136 THE ESSEX NATURALIST
can be obtained. In fact, such a "master grapii" has heen
prepared for the Medieval period A.D. 800-1500, and, with
a gap in the "dark ages", from the 3rd century A.D. back
to the, 2nd century B.C., although in this latter period the
dating of the timber is by associated pottery, and there may
be an error of not more than 20 years in the assigned dates.
The "master-graph" will look something like the
serrations of a Yale key, but having 2,000 such serrations,
and it is to be hoped that the sequence of hills and dales will
prove to be as non-repetitive in different periods as the keys
for different Yale locks.
The last step in this technique (from which I have
purposely omitted minor stages, which any statistically-
minded mathematician will observe) is to slide the short
graph, prepared on transparent paper from the results of
measuring our sample of unknown date, over the master-
graph until we find an epoch thereon which corresponds in
shape of hills and dales with the serrations of our shorter
sample graph. If a similar sequence does not occur else-
where, then it is reasonable to infer that the outermost ring
of our timber sample corresponds with the year marked on
the master-graph under the end of our sample graph. This
method has given reliable results, and a check with the
medieval timber chests in Westminster Hall, for some of
which the dates of construction are known, has confirmed its
potential accuracy.
I have applied the technique of dendrochronology to roof-
timber from Salisbury Hall. Walthamstow, Essex, and from
my own early 16th century house in Stoke-by-Nayland, a
mile over the Essex-Suffolk border, and the results agree
within about 15 years with the known dates of these
structures, obtained from quite other evidence.
The third and last new technique to which I shall refer,
is usually known as "C-fourteen dating", and owes its
inception and development to the intense effort expended on
nuclear research during the last war. Out of a drive to
produce atomic fission for weapons came, as one of many
by-products, this technique, as yet incomplete but of great
promise. I will endeavour to make clear its subtle ingenuity,
but I must beg your indulgence for short excursions into
fields of elementary chemistry and physics.