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 an inch in 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 



