That done, he retired the necessary distance, when, lying on the sand, his eye glanced at the same time at the top of the pole and the crest of the cliff. He carefully marked the place with a little stick.
Then addressing Herbert--"Do you know the first principles of geometry?" he asked.
"Slightly, captain," replied Herbert, who did not wish to put himself forward.
"You remember what are the properties of two similar triangles?"
"Yes," replied Herbert; "their homologous sides are proportional."
"Well, my boy, I have just constructed two similar right-angled triangles; the first, the smallest, has for its sides the perpendicular pole, the distance which separates the little stick from the foot of the pole and my visual ray for hypothenuse; the second has for its sides the perpendicular cliff, the height of which we wish to measure, the distance which separates the little stick from the bottom of the cliff, and my visual ray also forms its hypothenuse, which proves to be prolongation of that of the first triangle."
"Ah, captain, I understand!" cried Herbert. "As the distance from the stick to the pole is to the distance from the stick to the base of the cliff, so is the height of the pole to the height of the cliff."
"Just so, Herbert," replied the engineer; "and when we have measured the two first distances, knowing the height of the pole, we shall only have a sum in proportion to do, which will give us the height of the cliff, and will save us the trouble of measuring it directly."
The two horizontal distances were found out by means of the pole, whose length above the sand was exactly ten feet.
The first distance was fifteen feet between the stick and the place where the pole was thrust into the sand.
The second distance between the stick and the bottom of the cliff was five hundred feet.
These measurements finished, Cyrus Harding and the lad returned to the Chimneys.
The engineer then took a flat stone which he had brought back from one of his previous excursions, a sort of slate, on which it was easy to trace figures with a sharp shell. He then proved the following proportions:--
500 x 10 = 5000
5000 / 15 = 333.3
From which it was proved that the granite cliff measured 333 feet in height.
Cyrus Harding then took the instrument which he had made the evening before, the space between its two legs giving the angular distance between the star Alpha and the horizon. He measured, very exactly, the opening of this angle on a circumference which he divided into 360 equal parts. Now, this angle by adding to it the twenty-seven degrees which separated Alpha from the antarctic pole, and by reducing to the level of the sea the height of the cliff on which the observation had been made, was found to be fifty- three degrees. These fifty-three degrees being subtracted from ninety degrees--the distance from the pole to the equator--there remained thirty- seven degrees. Cyrus Harding concluded, therefore, that Lincoln Island was situated on the thirty-seventh degree of the southern latitude, or taking into consideration through the imperfection of the performance, an error of five degrees, that it must be situated between the thirty-fifth and the fortieth parallel.
There was only the longitude to be obtained, and the position of the island would be determined, The engineer hoped to attempt this the same day, at twelve o'clock, at which moment the sun would pass the meridian.
It was decided that Sunday should be spent in a walk, or rather an exploring expedition, to that side of the island between the north of the lake and Shark Gulf, and if there was time they would push their discoveries to the northern side of Cape South Mandible. They would breakfast on the downs, and not return till evening.
At half-past eight the little band was following the edge of the channel. On the other side, on Safety Islet, numerous birds were gravely strutting. They were divers, easily recognized by their cry, which much resembles the braying of a donkey.