There were several other evenings not very different from this one. The master would wait until all the pupils had gone, and then overtake Barbara. He solaced his conscience by carrying a book in his pocket, so as to study on the way back; but he found a strange wandering of the mind in his endeavors to read a dead language after a walk with Barbara. He still held to his resolution, or to what was left of his resolution, not to entangle himself with an early engagement. What visions he indulged in, of projects to be carried out in a very short time after his graduation, belong to the secrets of his own imagination; all his follies shall not be laid bare here. But to keep from committing himself too far, he drew the line at the boundary of Mrs. Grayson's farm,--the meadow fence. He gave himself a little grace, and drew the line on the inside of the fence. He was firmly resolved never to go quite home with his pupil, and never to call at her house. So long as he stopped at the fence, or within ten, or say twenty, or perhaps thirty, feet of it he felt reasonably safe. But he could not, in common civility, turn back until he had helped her to surmount this eight-rail fence; and indeed it was the great treat to which he always looked forward. There was a sort of permissible intimacy in such an attention. He guarded himself, however, against going beyond the limits of civility--of kindly politeness--of polite friendship; that was the precise phrase he hit on at last. But good resolutions often come to naught because of its being so very difficult to reckon beforehand with the involuntary and the uncontrollable. The goodman of the house never knows at what moment the thief will surprise him. One evening Mason had taken especial pains to talk on only the most innocent and indifferent subjects, such as algebra. On this theme he was the schoolmaster, and he felt particularly secure against any expression of feeling, for _x_, _y_, and _z_ are unknown quantities that have no emotion in them. Though Barbara was yet in the rudiments of the study, he was trying to make her understand the general principles involved in the discussion of the famous problem of the lights. To make this clear he sat down once or twice on logs lying by the roadside, and wrote some characters on her slate showing the relation of _a_ to _b_ in any given case, while Barbara sat by and looked over his demonstrations. But in spite of these delays, they got to the fence before he had finished, and the rest was postponed for another time. It didn't matter so much about the lights after all, whether they were near together or far apart; it does not matter to lights, but there _are_ flames much affected by proximity. As Mason helped Barbara down from the fence, his passion, by some sudden assault, got the better of his prudence, and looking intently into the eyes shaded by the sun-bonnet, he came out with: