“I know!  I know what it is!” gasped Harriet excitedly.  “Listen, Tommy, listen.  Don’t you know?”

CHAPTER XIX

SAILING THE BLUE WATER

“No, I don’t know what it ith.  If I did, I thhouldn’t be athking you,” answered Grace.  “It ith either lightning, fireworkth or a real fire.”

“It is wireless, Tommy.  Don’t you know now?”

Grace shook her head.

“Didn’t you ever hear a wireless machine work?”

“No; but there ithn’t any wireleth on the ‘Thilly Thue,’ ith there?”

“I—­I don’t know.  I mean, I did not see any when we were out there to-day.  I don’t understand it.  What can he be doing with wireless so late at night?”

“Maybe he ith telegraphing home to find out if the folkth are all right,” suggested Tommy.

Harriet did not smile.  Her face was very grave, her forehead wrinkled in thought.  For the greater part of an hour, with brief intervals between, the wireless on the sloop continued, the sparks at the masthead sputtering and snapping with marked regularity.  Had Harriet Burrell understood a little more of telegraphy she would have known, though unable to read the dots and dashes, that the operator was calling some one who did not answer.  After a long time he apparently gave it up, for the sparking at the masthead ceased suddenly, followed by a brief period of silence on board, then the creaking of block and tackle was renewed.  This was followed by a subdued thumping and rattling about on deck, this lasting only a few moments.  The “riding light”—­a light hung from the stern of the boat—­was hung out, a dim light appeared in the cabin, which after a time was extinguished, then silence settled over the sloop for the night.

“That is all for to-night, I think,” said Harriet aloud, but in a low voice.  “I do not know what it is all about, Tommy, but I do know that something queer is going on here.  Do you think you and I will be able to solve the mystery?”

“I think tho.  Don’t you?”

“I do.  This makes two mysteries for us to solve, one the finding of that mysterious box and the other the mystery of the wireless on the ‘Sister Sue.’  I would suggest that you don’t say a word about it to any one to-morrow.  Don’t ask any questions, either—­leave that to me—­but keep your eyes open while you are on board.  Perhaps we may discover something that we overlooked there to-day.  Wireless on the ‘Sister Sue’!  I don’t understand it at all.  Be very careful that you do not wake up the others when you go in.  Make sure that you don’t fall over a cot and startle the girls.”

“Yeth, I’ll be careful.”

Harriet remained outside while Grace was getting herself back to bed, but the former darted in quickly upon hearing a crash in the cabin, followed by a scream from Margery.  Tommy had stumbled against Buster’s bed and fallen across it and on the sleeping stout girl.  But Harriet, knowing it would not do for the girls to know that two of their number had been mooning out-of-doors, darted into her own cot, and before they realized that she had just got in, was sitting up in bed demanding to know what all the disturbance was about.