110     ON  THE  EFFECT OF  THE  INTERNAL  FRICTION  OF  FLUIDS
oscillation, whereas in the formula (148) it is supposed that the mass of which the weight is JcP or (n- 1)P is collected at the centre of the sphere. If h be the distance of the centre of the sphere from the axis of suspension, the observed value of re — 1 ought in strictness to be increased in the ratio of h^2 to Z^2, or the calculated value diminished in the ratio of I'^2 to h?, before comparing the results of theory and experiment. In the case of the loaded spheres especially, the theoretical value of n would thus be a little diminished; but except in a very few cases, in which either I or a — I is small, the diminution is hardly worth considering. After having been for a good while at a loss to account for the regular occurrence of rather large negative errors, the following occurred to me as the probable solution of the difficulty.
When a pendulum oscillates in water, the arc of oscillation rapidly decreases; this rapid diminution forms in fact the grand difficulty in experiments of this kind. In Dubuat's experiments, it will be remembered, the suspending thread was lengthened or shortened till the time of oscillation was an exact number of seconds, or occasionally half a second. Now, it is probable that the observer occasionally gave the suspending thread a slight push as the pendulum was commencing its return, in order to keep the oscillations going for a sufficient time to allow of tolerable precision in rendering the time of oscillation equal to what it ought to be. If so, these pushes would slightly accelerate the oscillations, and therefore cause the length of thread fixed on by observation to be a little too great, which would make the effect of the water in retarding the oscillations appear a little too small. On inspecting the table of differences, it may be observed that sometimes when the same sphere differently loaded is swung in the same time as before, the numbers in the table of differences are altered more than appears to be attributable to merely fortuitous errors of observation. This accords very well with the conjecture just mentioned, and seems difficult to account for in any other way, inasmuch as everything relating to the fluid must have been almost exactly the same in the two cases.
The occurrences of positive differences in the case of the large wooden sphere may be accounted for by the limitation of the fluid mass by the sides and bottom of the vessel, and by the free surface, which, except in the case of very short oscillations, would