The British imperial standard and the United States Troughton standard differ by only 0·000762 inch, which applied to the English reduction of 39·37079, would give 39·36996 as the relative value according to Troughton standard. The difference between these reductions is probably to be attributed to the use of different rates of expansion, in correcting for standard temperatures, which vary considerably, according to high authority as follows for brass at 1° F. Whitworth, 1876 0·00000956 = 0·00034416 in. per yard. Borda, 1799 0·000009913 = 0·00035687 “ Smeaton, 1750 0·000010417 = 0·00037501 “ Hassler 0·000010508 = 0·0003783 “ Ramsden, 1760 0·000010516 = 0·0003786 “ Faraday, 1830 0·00001059 = 0·00038124 “ And for the bronze of which the British imperial standards are made: Airy and Sheepshanks 0·0000095 = 0·000342 in. per yard. Fizeau 0·00000975 = 0·000351 “ The correction at Ramsden’s rate is nearly identical with Hassler’s, and gives 39·3684933; at Whitworth’s rate it would give 39·36962, very nearly the same as deduced from the difference between the British Imperial standard and the United States Troughton standard. The results of Sir Joseph Whitworth were obtained by use of all late improvements for scientific precision, and they must be accepted as most reliable. It would appear preferable to give comparisons at the same temperature in connection with the corrected result, so that international comparisons of scientific measurements may not be vitiated by accidental variations. COMPARISON OF ENGLISH AND FRENCH STANDARDS. When the mètre standard was established in France, 1799, it was compared with Sir George Schuckburg’s standard yard by Captain Kater. The quadrant of 10,000,000 mètres, or 5,130,740 toises, was determined to be 32,808,992 English feet, giving the mètre equal to 3·2808992 English feet, or 39·37079 inches, and the toise equal to 6·3945925921 English feet. In 1814 Wollaston and Playfair, by comparison with the platinum mètre standard at 55° F., deduced the mètre as equal to 39·3828 English inches. During the geodetic operations of General Roy in 1802, who used 60° F. as standard temperature, Pictet’s comparisons, using means capable of measuring the 10,000th part of an inch, gave the mètre standard, which is used at 32° F. as standard temperature, at 39·3828 English inches; this corrected for temperature by Dr. Young, gave 39·371 English inches at 62° F.; which result was confirmed by Bird, Maskelyne and Laudale. In 1823, by Act of Parliament on report of committee, the mètre is fixed as 39·37079 English inches. In 1800 the Royal Society, by comparison with two toise standards sent by Lalande to Maskelyne, deduced the mètre as 39·3702 English inches. Later comparisons by Clarke in the Ordnance Survey Office at Southampton, in 1866, give the mètre as 39·37043 inches.