oy 
| Aug. 14, 1873] 
ON THE SCIENCE OF WEIGHING AND 
MEASURING, AND THE STANDARDS OF 
WEIGHT AND MEASURE* 
II, 
i has already been mentioned that the gravitation or 
weight of bodies varies with their density, and the 
density of the medium in which they are placed. In order 
_ to ascertain the true relative weight, as well as the actual 
weight of standard weights differing in density when they 
are weighed in air, it is necessary to allow for the weight 
of air displaced by each. It thus becomes necessary to 
reduce these weighings to a vacuum, dy deducting from 
the apparent weight in air the weight of the volume of 
air displaced by each standard. 
But the weight of a given volume of air is necessarily 
more or less according to its temperature, the pressure of 
the atmosphere, and other conditions affecting it; and 
PS.1844.1.06 
= e 
ce 
Fic. 1.—Imperial Standard Fic. 2.—Official Standard Pound, Gilt 
Pound of Platinum. Gun Metal. No. 3r. 
Diameter = 1°15 inch. Size: Diameter at a = 1°25 inch, 
. A = 281572. 4 » 
Displaces 0'403 grains of 
air, 
P A = 8'5144 
Displaces 1’9or grain of air, 
Upper Surface of Gilt Gun Metal 
Standard Pound shown. 
Upeer Surface of Platinum 
itandard Pound shown. 
the following data are requisite for ascertaining the weight 
of air displaced by each standard. 
1. The mean temperature of the air during the weighings, 
2, The mean barometric pressure reduced to 32° Fahr. 
and corrected for the pressure of vapour and of carbonic 
acid gas in the air. 
3. The density of the metal of which each standard 
weight is composed. 
_4. The co-efficients of expansion of the metals and of 
air. 
5. The relative weight of each standard. 
From data 1 and 2 the ratio of the density of the air 
to the maximum density of water must be ascertained. 
This ratio is also affected by the height above the mean 
level of the sea, and the latitude of the place where the 
* Continued from p, 270, 
Ts a 
NATURE 
307 
comparison is made, as the force of gravity differs ac- 
cordingly. ut in practice the determination of the 
weight of air displaced in weighing is easily and quickly 
effected, either by the more accurate mode of making 
the computations from the above-mentioned data, with 
the aid of a logarithmetical formula and tables for re- 
duction of weighings, or approximately by special tables 
showing the mean weight of ordinary air displaced by 
standards of various densities. The mean ord nary air 
taken as the standard air in this country is of the normal 
temperature of 62° Fahr., the barometer being at 30 inches, 
with the mercury reduced by computation to the tempera- 
ture of 32° Fahr., the amount of aqueous vapour in the air 
being assumed to be two-thirds of the quantity in satu- 
rated air, and the amount of carbonic acid contained in it 
being taken at o'0004 of its volume. 
The actual mode of ascertaining the weight of air dis- 
placed by standard weights when compared by weighings 
in air, will be described more at length afterwards. 
But some illustrations may here be given of the 
effect of the difference of density in standard weights, 
upon their weight in ordinary air. The following 1lb, 
avoirdupois weights are of the actual form and size :— 
Fic. 3.—Quartz Pound in Standards Department, bearing no mark, 
Size = 2'17 inches cube, edges rounded. 
A=2'6505. Displaces 3'216 grains of air. 
It may here be seen that the difference of air displaced 
by the imperial standard lb. P.S. (Fig. 1), and the gilt gun 
metal lb. No. 31 (Fig. 2), is 0'598 gr.; and if they were 
equal in weight when in a vacuum, No. 31 would be 
0'598 gr. lighter in air of the given density. No. 31 is 
one of the gilt gun-metal secondary standard weights, 
intended to regulate the weighings in air of all com- 
mercial weights. As the primary platinum standard P.S. 
from its greater density displaced so much less air than 
ordinary brass and iron weights—the density of cast-iron 
being about 7'408, and a cast-iron lb, displacing about 
1150 gr. of air—the weight of all the gilt gun-metal lbs., 
of which No. 31 was one, was referred by Prof. Miller to 
a theoretical commercial standard lb. of brass of the 
average density of brass and bronze weights (A\=8. 143), 
and thus displacing 1'047 gr. of standard air. This com- 
mercial standard Ib. denoted as W. was assumed to be of 
the same weight in a vacuum as P.S., and consequently 
in standard air P.S. was 0°644 gr. heavier than W. 
The standard pound of quartz (Fig. 3) displaces 3'217 
grains of air. It was constructed as an auxiliary standard 
on account of the invariability of. quartz, and its apparent 
