing."
"I haven't had much chance!" was the somewhat grumpish answer, and Ned saw his chum's gaze following Mary and her partner. To do him justice, Barton was a fine dancer.
"Oh, ho!" mused Ned to himself, as he took in the situation. "Poor old Tom is jealous!"
CHAPTER XI
TRAILING THE MYSTERY
The next day, after a late, lazy sleep in the bunks of the House on Wheels, Tom Swift and Ned Newton, in fulfillment of a promise made the night before, called to take Mary and Grace for a ride in Tom's latest invention.
Tom and his chum called in a taxi to take the girls out to the House on Wheels, for the young inventor rightly guessed that Mrs. Winthrop would not appreciate the sensation that would be caused should the big auto stop in front of her house.
"Hello!" Mary called to Tom and Ned as she came downstairs with Grace to meet them. "Did you have a good time last night?"