Page:Popular Science Monthly Volume 22.djvu/218
THE POPULAR SCIENCE MONTHLY.
the several seasons and the growth made during the same, I selected from my meteorological records the maximum, minimum, and mean temperature, and the rain-fall, of the six growing months of spring and summer of each of the twelve years of growth. These extracts I have tabulated, and have also appended to each season the thickness of the ring formed, as measured on the oblique cut previously described.
An examination of this table shows a general relation of cause and effect between high temperature and large rain-fall, and greater growth. But it falls very far short of proving a general law of "so much heat and so much water during the growing season, to produce so much wood." For example, compare the years 1875 and 1878. The temperature of 1878 for the season is better than 4° in excess of the season of 1875, and the rain-fall only a little over four inches less; and yet the growth of 1875 is seven times what it was in 1878. This almost unparalleled growth of 1875—that is, as compared with the other years—can not be explained by the above general law. But I think the May and June record of that year throws light upon it. We see there a maximum heat in May of 96° (higher than I have ever known it in an observation and record of twenty-five years), and a mean temperature of the whole month, also unequaled, of 71°; and this great heat continued through the month of June, and no cold spells after the heat set in sufficient to check the growth. Then, in connection with this heat, the ground was well saturated with water when this heated term began (May 6th) by 1·62 inch of rain on the 4th. From this on, to the 26th of June, 15 inches more of rain fell, so apportioned over the time as to keep the ground saturated. This synchronous excess of heat and water evidently produced the abnormal growth. And probably, as this matter is further studied, it will be found that these agents, rightly proportioned, operating synchronously, produce these thicker rings; while as one or the other is in excess, or absent, the growth is checked, and thus has time to condense and harden, and form these sub-rings; and the more frequent these alternations, the greater the number of them.
By C. WILLIAM SIEMENS, F.R.S.