It may bring a grain of consolation to teachers who meet with troublesome elements in the discharge of their duties, to know that Thomson, great and brilliant as he was, had similar experiences now and again. At one time a book of mathematical data would be removed from the place assigned to it, upon which he would give orders that it should be chained to the table; at others, there would be no chalk near the blackboard, and then the assistant would be solemnly instructed to have one hundred pieces available next time. On one occasion, he settled in a very novel manner the case of a student who insisted on disturbing the class by moving his foot back and forth on the floor. Calling his assistant, Thomson told him in a whisper to go down into the room under the tiers of seats, to listen attentively, and locate the wandering foot by its distance from two adjacent walls of the building. On his return to the lecture-room, the triumphant assistant gave the desired coordinates to the Professor, who took out his tape at once and measured off the distances, by which the outwitted offender was mathematically located. In obedience to orders, the latter rose and left the room, muttering a few graceful epithets as he went, in honor of Descartes, the founder of a system of geometry that could serve so well the twofold purpose of the detective and the mathematician. It was the custom in Glasgow to open the daily sessions, morning and afternoon, with prayer, the selection of which was left to the discretion of the Professor. Thomson usually recited from memory the third collect from the morning service of the Church of England, to which he sometimes added reflections of his own for the spiritual benefit of his hearers. In his teaching, Prof. Thomson was particularly insistent that his students should not bow their intellects in mute admiration before an array of mathematical symbols; but that, on all occasions, they should seek the physical meaning behind them. Writing on his blackboard one day _dx/dt_, he was not satisfied when told that it represented the ratio of the increment of _x_ to the increment of the independent variable _t_ (time); he wanted the student to say it represents velocity. He himself was so wont to look for the physical meaning of symbols that, like the prophets of old, he saw many things that were hidden from the eyes of ordinary mortals. He had the rare gift of translating mathematical equations into real facts; and he strove all throughout his life, by word and writing, to purify mathematical theory from mere assumptions. He often said that he could not understand a thing until he was able to make, or at least conceive, a model of it.