Some would have liked to follow the motions of spinning bodies a little more closely, and to have made out clearly why they behaved as they did. Apparently Thomson imagined the whole affair was self-evident, for he never gave more than the simple parallelogram diagram showing the composition, with the already existing angular momentum about the axis of the top, of that generated about another axis, in any short time, by the action of gravity. As a matter of fact, the stability and instability of the gyrostat on the tray give the best possible illustration of the two different forms of solution of the differential equation, [:θ] + μθ = 0, according as μ is positive or negative; though it is also possible to explain the inversion very simply from first principles. All this was no doubt regarded by Thomson as obvious; but it was far from being self-evident to even good students of the ordinary class, who, without exception, were beginning the study of dynamics. Thomson's absorption in the work of the moment was often very great, and on these occasions he much disliked to be brought down to sublunary things by any slight mischance or inconvenience. Examples will occur to every old pupil of the great emphasis with which he commanded that precautions should be taken to prevent the like from happening again. Copies of Thomson and Tait's _Natural Philosophy_--"T and T'" was its familiar title--and of other books, including Barlow's Tables and other collections of numerical data, were always kept on the lecture-table. But occasionally a laboratory student would stray in after everything had been prepared for the morning lecture, and carry off Barlow to make some calculation, and of course forget to return it. Next morning some number would be wanted from Barlow in a hurry, and the book would be missing. Then Thomson would order that Barlow should be chained to the lecture-table, and enjoin his assistant to see that that was done without an hour's delay!