Page:Climatic Cycles and Tree-Growth - 1919.djvu/77
DETAILS OF CURVE PRODUCTION,
horizontally on the table at a distance of about 33 inches. The cathetometer telescope has a lens of such a focus that 1 mm. on the wood section becomes 0.25 mm. in the focus. The micrometer has a screw-thread with a pitch of 0.25 mm., so that one revolution of the micrometer head moves the thread through exactly 1 nun. as seen on the wood. The individual measures of rings are made on the micrometer screw by reading the graduation of the head to revolutions and hundredths, giving directly millimeters and hundredths. On commencing a set of readings the stationary thread of the micrometer is first placed on the zero-year ring of each decade, and the reading of the cathetometer is made and this is entered on the adding machine. A space is then inserted on the adding machine and thereafter the micrometer reading of each ring in the decade is added in column as fast as made. Then another space is made on the adding machine and the total is entered without clearing the machine. Immediately below this total the reading of the cathetometer in the new position 10 years advanced is made and inserted on the machine without addition. Then another space on the machine is given, followed by the individual readings of the next decade. In this way all the years are read individually by the micrometer and every 10 years the sum of these readings is checked against the cathetometer reading, which should come to the same amount.
The reading of the micrometer screw to 0.01 mm. is closer than the average setting can be obtained. The rule has been generally observed that in every decade the agreement between the sum of the readings obtained and the cathetometer reading should check within 0.20 mm. In the earlier measures, where the rings were irregular or the surface of the wood uneven, this accuracy of check was not obtained in a few cases. Yet even there the error in checking was not much larger than the figure mentioned, and it is expected that the results are sufficiently close for all purposes desired. The 25,000 measures on the first group of sequoias were begun by the writer, but after 2,000 had been done they were continued by Mr. Edward H. Estill, who did them with great care. In the second group, with 22,000 rings, the measuring had been done by Mr. J. F. Freeman, who has made some slight alterations in the method above described by which an increased accuracy is obtained. As a result, the check between the decades by measure and by cathetometer is nearly always within 0.10 mm.