There is a small irregularity in the line C D, at numbers 26 and 27; both these experiments were made with the same weight in the scale, and under similar circumstances, except that in 26, water was used to lubricate the friction pulley, and in 27 oil was used.
It has been stated, that, with heavy loads, the brake operates much more steadily with oil as a lubricator, than with water, and the change in the lubricator at experiment 27, was made in consequence of the difficulty experienced by the operator, in regulating the tension of the brake screws. In experiment 26, nearly his whole strength, applied to the extremity of a wrench about three feet long, was required to move the nuts, whereas, in experiment 27, the same operation was performed with great ease. Experiment 26 was of much shorter duration than experiment 27, and a portion of the discrepancy may be due to a proportionally less perfect observation of the data in 26.
The line C D shows that, with a velocity of the interior circumference of the wheel not less than 44 or more than 75 per cent. of that due to the fall, the useful effect is 75 per cent, or more, of the total power expended. Beyond these points, the change in the coefficient of effect is nearly equal for equal and opposite variations of speed; thus, the diagram indicates that the coefficient of effect is 70 per cent, of the power expended, at the velocities 0.360 and 0.834.
Taking the mean of the extreme velocities, that is, of 0, when the wheel was still, and 1.835, when the brake was removed, we have
which is not far from the velocity giving the maximum coefficient of effect; that is to say, when the gate is fully raised, the coefficient of effect is a maximum when the wheel is moving with about half its maximum velocity.
77. Experiments 43 and 44 were both made with the gate fully raised, but the wheel at rest, the brake being screwed up sufficiently tight to prevent the wheel from revolving; — they were made for the purpose of ascertaining the total effort that could be exercised by the wheel.
By reference to column 9, of the table of experiments, it will be seen that, in experiment 43, the weight sustained was 4213.38 pounds, and in 44, the weight was 3946.38 pounds. These experiments were made under circumstances nearly identical, except that in 43, the weight preponderated, and in 44, the power of
