Now that they had decided on an explanation of their schoolfellow's mysterious conduct, the chums felt that every circumstance seemed to point in its favour. They wondered they had never thought of it before. The importance of keeping a strict watch was realized by both. There was a certain satisfaction in doing so. They felt as if they were rendering their country a service, almost indeed as if they were members of a secret diplomatic corps, and had been told off for special duty. Who knew what England might have to thank them for some day? Possibly at no very far-off date the whole country might be ringing with their names, and the newspapers publishing portraits of the two schoolgirls who had averted a national disaster. Just to be prepared for emergencies, they took snapshots of each other with Dulcie's Brownie camera, and added a series of photographs of the school, all of which they thought would be very suitable to give to the enthusiastic reporter who would demand an illustrated interview. They were rather disappointed with the results of the portraits, which in their estimation scarcely did them justice. "I look more like forty than fourteen!" said Deirdre, regarding ruefully the dark shadows on her cheeks and the lines under her eyes. "It doesn't show my hair properly, either. No one could tell it was curly."

"And I look as fat as a prize pig, with no eyes to speak of, and an imbecile grin."

"I wonder how real photographers manage to touch things up, and make them look so nice?"

In spite of their best efforts it had proved impossible to do their developing and printing without their handiwork being seen by their companions. The photographs of the school were so good that the girls begged them shamelessly to send home. Gerda was particularly importunate, and even offered to buy copies when they were refused as a gift.

"We don't sell our things," said Dulcie bluntly. "You may go on asking till Doomsday, and you won't get a single print, so there!"

To the chums, Gerda's request was full of significance.

"It shows pretty plainly we're on the right track," said Deirdre. "Of course she wants them to send to her foreign government. They'd pay her handsomely."

"Don't she wish she may get them!" snorted Dulcie.

The affair made an added coolness in their dormitory. Gerda appeared to think them unkind, while they stood more than ever on the alert. They watched her unceasingly. For some days, however, they could find nothing of an incriminating nature in her conduct. Possibly she was aware of their vigilance, and was on her guard against them.

"I believe we're overdoing it," said Deirdre anxiously. "Best slack off a little, and seem as if we're taking no notice of her. Don't follow her about so continually. It's getting too marked altogether. We must be diplomatic."

Just at present Gerda's behaviour was perfectly orthodox. If she went on the warren, it was invariably as one of a "threesome", and the chums could detect her in no more solitary and clandestine excursions. She seemed to have assumed a sudden interest in salvaging, and particularly in the beacon which the girls were beginning to build upon the headland. No one was ready to work harder in carrying up the pieces of driftwood from the beach, and piling them on to the great stack which every day grew a little higher and higher, till it really began to be a conspicuous object, and could be seen from both the villages of Pontperran and Porthmorvan, and from the sea. It was at Gerda's suggestion that a Union Jack, fastened to a pole, was kept flying from the top—a little piece of patriotism which appealed to the school at large, though it roused suspicion in the minds of the chums.

"It's a signal, of course," said Dulcie.

"Some fine day she'll pull it down, and substitute the German flag," agreed Deirdre. "She's only waiting her opportunity."

"Unless we circumvent her. There are two Britishers here who mean to look after their country!" It was curious how many little things, really quite trivial in themselves, seemed to point in the direction of the chums' fears. Miss Birks greatly encouraged a debating society among her girls, and on her list of subjects for discussion had placed that of "National Truth versus Diplomatic Evasions". Gerda had certainly been chosen to speak for the opposition, and was therefore pledged to the side of diplomacy; but Deirdre and Dulcie thought she made far too good a case of it, and pleaded much too warmly the cause of the ambassador who on behalf of his country's honour is obliged to meet guile with guile, and outwit the enemy by means of stratagems and deeply-laid schemes.

"Any expedient is allowable for the sake of your fatherland," she had contended, and Dulcie quoted the words with a grave shake of her head as she talked the matter over with Deirdre.

"Notice particularly that she said fatherland! Now the Vaterland is always Germany. She didn't mean Britain, you may depend upon it. No—she's planning and scheming for another war!"

"Then we'll plan and scheme for King George! We'll accept her principles, and 'make use of any stratagem to outwit the enemy'."

So they waited and watched, and watched and waited, in what they flattered themselves was true Machiavellian style, till they were almost growing tired of so fruitless an occupation.

