[Ill.u.s.tration: _Fig. 33. The Portable Fort._]

"The sketch which I have made shows these features, in which you will see (Fig. 33), the side boards (A) hinged along the middle line, as at B. These would be held at the outer corners by posts C. In the sketch the side board of the wagon body has been let down, so access can be had to the wagon."

The plan, so ingeniously contrived, pleased the fancy of the boys.

"I imagine," said Harry, "this will be the first time people ever carried their fort with them. It will be fun to meet the savages with that kind of an outfit."

"You must not think, however," continued John, "that this is all we shall need. We must take in the wagon sufficient food for the yaks to withstand a siege, so that with our own provisions, guns and ammunition, we shall have a pretty good load."

"How heavy will the three sections of fence weigh?"

"I estimate that, as we shall want the boards made of good wood, the weight will be thirty pounds per cubic foot, and as all the boards will take fourteen cubic feet of lumber, the total weight, including the posts, can be brought within 450 pounds, and I do not think our other material will weigh much over 500 more."

"That would not be a hard load," observed the Professor, "as we have often hauled a ton, but it would be well to make a new set of wheels, and we can then take with us an extra wheel for the front and rear."

"I think we should take tools along also," said Tom, "because we can always make our own repairs, in that event."

"I am glad you referred to that. It is certainly a suggestion in the right direction. Are there any more observations from any of the other army engineers?"

"I may be wrong," answered Harry, "but it seems to me that one of the folding fences should be put on each side of the wagon body, and the third under the wagon."

"What is your reason for that?" asked Tom.

"Because we may want to put up the fort in a big hurry some time, and by having them at the three places, and have it understood who are to take out each section, it would be the work of a few moments only to set it up, because each set of workers could handle his section without interference from the others."

"That is really a stroke of genius. Certainly, that is the sensible way," responded John.

"It is simply another ill.u.s.tration," added the Professor, "how men, looking at things from different standpoints, will see the defects in each other's work. That is the story of every great invention."

These conferences were of the greatest value to the boys. It pointed out how men, through force of circ.u.mstances, were compelled to devise things for their need.

George had been an intent listener during these discussions. "It does seem that the old statement, 'that necessity is the mother of invention,' is true."

The Professor turned to him smilingly, as he replied: "That may be so in very many eases, it is true, but I imagine that in the vast majority of instances the necessity was in the mind of the inventor to get some money. The thought of that requirement was a more vivid thing to him than the real need of the article as an economic necessity."

"Do you really think that is the case?"

"If my memory is not at fault, the people of England howled with derision when the first locomotive was built; the men who put out the first sewing machine had their stores broken into and the machines smashed; and the telephone when first installed was considered simply as a plaything and curiosity, and not as a useful improvement. It has been the history of every age and of most of the great inventions. After the inventions were completed, and their value shown, the merchant and the manufacturer created the demand, and then the articles became a necessity, and not before. For this reason I think the proverb should be amended to say that 'the necessity of the inventor is the mother of invention.'"

Before starting on the trip the matter of clothing had to be attended to. A quant.i.ty of ramie had been cut, and put in water, for the purpose of rotting the woody fiber, and this was taken out of the water as fast as it was ready, and cleaned and combed, and at times worked up into threads, which were placed in the loom, and a coa.r.s.e cloth thus woven.

This was, necessarily, a slow process, and consumed considerable time.

This, together with the making of the percussion caps, was the tedious part of all the preparations. Every energy was put forth to get the different things required. Harry and Tom had made up the fort, and John suggested the idea of having a drill exercise in setting it up, so that the work could be performed without interference.

During the day, when the posts and the fastenings were all ready, the wagon was brought out and the yaks yoked up. The elements of the fort were attached to the wagon, in the manner that they were to be transported. As there were three sections of the fort, one on each side of the wagon body and one below the axles, it was provided that the six should form three divisions; the Professor and Ralph, John and Tom, and Harry and George the couples for setting up the fort.

As Harry and George were the most familiar with the animals, and knew better than the others how to handle them, it was made a part of their duty, when the signal was given, to unyoke and turn the yaks to the proper place at the side of the wagon.

While this was being done, the Professor and Ralph were to detach the section on the side of the wagon where the fort was to be set up, and carry it out at right angles and at the forward end of the wagon. At the same time John and Tom would take the section on the opposite side of the wagon and carry it around to form the end of the fort.

This would then give Harry and George the opportunity to take the part below the wagon and erect it at the rear angle of the wagon, while Tom and Ralph were getting the posts for the outer ends of the fort.

When all was ready, the signal was given, and the various divisions sprang to their allotted work. They marveled at the celerity with which the fort was put up.

"I see a serious defect in the arrangement at one point," said Tom.

"What is that?" was the question from all.

"You see we have the fort ready, but it is adapted for one side of the wagon only. It may be most important to have it arranged so that either side of the wagon can be used for the fort."

"A fine suggestion," answered John. "That change can be made by having both sides of the body so they can be let down."

George also had a good suggestion to make. "According to the calculations we have fourteen cubic feet of material, and it is light wood, at that. Why couldn't the fort be utilized as a raft, so as to save the cutting of green timber, which is so heavy?"

"Well," said the Professor, smiling, "we are developing this at an immense rate. The new idea is the starting point for an invention to so arrange the sections as to make them act as floats. Here is a great opportunity for the genius."

Tom and Harry conferred on this subject for some time, and after luncheon, announced the plan: "It is fortunate that the axles of our wheels project. It will be an easy matter to take the sections from the sides of the body and attach one edge of each section to the projecting ends of the axles, and then the two posts can be used as braces to run up from the outer edges on the sections to the upper edges of the body.

The third section can be left under the axles where it now is."

John was delighted at the simple solution of this problem.

"I really believe," said Tom, "that we can adjust the sections in that way while the wagon is moving, as it can be attached without any difficulty."

It is wonderful how one improvement marks the advance stride for the next. Invention is really nothing but a step by step movement; a little addition here, another accretion there, and so on, so that invention has been shown to be, not a matter of quant.i.ty, but of quality. The mere bending of a wire, if it produces a new and useful result, is just as much ent.i.tled to the dignity of an invention, as a room full of intricate mechanism.

CHAPTER XVIII

TRAILING A WARRING PARTY OF NATIVES

"Professor, won't you tell us what the difference is between weight and gravity? We have been discussing that matter this afternoon."

"Gravity is a force by virtue of which all bodies tend to approach each other constantly; and weight is the measure of the effect of gravity acting upon a body. The two are often confounded."

"Now, the next thing we want to know is, does gravity act in all directions?"

"Yes; and the attraction of one body for another is in proportion to its ma.s.s--that is to say, if two bodies, one weighing one and the other two pounds, should be hung near each other, the heavy object would pull the lighter one twice the distance it would itself move."

"Do you mean to say that everything on earth attracts everything else? I thought it was only the earth that had the power to attract."

"The earth is no different from any other object in that particular, but on account of its immense size everything goes toward it and its motion toward the small object is not noticed."

"Is there any way that such a statement could be proved?"

"I suggest one plan: select two walls, close together, or two tall trees, and run a wire across, as I show in the sketch (Fig. 32). From that cross wire, A, suspend three objects by cords, B, C, D. The cord B is exactly midway between the two walls, and the other cords C, D, and so attached that the objects at their lower ends hang close to the walls. It will be found that the cords C, D are farther apart at their lower ends than at the upper ends, and that the cord B is exactly plumb, as it is affected equally by the attraction of the opposite walls."