The completion of the Rogers-Bond comparator marks a long stride in advance over any method hitherto in use for comparison and subdivision of line-measure standards, combining, as it does, all the approved methods of former observers with others original with the designers. Comparisons can thus be checked thoroughly by different systems, so that the final result of the series may be relied on as being much nearer absolute accuracy than any hitherto produced. The calipering attachment to the comparator deserves special commendation, being simple in the extreme, and solving completely the problem of end measurements within the limit of accuracy attainable in line reading, by means of the microscope with the micrometer eye-piece. The standard to which the end measurements are referred is not touched, and each measurement is referred back to the same zero, so that error from end wear does not enter into the problem. This attachment is in advance of all hitherto known methods of comparing end measures, either with other end measures or with line standards, both as to rapidity of manipulation and accuracy of its readings, the strong point in its construction being that it refers all end measures to a carefully divided and investigated standard bar, which is not touched during its use, and cannot be in the slightest degree injured by this service, thus giving convincing assurance that the measures and gauges produced by its use will be accurate and interchangeable. In the opinion of this committee, the degree of accuracy already attained is such that no future improvements can occasion changes sufficiently great to affect the practical usefulness of the magnitudes here determined, or the interchangeability of structures based upon them with those involving further refinement. Prof. W. A. Rogers and Mr. George M. Bond are unquestionably entitled to great credit for the admirable manner in which they have solved the problem of exact and uniform measurement, while the enterprise of the Pratt & Whitney Company in bringing the whole matter into practical shape, is deserving of the thanks of the engineering community.[183] [183] Those interested may find detailed descriptions of the methods used and of the Rogers-Bond comparator in the following references: George M. Bond: Paper on “Standard Measurements,” Trans. A. S. M. E., Vol. II, p. 80. George M. Bond: Paper on “A Standard Gauge System,” Trans. A. S. M. E., Vol. III, p. 122. Report of Committee on Standards and Gauges, Trans. A. S. M. E., Vol. IV, p. 21 (quoted above). W. A. Rogers: Paper, “On a Practical Solution of the Perfect Screw Problem,” Trans. A. S. M. E., Vol. V, p. 216. Two lectures delivered by George M. Bond before the Franklin Institute, Philadelphia, in 1884, on: 1. “Standards of Length and their Subdivision.” 2. “Standards of Length as Applied to Gauge Dimensions.”