38 CLIMATIC CYCLES AND TREE-GROWTH 



is a similar head, also removable. The latter is attached to a small 

 drum with spiral thread about it, in which works a catgut string. 

 Between these two knurled heads, but not touching them, is an alumi- 

 num disk on the end of an arm, so made that by pressure on a lever 

 the disk comes into contact with both these knurled heads and thus 

 transmits the motion from one to the other and so from the main screw 

 to the catgut string. Several pairs of these knurled heads of different 

 relative sizes are supplied, so that motion in the catgut will be 20, 40, 

 or 100 times the motion of the carriage and telescope. By this means 

 change may be made in the vertical scale of the plot. 



Below the main screw and parallel to it is the plotting cylinder. 

 This is so arranged that the same lever-arm that brings contact between 

 the knurled heads moves this cylinder 2 mm. in rotation, measured 

 on the surface of the record paper. The ends of the catgut string pass 

 over rollers and extend parallel to the recording cylinder, and after 

 one end turns back on a small wheel the two ends meet and are attached 

 to the pen carriage, which travels on its own track parallel to the 

 recording cylinder. Thus, when the lever-arm is pressed and the 

 micrometer screw moves the telescope thread across a ring, from one 

 sharp outer edge to the next, the pen draws a line in proportion trans- 

 versely on the record sheet. The release of the lever-arm at the left 

 moves the cylinder, and the pen is restored to zero position. Thus a 

 columnar plot, here called "auto-plot", is made by setting on one ring 

 after another. 



Accuracy — The rapidity and mechanical accuracy of this instru- 

 ment are high. The graduations of a steel ruler were measured with 

 a very small percentage of error; that is, the accuracy is greater than 

 the accuracy of setting on a ring. 



Advantages — The instrument saves much time, because it makes 

 automatically the plotted records which in the cathetometer method 

 were plotted from the readings: These automatic records are called 

 auto-plots. The distance of the wood from the telescope does not 

 have to be fixed. In fact, I have measured rings in wood lying in 

 glass cases by placing the instrument on the outside of the case. 

 The records are in a convenient form and may be very long. They are 

 made on coordinate paper to definite scale, so that values may be 

 read off from the plots for use in tabulation. The plot is also ready 

 at once for a standardizing line, such as will be discussed below. 



Disadvantages — While the rapidity and accuracy of this method 

 exceed any other, its disadvantage lies in the difficulty of checking and 

 correcting the work after it is done. Coarse rings are readily handled 

 by inexperienced helpers, but the fine ones under 0.5 millimeter are 

 subject to mistakes. This is usually a question of identification, but 

 the difficulty in checking work immediately after it is done (without 



