orrow."

"Not without a light. There ith thomething else going on. Oh, look!"

Following a period of silence, blue sparks began sputtering from the
masthead of the "Sister Sue." The girls could hear the sparks crackle
and snap spitefully.

"Oh, look at the fireworkth!" cried Tommy out loud. "The thhip ith on
fire!"

Harriet laid a firm hand on her arm. "Keep still!"

A faint squealing sound was now distinguishable, while the sparking
at the masthead continued with almost rhythmic regularity.

"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 myster