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.