Then one day, quite unexpectedly, something happened. It was a wild, windy March morning, and the girls were taking a hasty run on the warren between morning school and dinner, to "blow away cobwebs" and give them an appetite. There was not time to go far, but they dispersed in all directions, trying which could make the biggest distance record available. Gerda had started with Annie Pridwell and Betty Scott, but under pretence of beating their speed she had got considerably ahead and left them panting in the rear.

"Where's Gerda?" asked Deirdre, who, with Dulcie and Evie Bennett, had followed the first "threesome".

"We simply can't keep up with her! She walked as if she had seven-leagued boots. She's gone over the hill there. I'm going to wait till she comes back."

"There's no sense in flying like the wandering Jew!" protested Betty. "I hope she won't be long, because I don't want to walk back as fast as I came."

"Dulcie and I'll go after her," said Deirdre promptly. "We don't mind running. You two can be toddling along with Evie as leisurely as you like."

It only meant a change of "threesomes", so the girls agreed readily and departed at once, leaving the chums to act escort to the truant.

"She's done it on purpose," gasped Dulcie as soon as they were alone.

"Of course. It's a perfectly transparent dodge. Now we must do Secret Service work again and not let her see she's being followed."

The chums really congratulated themselves that they were getting on in the matter of scouting, they availed themselves so cleverly of the cover of rocks and bushes and proceeded with such admirable caution and care. Their efforts were successful, for after a few minutes of skilful stalking they caught sight of their quarry.

Gerda was climbing down the cliff side, fully a hundred feet below them, and had nearly reached the level of the beach. She descended quickly, almost recklessly, scrambling anyhow over rocks and through brambles, and splashing through a boggy piece where a trickle of water had formed a pool. Arrived on the shingle, she went straight to a hole among the rocks, searched in the seaweed, and produced a bottle. Taking a piece of paper from her pocket, she folded it into a long narrow slip and put it inside, replacing the cork tightly. Then she ran towards the crag at the mouth of the cove, and climbing up higher than was compatible with safety she hurled the bottle as far as she could throw it into the sea. She stood looking for a moment or two as it bobbed about on the surface of the water, then, turning round, began to scramble back with more haste than care.

"We've seen enough! Come quick before she spies us!" whispered Deirdre, dragging Dulcie away. "We mustn't let her know we were anywhere near. Let us run and be a long way off before she gets to the top of the cliff and sees us."

The clanging of the first dinner bell, which could plainly be heard in the distance, certainly offered a reasonable excuse for hurry. The chums fled like hares, and even with their best efforts only took their places at table when grace was said and the beef carved. Gerda was later still and scurried in, hot and breathless, after the potatoes had been handed. She drank her whole glassful of water at a gulp. Deirdre and Dulcie avoided looking at her, but they nudged each other secretly. It was a satisfaction to know what she had been doing, though they could not openly proclaim their rejoicing. The penalty for lateness at meals was a fine, but they put their pennies in the charity box with the feeling of philanthropists. They considered them as contributions to a most excellent cause.

It was Wednesday, and a half-holiday. At three o'clock the whole school was to start for a walk to Avonporth, and in the meantime the girls were expected to busy themselves with minor occupations. A certain number were due at the pianos for practising or music lessons, and from the rest stocking-darning, mending, and the tidying of drawers would be required. Gerda marched off with a volume of Beethoven, and was soon hard at work on the Moonlight Sonata under Mademoiselle's tuition. She played well, for she had been carefully taught in Germany, and had a good execution and sympathetic touch.

Deirdre and Dulcie stood outside the door for a moment or two listening to her crisp chords.

"She's boxed up there safe for an hour," commented Deirdre.

"Yes, Mademoiselle won't let her off," agreed Dulcie.

"I could do my darning after tea, and my drawers are as tidy as tidy."

"So are mine!" "Should we? Do you think we dare?"

"Yes, yes. I'm game if you are."

Then the pair did a scandalous deed, such as they had never even contemplated in all their schooldays before. They took French leave and went out on to the warren. They knew the consequences would be disastrous if they were caught, for they were breaking three rules all at once, absenting themselves without permission, going two together instead of in a "threesome", and being on the headland at a forbidden hour. Perhaps the very riskiness of the undertaking added to its enjoyment.

"We must try and get that bottle, and here's our opportunity," said Deirdre.

"We can't explain to Miss Birks now, but we can tell her some day that we went out of sheer necessity," argued Dulcie.

"Of course; it's only our duty. Even the best of rules have to be broken sometimes when it's a matter of expediency. Miss Birks will quite appreciate that."

"Yes—when she knows the whole."

Meantime Miss Birks did not know, and the sense that their disinterested motives might be liable to misinterpretation caused the chums to proceed warily and avoid exposing themselves to any observer from the upper windows. They tacked along bypaths and went rather a roundabout route to reach their destination. Their hope was that the rising water might have washed the bottle back on to the beach, for Gerda's arm had not been strong enough to throw it sufficiently far to carry it into the open sea, and when they last saw it it had been whirling round and round at the mouth of the creek. They climbed down the cliff side by the same track that she had followed, and ran eagerly to the edge of the waves.

The tide was much higher than it had been before dinner, and was rolling up its usual toll of sticks, seaweed, and miscellaneous debris. What was that dark-green object that kept appearing and disappearing, half-hidden by a mass of floating brown bladderwrack? One moment it had vanished, and the next it bobbed up persistently. Deirdre and Dulcie did not wait to ask. With one accord they whisked off shoes and stockings (a proceeding utterly and entirely forbidden except in the months of June and July) and plunged into the water. They were both adepts in the art of salvaging, but no piece of driftwood ever gave them more trouble than that elusive bottle, which dipped and dived and evaded them with the skill of an eel. The beach was shingly, not sandy, which made their fishing not only a slippery but a most agonizing performance. They were obliged to grip each other's hands to keep their foothold at all. At last a larger wave than usual proved helpful, and indeed did its office so thoroughly that it dashed the bottle against Dulcie's shins. With a squeal of pain she caught it, nearly upsetting herself and Deirdre in the process, and the pair hobbled back to where they had left their shoes and stockings.

"Ugh! I'm absolutely lame! I didn't know stones could cut so," complained Deirdre. "Look at my leg! It will be black and blue, I know," groaned Dulcie.

The possession of the bottle, however, was ample compensation for any scars they might have won in the struggle for its acquisition. They tried with impatient fingers to pull out the cork, but as that proved obdurate they cut the Gordian knot by breaking the neck on a stone. The thin piece of foreign note-paper was quite untouched by wet. Together they unfolded it, knocking their heads in their eagerness to read it both at once. At last, surely, they were within reach of Gerda's secret. But the letter was written in German, and alas! the chums were still in the elementary stages of the language, so that except for a chance word here and there they could not decipher a line of it. Their disappointment was keen.

"What does she mean by writing in her wretched old Deutsch?" demanded Dulcie indignantly.

"Oh, bother her! I wish I could read it!" moaned Deirdre.

Never had the advantages of education appealed to the girls more strongly. They began to think quite seriously of the necessity for studying foreign languages.

"Why didn't I have a FrÄulein in my babyhood instead of an ordinary English nursery governess?" lamented Deirdre.

"We may be able to do something with a dictionary," said Dulcie more hopefully.

The idea was consoling enough to prompt them to put on their shoes and stockings, pocket the document, and climb the cliff. After all, if they could make little out of it themselves, they had at least prevented the message from falling into the hands of the person for whom it was destined, and so had frustrated Gerda's intention. That was sufficient reward for their trouble, even without the chance of learning its contents.

"We can keep asking separate words or even sentences until we can piece it all together," said Dulcie sagely.

"Right you are! and now we'd best rush back as fast as we can."

Time waits for nobody, and during their excursion to the beach it had seemed to roll on above the speed limit. Unless they meant to be late for the walk, they must hurry. They were obliged to skirt the cliffs, for they did not dare to show themselves on the open tract of the warren. It was not particularly easy to make haste along a narrow path beset with briers and riddled with rabbit holes. Deirdre went first, because she always naturally took the lead, and Dulcie, whose physical endurance was less, panted after her a bad second. Suddenly Deirdre stopped, and, shading her eyes with her hand, looked intently over the sea at a small object in the far distance.

"What's that?" she asked sharply.

For a moment or two it had the semblance of a huge bird, then a strange whirring noise was heard, and as it drew rapidly nearer and nearer they could see it was an aeroplane flying at no great height over the water. Apparently it was aiming for the exact spot where they were standing, and, quite scared, the girls crouched down beside a gorse bush. With a loud whirr it passed over their heads, and, steering as easily as a hawk, alighted gently on the moorland only about a hundred yards farther on.

Here was a pretty state of things! Had the vanguard of the German army arrived already? And did the enemy mean to swoop down on the school? They peeped timorously from behind the bush and saw two airmen in full oilskins dismount hastily and make an examination of the machine. Whether they were Germans it was impossible to tell; they spoke in tones too low for their words to carry, and certainly their garments gave no hint of their nationality. They looked round searchingly, as if verifying their whereabouts, glanced in the direction of the girls who cowered under their gorse bush, devoutly hoping they were not visible, and consulted a map; then, after an earnest conference, entered their machine again and started off in a northerly direction, flying over the warren towards Avonporth. The chums, almost spellbound, watched the aeroplane till it waned into a mere speck in the sky; then fear lent them wings and they scuttled back to school at a pace they had never attained even at the annual sports. Fortune favoured them, and they managed to dodge unnoticed into the garden, run round to the front, and just in the nick of time take their places among the file of girls assembled on the drive.

Nobody mentioned the aeroplane, so evidently nobody but themselves could have seen it. Whence it came and where it was going remained a mystery, though Deirdre and Dulcie had a settled conviction that Gerda could have enlightened them on that point. She was quite unconscious of the trick they had played her, and as they walked just behind her they chuckled inwardly at the knowledge that her cherished letter lay in Deirdre's pocket. Outward and visible triumph they dared not venture on: it was too dangerous an indulgence for those who wished to keep a secret. As it was, they found it difficult to evade the enquiries of their friends.

"What became of you two just now?" asked Evie Bennett. "Miss Harding was inspecting drawers, and she sent me to fetch you. I'd such a hunt all over the place and couldn't find you anywhere."

"You're a notoriously bad looker, you know, Evie," returned Deirdre, laughing the matter off.

"So Miss Harding said; but it isn't fair to expect one to find people who aren't there."

"Perhaps Betty had mesmerized us into the hypnotic state and rendered us invisible to mortal eyes such as yours!"

"Now, don't rag me! Oh, wasn't that joke spiffing! I shall never forget Va with their faces all streaked with black! I laughed till I nearly died. They haven't forgiven us, and I believe they're plotting something to pay us back in our own coin."

"Let them try, if they like. We're not easily taken in."

"By the by, I was hunting for you two just now," Annie Pridwell broke in. "I wanted to borrow some darning wool, and as I couldn't find you I helped myself off your dressing-table. I don't know whose basket it was I rifled. I took the last skein."

"Mine, but you're welcome," said Dulcie. "My stockings are darned for this week, and shown to Miss Harding and put away. I'll get some more wool on Saturday, if we go to the village."

"But I couldn't find you when I looked for you," persisted Annie.

"Yes, where were you?" asked Evie again.

But to such an inconvenient question the chums prudently turned deaf ears.

Deirdre and Dulcie were determined to leave no stone unturned until they had obtained a translation of the letter which they had purloined from the bottle. They did not care to show the manuscript itself to any of the elder girls, as to do so might be to betray their secret, but by dint of asking odd sentences and words they made it out to run thus: "Very little to report. No progress at all just at present. Extreme caution necessary. Better keep clear of headland for a while, and let all plans stand over." There was neither beginning nor signature, and no date or address.

To the chums the communication had only one meaning. It must refer to a German attack upon the coast. The aeroplane had probably been prospecting for a suitable place to land troops. It was Gerda who was to supply the information needed by the foreign government as to a favourable time for executing a master-stroke.

Evidently she did not consider the hour was yet ripe. For the present England was safe, but who knew for how long?

"It's that man in the brown jersey who's engineering the mischief," said Deirdre. "When we see him sneaking about in his boat we may know there's something on foot."

"What ought we to do?" asked Dulcie doubtfully.

"Nothing can be done just now, if they're on their guard and lying low. We must be vigilant and keep a general eye over things. If anything unexpected crops up we can warn the police. But, of course, we should have to have very good grounds to go upon in that case, a perfectly circumstantial story to tell."

"We've nothing but suspicions at present."

"That's the worst of it. We want more direct evidence. They might only laugh at us for our pains, and we should get into trouble with Miss Birks for interfering in concerns that aren't ours. No; we'll keep the police as the very last resource, and only tell them what we know in the face of a great emergency."