But when he had reached home he recollected that he ought to know the latitude, and that there were Arabs or some other people in Africa who found out the latitude of the place they were in by gazing at the sun through a tube. Bevis considered a little, and then went to the rick-yard, where there was a large elder bush, and cut a straight branch between the knots with his knife. He peeled it, and then forced out the pith, and thus made a tube. Next he took a thin board, and scratched a circle on it with the point of the compasses, and divided it into degrees. Round the tube he bent a piece of wire, and put the ends through a gimlet-hole in the centre of the board. The ends were opened apart, so as to fasten the tube to the board, allowing it to rotate round the circle. Two gimlet-holes were bored at the top corners of the board, and string passed through so that the instrument could be attached to a tree or post.
He was tying it to one of the young walnut-trees as an upright against which to work his astrolabe, when Mark arrived, and everything had to be explained to him. After they had glanced through the tube, and decided that the raft was at least ten degrees distant, it was clearly of no use to go to it to-day, as they could not reach it under a week's travel. The best thing, Mark thought, would be to continue their expedition in some other direction.
"Let's go round the Longpond," said Bevis; "we have never been quite round it."
"So we will," said Mark. "But we shall not be back to dinner."
"As if travellers ever thought of dinner! Of course we shall take our provisions with us."
"Let's go and get our spears," said Mark.
"O, I know--and I must make a map; wait a minute. We ought to have a medicine-chest; the savages will worry us for physic: and very likely we shall have dreadful fevers."
"So we shall, of course; but perhaps there are wonderful plants to cure us, and we know them and the savages don't--there's sorrel."
"Of course, and we can nibble some hawthorn leaf."
"No, certainly not; they're not ripe," said Bevis, "and unripe fruit is very dangerous in tropical countries."
"We ought to keep a diary," said Mark. "When we go to sleep who shall watch first, you or I?"