Page:Climatic Cycles and Tree-Growth - 1919.djvu/73

 Preparation for measurement.—These pieces were then examined to find the longest sequences of clear and large rings, and guide-lines for the subsequent identification and measurement were selected as nearly as possible perpendicular to the rings. Such lines having been decided on, two straight pencil lines, half an inch apart, were drawn and the surface between these was "shaved." For this purpose, after the trial of many other methods, a common safety-razor blade was clamped to a short brass handle. With this very sharp blade the rough surface of the wood is removed and the rings stand out very clear and distinct. Besides the space between the lines, the region close outside is usually shaved also for a preliminary trial at cross-identification, the final marks being the only ones permitted between the guide-fines.

The best light for observing the rings is a somewhat diffused light coming sharply from the side. A light falling on the wood perpendicularly is apt to be very poor, either for visual work or photography. Light from each side must be tried, for there is often a great difference between the two directions, due probably to the way in which the knife passed over the wood and bent the ragged edges of the cells. In photographing, the colors involved and the result sought (i. e., to show the red rings as black) require an ordinary plate and a blue color-screen.

When the surface is well prepared it is placed in a suitable light and wet with kerosene applied by means of a bit of cotton on the end of a small stick. This deadens the undesired details of the surface, and brings the rings into greater prominence. The identified section is now supported over the unknown and with watchmaker's glass in eye and long needle in hand, the observer can make rapid comparison and quickly put on the required marks.

IDENTIFICATION OF RINGS. In the early Flagstaff work the rings were first numbered, beginning at the outside without regard to the year in which they grew. But this was found to add complexity and involve the use of a separate reduction from the provisional numbers to the true dates of the rings. Accordingly the rings are dated at once as well as possible on some selected section that gives promise of an accurate record. The identification mark is a pin-prick or very small hole placed on the last ring of each decade. The middle year of each century has 2 pin-pricks and the centuries are marked with 3; the 1,000-year mark is 4. Marks found in error are "erased" by a scratch through them. After the selected section is dated with the greatest care not to overlook or mistake any rings, others are dated by direct comparison with it. The common practical test in such comparison is the relation of width of a ring to its half-dozen near neighbors. For some unknown reason, rings of diminished size seem to carry more individuality than enlarged rings, and so they are usually picked out for cross-comparison.