140 THE ESSEX NATURALIST
tribution from the specimen will be but a fraction of the
adventitious background, and will be lost in it. This places
an upper limit on the age which can at present be determined
by this method : some 20,000 years. The lower limit is set
by the. difficulty that after, say, only 1,500 years, the count
will differ from that at the beginning by about 10 per cent,
and small differences of this order again tend to be masked
by irregularities unless the counting be continued for many
days. The remedy of using large samples presents other,
technical difficulties, even when the amount of material
available is large, which is often not the case. The accuracy
obtainable is within about 200 years in 3,000. 300-400 year's
in 10,000 years, and perhaps 1,000 years in 22,000. 'The
greatest relative reliability is obtained with specimens from
three to six thousand years old.
A few months ago, in May, 1952, some charcoal from a
small ritual pit at Stonehenge was examined and dated by
this radio-carbon or C14 method; the result was 1818
B.C., with a probable error of not more than plus or minus
250 years. This date agrees remarkably with other estimates
for the early phase of Stonehenge, including that rather
outmoded estimate of Sir Norman Lockyer, who dated the
"altar" stone of Stonehenge by an astronomical calculation
to 1080 B.C.