This was somewhat embarrassing; but the school was famous and visitors were now and then shown round.
She paused, and the stranger turned to the principal.

"If you will allow me--" The principal smiled and he came up to Agatha, holding out his hand for the chalk.
"Suppose we alter the formula this way ?" he said and wiped out the letters and figures.
Agatha studied him as he wrote fresh symbols.

He was plainly dressed and about sixty years of age.

There was nothing else worth noting, but he obviously knew his subject and she liked his face.

She saw that the girls could follow his explanation, but while suited to their understanding, it was, in one respect, not quite accurate.
"I don't know if I've made it much plainer," he said deprecatingly when he stopped.
Agatha indicated a group of letters.