II~. Barrus. amount of distortion which these defects produced were applicable to good practice. These conclusions were based on investigations made with a Richards indicator. In this instrument the moving parts were all unnecessarily heavy. Not only was this true of the pencil-mechanism with its arrangement of parallel bars, but it was also true of the drum-mechanism. The Richards indicator had been superseded, a t least in the United Stat,es, by instruments which possessed these defects in a much less degree, and it was to be regretted that the investigations were not made on one of these instruments, so as to bring the results into accord with the latest practice. This was the only adverse criticism he had to offer. Unnecessary weight in the moving parts appeared to be the cause of the principal errors to which indicators were liable. Since the time of the Richards indicator, nearly all the improvements had been in the direction of overcoming this defect, and it was in this field principally that further improvement might be expected. Much had already been accomplished. A comparison of the weights of the Richards indicator with those of one of the prominent modern instruments having the same size of drum, showed that a marked advance had been made. In the one, the weight of the several bars, which carried the pencil, was 11 3 grams (174 grs. troy) ; in the other, the single pencil-bar weighed 1 * 8 gram (28 grs.). In the one the weight of the drum-mechanism was 594 grams (9,166 grs.) ; in the other 207 grams (3,085 grs.). Further than this, it was probable that little reduction in weight could be effected without a change of proportions, or a change in the metal of which the parts were constructed. Something had been accomplished in the first direction by reducing the diameter of the paper-drum, and this was on the whole an improvement. Changes of this nature reduced the size of the diagram, which was undesirable, and consequently improvement in this direction was limited. Little had been done in the second direction, except in the line of experiment; but there was a field here for much progress. The metal aluminium furnished a material for greatly reducing the weight of nearly all the moving parts, and its use for this purpose promised excellent results. He had experimented with it when used for piston and piston-rod, and had compared its action with that of the composition ordinarily adopted. The diagram taken with aluminium showed that some of the oscillations of the pencil were entirely suppressed, and the amplitude of the remaining ones was reduced one-half. The result was a diagram nearly free from wa,vy lines a t a speed of 310 revolutions per minute. The weight of the aluminium piston and rod was 4 grams