That same evening an extraordinary thing happened. It was the custom for glasses of milk, dishes of stewed fruit, and plates of bread and butter to be placed on the table in the dining-hall about eight o'clock. This was done as usual, but when the girls arrived for supper they found a large proportion of the bread and butter had vanished. At first the suspicion fell on Spot, the fox-terrier, but the cook pleaded an alibi on his behalf, proving that he had been in the kitchen the whole time; also, the rifled plates were in the middle of the table, so no dog could have purloined their contents without knocking over glasses, or disturbing spoons and forks.

"I'm afraid it's a two-legged dog," said Miss Harding gravely. "The French window was open, and it would be easy for anyone to walk in and help himself. I'm glad nothing more valuable was taken. I wish Miss Birks were here! It's most unfortunate it should happen on the very evening she's away."

The incident gave cause for serious apprehension. Miss Harding made a most careful round of the house before bedtime, to see that all bolts and shutters were well secured. Though she would not betray her alarm to the girls, she was afraid that a burglary might be committed during the night. Both she and Mademoiselle kept awake till dawn, listening for suspicious footsteps on the gravel outside. All was as usual, however, in the morning; there were no evidences of attempts to force locks or windows, and no trace of the mysterious thief who had taken the bread and butter. Mademoiselle reported indeed that she had again heard the curious sounds which for some nights past had disturbed her. She had risen and patrolled the house, and had come to the unmistakable conclusion that they issued from the barred room. The closed chamber was as much a riddle to teachers as to girls, so Miss Harding merely shook her head, and recommended Mademoiselle to tell her experiences to Miss Birks as soon as the Principal returned.

At five o'clock that afternoon Elyned Hughes came running downstairs with a white, scared face. She solemnly averred that, when passing the door of the mysterious room, she had heard extraordinary noises within.

"It was exactly like somebody moving about and frying sausages. I smelled them too!" she declared.

The report was in part confirmed by several other girls, who pledged their word that they heard stealthy movements when they listened at the barred door.

"Are you absolutely certain, or is it only mice?" queried Gerda. "We've so often fancied things."

"Mice don't clink cans, and strike matches, and clear their throats!" retorted Rhoda.

"But you may have thought it sounded like that."

"I couldn't be mistaken."

"Somebody's there, beyond a doubt," said Agnes.

"Perhaps it's a ghost?" queried Elyned.

"It's nothing supernatural this time, I'll undertake to say—whatever may have made the noises before."

"It ought to be enquired into," declared Doris. "Miss Birks ought to insist on having the bars taken down, and seeing what's going on."

"Oh, no, no! It's best to leave things as they are."

Gerda was looking white and upset and spoke almost hysterically.

"Do you expect the ghost to bolt in amongst us the moment the door is unlocked?" mocked Rhoda.

"No, of course, I'm not so silly! But it's often better to let well alone."

"Mrs. Trevellyan is still away, so Miss Birks couldn't ask her to have the bars taken down now," volunteered Betty Scott.

"So she is," exclaimed Gerda, with an air of relief.

"Ah! You're afraid of the ghost," repeated Rhoda. "I'm more inclined towards the burglar theory. In the circumstances, I think Miss Birks would be quite justified in making an investigation, even without Mrs. Trevellyan's permission."

"I shouldn't wonder myself if Miss Birks called in the police," said Betty Scott.

The girls were in a ferment of excitement over the affair. Deirdre and Dulcie felt that in view of yesterday's discovery they had a strong clue to the mystery. They hesitated as to whether they ought at once to tell Miss Harding, but, as Miss Birks was expected home within an hour or two, they decided it was better to wait till they could deliver their news at head-quarters.

Gerda, during the whole day, had been very abstracted and peculiar in her manner. She was nervous, starting at every sound, and seemed so preoccupied with her own thoughts that she often took no notice when spoken to.

"What's wrong with the Sphinx?" commented Deirdre. "She's absolutely obsessed."

"Yes, I can't make her out. She's disturbed in her mind. That's easy enough to see. There's something queer going on in this school. I hope she's not mixed up in it."

"We'd decidedly better watch her. After all that's happened before, one can't trust her in the least. Until Miss Birks is safely back in the house I feel we oughtn't to let Gerda out of our sight. Who knows what she may be going to do, or whom she's in league with?" Coupled with the mysterious happenings of last night and to-day, Gerda's palpable uneasiness gave strong grounds for suspicion. The chums watched her like a couple of detectives. They were determined to warn Miss Birks directly on her return. Meanwhile nothing their room-mate did must escape their notice. They were to perform a duet at the musical examination, therefore they had the extreme felicity of doing their practising together. For the same half-hour Gerda was due at the instrument in the next room. They waited to begin until they heard the first bars of her "Arabesque". At the same moment came from the hall the sounds of the bustle occasioned by Miss Birks's arrival home. Deirdre and Dulcie looked at one another in much relief.

"She'll just be downstairs again by the time we've finished practising, and then we'll go straight and tell her," they agreed.

I am afraid neither in the least gave her mind to the piano. Mademoiselle, had she been near, would have been highly irate at the wrong notes and other faults that marred the beauty of their mazurka. Both girls were playing with an ear for the "Arabesque" on the other side of the wall.

"She's stopped!" exclaimed Dulcie, pausing in the middle of a bar. "Now, what's that for, I should like to know? I don't trust you, Miss Gerda Thorwaldson."

But Deirdre was already at the window. "Look! look!" she gasped. "Gerda's off somewhere!"

The window of the adjacent room was a French one, and the girls could see their schoolfellow open it gently and steal cautiously out on to the lawn. She glanced round to see if she were observed, then ran off in the direction of the kitchen-garden. In a moment the chums had thrown up the sash of their window and followed her. All their old suspicions of her had revived in full force; they were certain she was in league with somebody, and for no good purpose, and they were determined that at last they would unmask her and expose her duplicity. They had spared her before, but this time they intended to act, and act promptly too.

