"It does seem to me that we should not delay an hour in making some effort to explore the direction the boat came from," was George's view of the situation as they canvassed the subject.
"That is my idea, also, and I am not in favor of giving much more time to hunting or other forms of recreation until we know how that boat came to South River."
"Yes; I can appreciate how anxious you are," said the Professor, after the boys had given their views. "What we are doing, however, is essential from every point of view. We must prepare provisions, so that we shall be able to know where we can get them in case of need. On the other hand, weapons are necessary, which take time to construct. If, however, it is thought advisable, we might make a trip of explorations along the South River, beyond the falls, the time to be limited to a week; but I have my doubts of the wisdom of such a course."
This suggestion appealed strongly to the boys, who were always keen for anything which savored of adventure, and it was some time before the boys could reconcile themselves to the saner and more business-like course of completing the boat and making the trip by water.
The weather was beautiful, and vegetation was springing up in abundant profusion everywhere. Magnificent showers fell at intervals, and the rainbows, more beautiful than any they had ever heretofore seen, spanned the heavens after the showers.
This had been noticed during the previous year, but now, after nine months of their life, with the wonderful insight which their needs had instilled into them, made them very observant of every phenomenon.
"I have often wondered," observed George, as he gazed at the beautiful broad band which formed a crescent across the heavens, "why there are never any rainbows in the middle of the day. They are never seen except in the morning or in the evening, and usually only in the evening."
"In order to understand that it will be necessary to explain what a rainbow is. As I stated previously, light is merely vibration. Now colors are formed by the different lengths of the vibrations, just the same as the different musical notes are made by the different vibratory lengths. To understand this more fully, I make a sketch (Fig. 7), which shows just what I mean. You will see that red is the lowest musical pitch, which we will call C, and to the right is a long, wavy line. D, the next pitch higher, might resemble orange, with the wavy line a little shorter, and so on, until we reach the highest note in the scale, where the wave lengths are very short. You have probably noticed that a drop of water in the sunshine glistens, and, if closely observed, may have seen that it was colored, particularly blue or green. As the rays of the sun strike the globe of water, they produce different wave lengths, and in that way make it appear to you as being possessed of colors. Now, a rainbow is nothing more nor less than sunlight passing through the drops of water which are suspended in the air and causing a refraction of the light. At noon the sun shines down from overhead, and we are not in the proper position to see this refracted light; but in the morning or in the evening the sun shines against the earth at an angle. At those times we are able to see the effect of refraction by the colors produced.