(a) The unit of volume for determinations of a high degree of accuracy is the volume occupied by the mass of 1 kilogram of pure water at its maximum density and under the normal atmospheric pressure; this volume is called litre.
(b) In determinations of volume which do not admit of a high degree of accuracy the cubic decimetre can be taken as equivalent to the litre; and in these determinations expressions of volumes based on the cube of the unit of linear measure can be substituted for expressions based on the litre as defined above.
(c) The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram.
(d) The term “weight” denotes a magnitude of the same nature as a force; the weight of a body is the product of the mass of the body by the acceleration of gravity; in particular, the normal weight of a body is the product of the mass of the body by the normal acceleration of gravity. The number adopted for the value of the normal acceleration of gravity is 980·665 cm/sec².
2. Standards.—The metre (metre à traits) is represented by the distance marked by two fine lines on an iridio-platinum bar (t = 0° C.) deposited with the Standards Department. This metre (m.) is the only unit of metric extension by which all other metric measures of extension—whether linear, superficial or solid—are ascertained.
The kilogram (kg.) is represented by an iridio-platinum Standard weight, of cylindrical form, by which all other metric weights, and all measures having reference to metric weight, are ascertained in the United Kingdom. From the above four units are derived all other heights and measures (W. and M.) of the two systems.
The gallon is the standard measure of capacity in the imperial system as well for liquids as for dry goods. In the United Kingdom the lAetric standard of capacity is the litre, represented (Order in Council, 19th May 1890) by the capacity of a hollow cylindrical brass measure whose internal diameter is equal to one-half its height, and which at 0° C, vi-hen filled to the brim, contains one kg. of distilled water of the temperature of 4° C, under an atmospheric pressure equal to 760 millimetres at 0° C. at sea-level and latitude 45°; the weighing being made in air, but reduced by calculation to a vacuum. In such definition an attempt has been made to avoid former confusion of expression as to capacity, cubic measure, and volume; the litre being recognized as a measure of capacity holding a given weight of water.
For the equivalent of the litre in terms of the gallon, see below III. Commercial.
In the measurement of the cubic inch it has been found that the specific mass of the cubic inch of distQled water freed from air, and weighed in air against brass weights (A = 8-i3), at the temperature of 62° F.,
and under an atmospheric pressure
equal to 30 in. (at 32° F.), is equal to 252-297 grains weight of water at its maximum density (4° C). Hence a cubic foot of water would weigh 62-281 lb avoir., and not 62-321 lb as at present legally taken.
For the specific mass of the cubic decimetre of water at 4° C, under an atmospheric pressure equal to 760 mm., Guillaume and Chappuis of the Comite International des Poids et Mesures at Paris (C.I .P .M.) have obtained 0-9999707 kg., ' which has been accepted by the committee.
The two standards, the cubic inch and the cubic decimetre, may not be strictly comparable owing to a difference in the normal temperature (Centigrade and Fahrenheit scales) of the two units of extension, the metre and the yard. Troisihne Conference Gcncralc des Poids ct Mesures (Paris, 1901). Metric Units Com. Roy. Soc. (1898).
Phil. Trans. (1892): and Proc. Roy. Soc. (1895), p. 143. ^ Prnc. Vc'b. Com. Intern, des Poids el Mesures (1900), p. 84. Congrcs International de Physique r^uni k Paris en 1900. (SCIENTIFIC
For the weight of the cubic decimetre of water, as deduced from the experiments made in London in 1896 as to the weight of the cubic inch of water. D. Mcn<icc(i {Proc. Roy. Soc, 1895) has obtained the following results, which have been adopted in legislative enactments in the United Kingdom:
—
Temperature on
the Hydrogen
Thermometer
Scale.