She looked at the red lines, and thought, as she always did when she saw a ruled track upon a chart, of a kitchen table in Lambeth Marsh, and a lamp, and her father's hand moving very slowly down a column of figures because he could no longer see the figures he was moving it down.
* * *
Sir Augustus Threlkeld came in at seven exactly, to applause. He was a big man of sixty, handsome still in the heavy way of a Roman bust, with a great deal of white hair worn long at the back and a voice that had been trained in a college chapel and improved at the dinner tables of three Lord Chancellors. He bowed to the President of the Institution, bowed to the Fellows, bowed to the gallery, where the ladies were thickest, and laid one large hand upon the green baize as if he were taking an oath upon it.
"My lords," he said, "gentlemen; and ladies, whose presence lends to the dry pursuits of science an ornament they could not otherwise boast. I propose this evening to show you how a sailor may find his longitude in a quarter of the time he now spends upon it, and with no loss of exactness whatever."
There was a murmur of pleasure. Judith's head tilted.
He began well. She would always say afterwards that he began well, and that it was this which made the rest so dreadful. He explained, in language a schoolboy could follow, the problem every navigator faced on every clear night at sea. The moon moves among the stars, he said, as the hand of a clock moves across its face, about her own breadth in an hour; and the Nautical Almanac, computed under the eye of the Astronomer Royal, foretells for every third hour of every day at Greenwich what her distance shall be from the sun and from nine bright stars. Let a sailor measure that distance with his sextant, and compare it with the Almanac, and he has Greenwich time; let him compare Greenwich time with the time of his own ship, got from the sun, and he has his longitude.
"Simple," said Sir Augustus, and smiled at the gallery. "Simple as a sum in a copybook. And yet, my lords, it is not simple at all, for the distance the sailor measures is not the distance the Almanac gives. He stands upon the surface of the earth, and the Almanac is computed for its centre. The moon, being near us, is thrown down in the sky by parallax, sometimes by a whole degree. The air bends the light of every body near the horizon and lifts it up by refraction. The sailor's distance is