This calculation was postponed until the next day, and by 10 o’clock everybody slept profoundly. CHAPTER XIV. THE MEASURE OF THE GRANITE WALL—AN APPLICATION OF THE THEOREM OF SIMILAR TRIANGLES—THE LATITUDE OF THE ISLAND—AN EXCURSION TO THE NORTH—AN OYSTER-BED—PLANS FOR THE FUTURE—THE SUN’S PASSAGE OF THE MERIDIAN—THE CO-ORDINATES OF LINCOLN ISLAND. At daybreak the next day, Easter Sunday, the colonists left the Chimneys and went to wash their linen and clean their clothing. The engineer intended to make some soap as soon as he could obtain some soda or potash and grease or oil. The important question of renewing their wardrobes would be considered in due time. At present they were strong, and able to stand hard wear for at least six months longer. But everything depended on the situation of the island as regarded inhabited countries, and that would be determined this day, providing the weather permitted. The sun rising above the horizon, ushered in one of those glorious days which seem like the farewell of summer. The first thing to be done was to measure the height of Prospect Plateau above the sea. “Do you not need another pair of compasses?” asked Herbert, of the engineer. “No, my boy,” responded the latter, “this time we will try another and nearly as precise a method.” Pencroff, Neb, and the reporter were busy at other things; but Herbert, who desired to learn, followed the engineer, who proceeded along the beach to the base of the granite wall. Smith was provided with a pole twelve feet long, carefully measured off from his own height, which he knew to a hair. Herbert carried a plumb-line made from a flexible fibre tied to a stone. Having reached a point 20 feet from the shore and 500 feet from the perpendicular granite wall, Smith sunk the pole two feet in the sand, and, steadying it carefully, proceeded to make it plumb with the horizon. Then, moving back to a spot where, stretched upon the sand, he could sight over the top of the pole to the edge of the cliff, bringing the two points in line, he carefully marked this place with a stone. Then addressing Herbert, “Do you know the first principles of geometry?” said he. “Slightly, sir,” answered Herbert, not wishing to seem forward. “Then you remember the relation of similar triangles?” “Yes, sir,” answered Herbert. “Their like sides are proportional.” “Right, my boy. And I have just constructed two similar right angled triangles:—the smaller has for its sides the perpendicular pole and the distances from its base and top to the stake; the second has the wall which we are about to measure, and the distances from its base and summit to the stake, which are only the prolongation of the base and hypothenuse of the first triangle. “I understand,” cried Herbert. “As the distance from the stake to the pole is proportional to the distance from the stake to the base of the wall, so the height of the wall is proportional to the height of the rod.”