“What makes an ice-boat go?” asked Freddie.

“The wind blows it, just as the wind blows a sailboat,” explained Bert, looking down the lake after the ice-boat.

“But it hasn’t any cabin to it like a real boat,” went on Freddie.  “And it doesn’t go in the water.  Where do the people sit?”

“An ice-boat is like this,” said Bert, and with the sharp heel end of his skate he drew a picture on the ice.  “You take two long pieces of wood, and fasten them together like a cross—­almost the same as when you start to make a kite,” he went on.  “On each end of the short cross there are double runners, like skates, only bigger.  And at the end of the long stick, at the back, is another runner, and this moves, and has a handle to it like the rudder on a boat.  They steer the ice-boat with this handle.

“And where the two big sticks cross they put up the tall mast and make the sail fast to that.  Then when the wind blows it sends the ice-boat over the ice as fast as anything.”

“It sure does go fast,” said Tommy Todd.  “Look!  He’s almost at the end of the lake now.”

“Yes, an ice-boat goes almost as fast as the wind,” said Bert.  “Maybe some day——­”

“Oh, come on!” cried Flossie.  “I want to go home!  I’m cold standing here.”

“Yes, we had better go on,” said Nan.  “I’m all right now.”

As the five children skated off, no longer thinking of the race, Nan asked Bert: 

“What are you going to do some day?”

“Oh, I don’t know.  I haven’t got it all thought out yet.  I’ll tell you after a bit.”

“Is it a secret?” asked Nan, eagerly.

“Sort of.”

“Oh, please tell me!”

“Not now.  Come on, skate faster!”

Bert and Nan skated on ahead, knowing that Flossie and Freddie would try to keep up with them, and so would get home more quickly.  But they did not leave the smaller twins too far behind.

A little later the Bobbseys were safe at home.  Tommy Todd went to his grandmother’s house, and Flossie and Freddie took turns giving their mother an account of their escape from the ice-boat.

“Was there really any danger?” asked Mrs. Bobbsey of Bert.

“Well, maybe, just a little.  But I guess Mr. Watson would have stopped in time.  He’s a good ice-boat sailor.”

“But don’t let Flossie and Freddie get so far away from you another time.  They might have been hurt.”

Bert promised to look well after his little sister and brother, and then, having asked his mother if she wanted anything from the store, he said he was going down to his father’s lumberyard.

“What for?” asked Nan, as she saw him leaving.  “Is it about the secret?”

“Partly,” answered Bert with a laugh.

Two or three days later the Bobbseys were again out skating on the ice, Nan and Bert keeping close to Freddie and Flossie.  They had not been long gliding about when Freddie suddenly called: