"For your guidance I make a drawing (Figure 24), in which A is the base, about five inches long, three inches at its widest end, and an inch wide at the narrow end. This should be made of a thin piece of hard wood.

Bore a small hole in each end of the C-shaped piece. The next thing is to make a pointer (B) nearly as long as the base, pointed at one end, and provided with two holes at the other. The pointer is attached to the base by a pin (D). One end of the C-shaped piece of metal is then hinged to the other hole (E), and the other end of the C-shaped piece is hinged, as at F, to the base. You will now see that if the ends of the C-shaped piece spread apart the least bit the long end of the pointer will swing over to the other side of the base."

[Ill.u.s.tration: _Fig. 24. THERMOMETER_]

"Do you intend to make the thermometer show the exact degrees of heat we really have?"

"Yes; as nearly as possible."

"Why can't we make it exact!"

"For the reason that to make what is called the Fahrenheit scale we should have freezing weather. The scale adopted by Fahrenheit was an arbitrary one. He determined it in this way: The temperature of his body was taken as one point in the scale. For zero he took the lowest temperature observed by him in the year 1709. As the temperature of his body was 86 he made a scale with 86 degrees on it, and then when he observed ice melting in water he put his thermometer in and found it registered at 32 degrees. It was not a very scientific way of doing it, but it answered the purpose, as, of course, temperature is merely a relative thing."

"But isn't there another scale to measure heat by?"

"The Centigrade method is on a more scientific basis. It is determined by taking the freezing and the boiling points of water, and dividing the scale into 100 degrees between those points."

It required the work of several hours to make the device as here shown, so that the tension of the C-shaped piece would hold the point to one side. The temperature of the atmosphere was about 65, as nearly as the Professor could judge, but when the C-shaped piece was held in the palm of the hand, the pointer moved to the lower edge of the base piece, and a mark of 80 was put there as the starting point. As they had no immediate use for a scale beyond 110 degrees, the end of the base was marked off, as you see in regular divisions.

The C-shaped piece was put under the tongue or in the armpit, so that the temperature of Harry could be determined, and it registered 102 degrees. It might be that Harry's temperature was really much higher, as the thermometer, for the reasons stated, was not accurate.

"I wish you would test the thermometer, George, so we can pretty well determine, within a range of two or three degrees, how nearly right we are."

George's temperature was found to register 98, whereas the Professor's was only 90. He explained that the temperature of youth is normally greater than old age, but it was remarkably close to the average normal temperature of two healthy persons.

"The advantage of the instrument now is to enable us to ascertain whether Harry's temperature will increase or decrease."

"Is there no other way in which we can find out about a fever?"

"When the fever comes on the pulse is usually quickened, as well as the breathing; the bowels are apt to be constipated; and thirst, loss of appet.i.te, headache, and vague pains are felt. When the temperature goes beyond 105 it is very dangerous, and it is for this reason that physicians want to know the temperature."

"I am anxious to know why that C-shaped piece should try to straighten itself out when heat is applied to it?"

"When you bent the metal piece of which it is made you crowded the metal on the inside of the piece together, and also stretched the metal on the outside of the bend. As the application of heat expands the metal, the contracted particles of the metal on the inside of the piece pushed against each other with greater force than those on the outside, and the bar tried to straighten itself out again."

"I have noticed that if a hose is coiled up and water is forced through under pressure it will straighten out the hose. Is that also the case with the hose?"

"To a certain extent only. Another principle comes into play in that case. Water under pressure acts as a solid, and has a tendency to move along the shortest route or in the most direct way. If, therefore, there is a crook in the pipe the water tries to straighten it out. Steam gauges are made of flattened spirally coiled tubes. One end of the tube is open and the other has an inlet for the steam. The dial finger has a connection with the moving end, and by that means pressure is indicated."

The next morning Harry's temperature was fully one degree higher than the previous day, and the Professor advised that it would be necessary to administer some fever medicine.

"Last week you found several samples of the gentian flower. It is a first-cla.s.s fever medicine and tonic. Do you think you could distinguish it by its large blue-colored, fringed flowers?"

"I know what you mean; it has one central stalk, with big leaves at the bottom which gradually grow smaller, and in which the stem seems to go up through the flowers."

"That describes it exactly. Get some of the roots, and peel them, then sc.r.a.pe a quant.i.ty, so we can give some to Harry."

This is a simple remedy, in the absence of regular fever medicines which were not available to them.

To their great relief the fever abated before morning, and by persistently taking the gentian tonic Harry was soon well again.

This little experience was sufficient notice that in health at least some preparation should be made for illness, which is sure to come to all at most unexpected times. It had also a stimulating effect in more pointedly directing the attention of the boys to the wonders of the vegetable world.

It was now the latter part of June, and the weather was not at all cold.

Plenty of rain had fallen, and the Cataract stream had risen so high that their water wheel had been out of commission for several weeks, and Harry's illness or indisposition had somewhat r.e.t.a.r.ded the work in the factory.

"Wouldn't it be a good idea to look up that animal over in the cave?"

was George's first suggestion, one morning, shortly after Harry's recovery. "We might put in a little time there, and then go down around the bay on a little tour."

This was agreeable to all, and then George remembered the want of the candles. Zinc had been turned out, as previously told, but no steps had been taken toward making a battery which would be the starting point for an electric lighting system, as Harry and George both hoped for.

"It seems to me," said Harry, "that we ought to explore the coast line to the southeast of us, as we have never been in that direction, and then work our way around to the cliff cave."

Without further words the yaks were yoked up, and taking a hearty luncheon they were off for the east coast, where the bay indented the land. The coast was reached within two hours, a great deal of the time en route being spent in gathering samples of plants, flowers, and fruit, of which some species of trees were filled.

To the right of the place where they struck the sea was a cape which ran out into the sea for fully a quarter of a mile, and to the south of this was the mouth of the South River. As they had definitely planned to go north along the coast line to the cliff rocks, the explorations to the south must be reserved for some future day.

The sea front showed delightful stretches of beach, but at intervals small trees and bushes grew close to the ocean on the elevated spurs which broke up the otherwise smooth beach line.

The clam, as a source of food supply, had practically been neglected, because it was quite a distance from the Cataract home to the beach, and princ.i.p.ally for the reason that other foods were so plentiful. Harry wanted some clams, and with one of their bags the beach was scoured for fully a mile, until he gathered a staggering load.

As he reached one of the little knolls which broke off abruptly close to the sea, Harry dropped the bag and ran to the brush. The Professor looked on in wonder. When Harry disappeared in the bush George and the Professor both hurried forward. Harry reappeared in an instant.

"What do you think I have found?"

"Another cave?" queried George, without stopping.

"No; our boat."

There, perched less than five feet above the level of the ocean, was the boat which they had left at the foot of the falls in the South River, fully three months before. One of the puzzling mysteries was solved.

It was some labor to dislodge the boat from its position between the cleft branches of shrubbery which also held other debris, and furthermore the boat was full of all sorts of rubbish. This was laboriously removed.

"You will remember I stated at the time of the disappearance the most likely explanation would be that high water coming on suddenly would wrench the boat from its fastenings, and----"

The Professor got no further; he suddenly stopped and glanced to the forward end of the boat. "Who tied on that rope? It really does look as though some one has used the boat. That is not one of the oars we made."

"But where are the lockers we had on the boat, in which we put our provisions?"

"They have been removed by some one. This is a rope entirely unlike any we have had, and it is a native, or rather, hand-made article."

"Well, we have struck a greater mystery now than when we lost the boat."

This discovery brought up several other questions which, as it now appeared, might be linked together. The removal of the flag and flagstaff; the "hole" in the hill; the fire in the forest; the branded bull, all indicated that people had, at various times, visited the island. But the finding of their boat, with the positive evidence furnished by the oar and the rope, was conclusive, and what made it the more interesting was the fact that the island must have had such a visit within two, or at the most, not over three months ago.

Each was too busy to give much time for discussing the probabilities.

They had entirely forgotten the cave. It was, by common consent, agreed that the boat must be taken home, and it was finally decided that the boys should pilot it around the point, past the cliffs, and in that way reach the mouth of the Cataract River, where it would always be convenient for cruising purposes.

It was fortunate that the sea was calm when the boys pushed the boat from the sh.o.r.e. It showed signs of leaking here and there, but the Professor a.s.sured them that the water would close up the joints before long. The Professor, himself, drove the team to the Cataract, and after unyoking them, followed the course of the river down to its mouth, to await the coming of the boys. He waited there until sundown, but the boys did not appear.