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!"