Gerda opened the gate of the kitchen-garden as confidently as if she were not transgressing a rule, and rushed away between the strawberry beds. Pilfering was evidently not her object, for she never even looked at the fruit, but kept straight on towards the end where the horse-radish grew. Keeping her well within sight, the chums went swiftly but cautiously after. She stood for a moment on the piece of waste ground that bounded the cliff, looked carefully round—her pursuers were hidden behind a tree—then plunged down the side of the rock and out of sight. Deirdre and Dulcie each drew a long breath. The conclusion was certain. Without doubt she must be going to pay a visit to the cave which communicated with the mysterious chamber. Whom did she expect to find there?

"To me there's only one course open," declared Deirdre solemnly. "We must go straight to Miss Birks and tell her this very instant."

The Principal, disturbed in the midst of changing her travelling costume, listened with amazement to her insistent pupils' excited account.

"This must be investigated immediately," she declared. "Dulcie, fetch a candle and matches, and you must both accompany me to this cave. You say Gerda has gone on there alone?"

Miss Birks took the affair gravely. She appeared very much concerned, even alarmed. She hurried off at once with the girls to the kitchen-garden.

They led the way down the narrow staircase cut in the cliff, and across the beach and over the rocks. At the entrance to the cave they both uttered a sharp exclamation, for Gerda stood there in an attitude of hesitation, as if unable to make up her mind whether to enter or no. She turned red, and white, and then red again to the tips of her ears when she saw that she was discovered, but she offered no explanation of her presence there. She did not even speak.

"Girls," said Miss Birks, "I think it is highly desirable and necessary that we should follow this passage into the room which I am told is beyond. Deirdre, you go first, with this candle, then Dulcie—Gerda, give me your candle, and walk just in front of me."

Policing the three in the rear, the Principal gave nobody an opportunity to escape. She had her own reasons for her conduct, which at present she did not choose to explain. With a hand on Gerda's shoulder, she forced that unwilling explorer along, and she urged an occasional caution on Deirdre. They had reached the cavern, and now, opening the small inner door, flashed their candles into the room. The result was startling.

On the bed reclined a figure, which, at sight of the light, sprang up with the cry of a hare in a trap—a man, unkempt, ragged, and dirty, bearing the impress of tramp written plainly upon his haggard, unshaven countenance. He darted wildly forward, gazed up at the strangers regarding him, then threw himself on a chair, and buried his face in his hands.

Gerda gave a long sigh of supreme relief. It was evidently not at all what she had expected to see.

"I'm done!" whimpered the tramp. "Send for the bobbies if you like. I'll go quiet."

"You must first tell me what you are doing here," said Miss Birks, stepping down into the room. "Then I can decide whether or no it is necessary to call in the police. Who are you? And where do you come from?"

"I knowed this passage when I was a boy," was the whining reply. "We used to dare each other to go up it, but the door at the end was firm shut. Then when I come back, down on my luck, and without a penny in my pocket to pay for a lodging, I thought I'd at least spend a night there under cover. I'd a bit of candle and a few matches, so I found my way along easy, and there! if the door at the end wasn't broke open, and the place waitin' all ready for me—bed, kettle, cooking-stove, frying-pan, cup and saucer, and all the rest of it, just as if someone 'ad put 'em there a purpose. I wasn't long in takin' possession, and I've lived here five days, and done nobody no harm. I didn't take nothing from the house either, except a bit of bread and butter last night when I felt starving. T'other days I'd found a job on the quay, and was able to buy myself victuals."

"Did you cook sausages?" quavered Dulcie, with intense interest.

"Aye, I'd earned a bit this morning to buy 'em with. Don't know who set up a stove here, but it come in handy for me, all filled ready with oil, too."

"But you know you've no right here," said Miss Birks severely.

"No, mum," reverting to his original whine. "I know that, but I'm a poor man, and I've been unfortunate. I came back to my native place looking for a bit of work. I've bin half over the world since I left it."

"If you're a Pontperran man, somebody ought to be able to vouch for you. What's your name?" "Abel Galsworthy."

Then Gerda sprang forward with intense, irrepressible excitement on her face.

"Not Abel Galsworthy who was at one time under-gardener at the Castle?" she queried eagerly.

"The same—at your service, miss."

"And you were dismissed for—for——"

"For borrowing a matter of a few pears, that made a little disagreement betwixt me and the head gardener. I swore I'd try another line of life, and I shipped as a fireman on board a steamer bound for America, and worked my way over the continent to California. I didn't get on with the Yankees, so I took a turn to Australia, but that didn't suit me no better, and after I'd knocked about till I was tired of it, I come home."

"Do you remember that when you were at the Castle you witnessed a paper that the old Squire signed?"

"Aye, I remember it as if it was yesterday. Me and Jim Robinson, the under-groom, was the witnesses, but Jim's been gone this many a year."

"Should you know your own handwriting again? Could you swear to it?"

"I'd take my Bible oath afore a judge and jury, if need be."

"Then—oh! thank Heaven I have pieced the broken link of my chain!" cried Gerda. "Oh! can I really clear my father's name at last, and wipe the stain from the honour of the Trevellyans?" "What does she mean?" asked Dulcie. "I don't understand!"

"It's all a jig-saw puzzle to me!" said Deirdre. "What does Gerda know about the Castle, and the old Squire, and a paper? And what has she to do with the honour of the Trevellyans?"

"I guessed the riddle long ago," smiled Miss Birks, laying a friendly hand on Gerda's arm. "The likeness to Ronnie was enough to tell me that she was his sister."