Weighing of water. With reference to the use of the expression "mass" it would appear that the expression "weight" is generally used in legal enactments. The weight of a cubic foot of water has been fixed by the present Sale of Gas Act, 1859, but the Weights and Measures Acts do not fix the actual weight of a given volume of water, as of a cubic foot. The weight adopted in the Sale of Gas Act was derived from the evaluation of the mass of a cubic inch of distilled water based on weighings made in 1798 by Sir G. Shuckburgh (Philosophical Transactions, Royal Society, 1798, p. 133); and on measurements made in 1821 by Captain Kater (Phil. Trans., 1821, pp. 316 and 326). According to these measurements a cubic inch of distilled water weighed 252-458 grains, and the capacity of the Imperial Gallon was taken at 277.27384 cubic inches. The Imperial Standard Bushel, which is equal to 8 gallons, had, therefore, an apparent capacity of 2218.19072 cubic inches. As the diameter of the standard bushel is double its internal depth, its diameter would be 17.80927948 and its depth 8.90463974 inches. A more recent evaluation of the mass of a cubic inch of distilled water has been stated in the Transactions of the Royal Society, pp. 331-354, 1892, showing that the mass of the cubic inch of distilled water, freed from air, weighed in air at the temperature of 62° F., the barometer being reduced to 30 inches, against brass weights of the density of 8:143, was found to equal 252.286 grains ± 0·0002 grain. A cubic foot of such water under similar conditions would weigh 62.278601 lbs. avoirdupois. It appeared that a cubic foot of distilled water freed from air weighed about 321 grains more than when "saturated" with air. The value 252.286 is adopted in an Order in Council dated 28th November 1889, legalising certain new measures of capacity. Where distilled water is not available, or pure rain water, then ordinary water may be used: Professor D. Mendeléeff has also given the following as the weight of a cubic decimetre of distilled water freed from air : (Proceedings, Royal Society, London, February, 1896, p. 155.) In ancient times, although some attention was given to the purity of the water used when determining the capacity of a standard measure of capacity, Dr. Wybard was probably the first English experimenter who took especial care in the selection of pure water (Wybard's Tactometria," London, 1650), as in his experiments in London on the capacities of the standard ale and wine gallons of 1650, he used rain water "as it fell from the clouds," and snow water, as well as the waters of the Thames and the "new river of Ware," and the springs of Lamb's Conduit, Tower Hill, and Cripplegate. Com parisons of standard linear measures. In the verification of the standard of length, or in the comparison of graduated or linear standards of length, the form of comparator generally used is one fitted. with two micrometer microscopes, which are placed over the two graduations or lines between which any given distance or interval of length is to be measured. Such microscopes are firmly fixed so that when an interval of length has been measured on one comparing standard the other standard measure can be brought under the microscopes and the corresponding interval of length marked on it can be thus compared with that on the comparing standard (vide Fifth Report, Standards Commission, Appendix vii.) The two micrometer microscopes are thus used much in the same way as a pair of beam compasses might be, with the important exception that the standards are not touched or in any way interfered with whilst under the microscopes: both standards being placed in a box so that a uniform temperature may be secured during the comparison. In the following figure (Fig. 17) the two vertical micrometer microscopes are shown as mounted in position and fixed to a stone shelf which is supported by a platform of masonry standing on a concrete base. Under the microscopes is a travelling platform on which rests the long iron box or trough shown in the middle if the figure (Fig. 17). The following will show the amount of accuracy within which standard yards can be constructed. In 1882 the three Parliamentary copies of the Imperial Yard were compared one with the other as follows: Yard No. 2 Yard No. 3-0.000091 inch. Yard No. 2 = Yard No. 5-0.000029 inch. the difference in no case reaching one ten-thousandth of an inch. For the above comparison a large number of |