used. No. S 17, Picea excelsa, marked "4105-5," was cut in May 1909, in the same locality as S 14 and shows a similar rhythm. Its center was in 1777. No. S 18 was a small section marked "4131-a-12." It was cut in October 1910, in latitude 58°, well to the west of the others. Its radius measured only 2 inches. The average diameter of the other sections was about 12 inches.
On page 115 will be found the means of these sections, with two extrapolations, one from 1820 to 1848 and the other from 1820 to 1878. This curve will be found plotted in figure 8. It has been corrected for age and reduced to percentages by dividing by the readings of a straight line extending from 1.90 mm. in 1820 to 0.70 mm. in 1910. This corrected set has been smoothed by Hann's formula and will be found plotted in figure 23.
EBERSWALDE (PRUSSIA) GROUP.
These 13 trees were cut and sections prepared for me by the kindness of Professor A. Schwappach of Eberswalde. They were all Pinus silvestris planted about 1820 to 1830, exactly alike in height and size, with tall, straight, clear trunks about 10 inches in diameter and bushy tops. The land is a gently rolling country with a slight northerly slope, leaf-covered ground, a sandy soil with loam on top, and an elevation above the sea of 200 to 300 meters. The height above the city level was 200 feet or so; the locality was south and west of Eberswalde station. The trees cut were scattered along a quarter of a mile and so did not represent any close grouping. Their rings show almost identical records; 2 to 10 in every decade have enough individuality to make them recognizable in every tree.
On the first examination of these sections in November 1912, it was evident that their growth follows with fidelity the sunspot curve since 1830. This may be traced in the curves below and in the accompanying photographs of two of the sections in plate 8. It will be seen at once that there is a rhythmic sway in the growth, groups of large rings alternating with small ones. The arrows placed in the photographs mark the years of maximum sunspots. Taking the group as a whole, the maximum growth comes within 0.6 year of the sunspot maximum. To one maximum alone they fail to respond, namely, 1894; instead of rising, the curve drops in 1892, 1893, and 1894. I have tried to find cause for this, but was informed by Professor Schwappach that there were no fires, pests, or other known causes for it except climatic conditions.
- Schwappach. Zeitschrift Forst- und Jagdwesen, September, 1904. A recent bulletin of the Mellon Institute, by J. F. Clavenger, entitled, "Effect of the soot in smoke on vegetation," suggests at least a possibility. Clavenger shows photographs of tree sections in the neighborhood of iron mills, in which the growth is normal until the smoke from the mills pours over the forest, and then the rings rapidly decrease in size. It would solve the puzzle if it could be shown that smoke from the iron and brass works in the neighboring city came over the forest of Eberswalde more abundantly at about this time. Dr. Schwappach writes that the manufactories and repair shops are 3 km. distant and in his opinion the decrease in growth can not be due to smoke from them.