192
To obtain annual rates of growth in metals in use
for 1920-1960, the basic data were modified to take
increases in apparent consumption for the period 1860-1900
into account. Since no equivalent Bureau of Mines data
exist for 1860-1900, changes in iron and copper production
for the period were used to obtain an approximate figure
for total metals added.
Copper production for 1861-1900 was approximately
50 percent of total copper production for 1900-1920.
Therefore, the figure for copper in use in 1920 was
increased by 50 percent and the figure for 1960 by the
same absolute amount. Exports for 1861-1900 and 1900-1920
were assumed to be the same percentage of total production
for both periods. Pig iron shipments for the period 1860-
1900 were approximately 36 percent of shipments for 1900-
1920. Therefore, the figure for iron in use in 1920 was
increased by 36 percent and the figure for 1960 increased
by the same absolute amount. As with copper, exports
were assumed to be the same for both 1860-1900 and
1900-1920. The amounts by which 1920 and 1960 copper
and iron figures were increased represents an estimate
of copper and iron in use in 1900.
Absolute quantities are overstated to the degree
that metals have been lost from the stock of metals in
use. However, if the percent lost per year were