DETAILS OF CURVE PRODUCTION. 59 
horizontally on the table at a distance of about 33 inches. The cathe- 
tometer 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 microm- 
eter head moves the thread through exactly 1 mm. 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. 
TABULATING. 
The paper used for the tables throughout has been a cross-ruled 
paper with squares about three-eighths of aninchin size. This paper 
is 8 by 10 inches in size and suffices admirably for small tables. Usually 
20 numbers are placed on a horizontal line with the beginning year at 
the left and with numbers from 1 to 20 at the top. Thus 1820, 1840, 
etc., will be placed at the left, and 1821 will be the first date given in 
