On one occasion, after working out part of a calculation on the long fixed blackboard on the wall behind the table, his chalk gave out, and he dropped his hand down to the long ledge which projected from the bottom of the board to find another piece. None was just there; and he had to walk a step or two to obtain one. So he enjoined McFarlane, his assistant, who was always in attendance, to have a sufficient number of pieces on the ledge in future, to enable him to find one handy wherever he might need it. McFarlane forgot the injunction, or could not obtain more chalk at the time, and the same thing happened next day. So the command was issued, "McFarlane, I told you to get plenty of chalk, and you haven't done it. Now have a hundred pieces of chalk on this ledge to-morrow; remember, a hundred pieces; I will count them!" McFarlane, afraid to be caught napping again, sent that afternoon for several boxes of chalk, and carefully laid the new shining white sticks on the shelf, all neatly parallel at an angle to the edge. The shelf was about sixteen feet long, so that there was one piece of chalk for every two inches, and the effect was very fine. The class next morning was delighted, and very appreciative of McFarlane's diligence. Thomson came in, put up his eye-glass, looked at the display, smiled sweetly, and, turning to the applauding students, began his lecture. From time to time there were special experiments, which excited the interest of the class to an extraordinary degree. One was the determination of the velocity of a bullet fired from a rifle into a Robins ballistic pendulum. The pendulum, consisting of a massive bob of lead attached to a rigid frame of iron bars turning about knife-edges, was set up behind the lecture-table, and the bullet was fired by Thomson from a Jacob rifle into the bob of the pendulum. The velocity was deduced from the deflection of the pendulum, its known moment of inertia about the line of the knife-edges, the distance of the line of fire from that line, and the mass of the bullet. In some of the notices of Lord Kelvin that have appeared in the newspapers, the imagination of the writers has converted the Jacob rifle into one which Professor Thomson carried in the early years of the volunteer movement, as a member of a Glasgow corps. It is still used in the Natural Philosophy Department for the same experiment, and is a muzzle-loading rifle of large calibre, which throws an ounce bullet. It was invented by the well-known Indian sportsman, Colonel Jacob, for big-game shooting in India. Thomson held a commission as captain in the K (or University) Company of rifle volunteers, and so did not shoulder a rifle, except when he may have indulged in target practice.