Page:Tales of the long bow.pdf/222

 goes round the sun, and the moon goes round the earth. Well, in my formula, we first treat it as if the sun went round the earth"

She looked up radiantly. "I always thought it looked like that," she said emphatically.

"And you will, of course, see for yourself," he continued triumphantly, "that by the same logical inversion we must suppose the earth to be going round the moon."

The radiant face showed a shadow of doubt and she said "Oh!"

"But any of the things you mention, the milking-stool or the cow or what not, would serve the same purpose, since they are objects generally regarded as stationary."

He looked up vaguely at the moon which was steadily brightening as vast shadows spread over the sky.

"Well, take those things you talk of," he went on, moved by a meaningless unrest and tremor. "You see the moon rise behind the woods over there and sweep in a great curve through the sky and seem to set again beyond the hill. But it would be just as easy to preserve the same mathematical relations by regarding the moon as the centre of the circle and the curve described by some such object as the cow"