end and a projection at the other for holding in a common brace. A chain drill attachment is also provided to help in forcing the drill into the wood. The difficulty with this borer is the disposal of sawdust and the extraction of the core. For the former, a separate hole is bored with a common auger just below the core (if in an upright tree) and in advance of it to catch the sawdust. The core is broken off every three inches and pulled out to make more room for the sawdust. To extract the core a small steel rod is provided with a wedge at one end and a screw at the other. One- and two-foot tubes are carried so that it is possible to reach the centers of most pine trees. It would not be difficult to develop an instrument much more efficient than this and it should be done. Soon a borer will be needed to pass through a 35-foot tree or to sound the depths of the great Tule trees of southern Mexico.

The tools just mentioned are technical, yet in no sense complex. A measuring instrument has just been completed whose usefulness will be extensive and whose details of construction are too complex for present description. It is for measuring the width of rings. It makes a record as fast as one can set a micrometer thread on successive rings. The record is in the form of a plot drawn in ink to scale on coordinate paper so that the values can be read off from it at once for tabulation. This form of record was desired because individual plots have long been made to help in selecting the best trees and in studying their relation to topography. The instrument as constructed magnifies 20, 40 or 100 times, as desired. It can be attached to the end of an astronomical telescope and used as a recording micrometer capable of making a hundred or more settings before reading the values. It seems possible that it will have other applications than the ones here mentioned.

Another instrument of entirely different type has been developed here since 1913. Its general principle has been published and will not be repeated, but in the last three years it has been entirely rebuilt in a more convenient form through the generosity of Mr. Clarence G. White of Redlands. This instrument is now known as the White Periodograph. It could be called a cycloscope or cyclograph. Its purpose is to detect cycles or periods in any plotted curve. It differs from previous instruments performing harmonic analysis in that it is designed primarily to untangle a complex mixture of fairly pronounced periods while others determine the constants of a series of harmonic components. For example, the periodograph can be applied to a series of rainfall records to find if there are any real periods operating in a confused mixture. It is also designed to get rid of personal equation and to get results quickly. The instrument as reconstructed is far more convenient

and accurate in use and has already given important results. It enables one to see characteristics in tree growth variation which are not visible to the unaided eye. It is specially arranged now to give what I have called the differential pattern or cyclogram because this pattern not only tells the periods or cycles when properly read but shows the variations and interferences of cycles and possible alternative readings. Tests on the accuracy of solutions by this instrument show that its results correspond in precision to least square solutions.

VI. CORRELATIONS

It is no surprise that variations in climate can be read in the growth rings of trees, for the tree ring itself is a climatic product. It is an effect of seasons. The geologists use the absence of rings in certain primitive trees as an indication that no seasons existed in certain early times. Whatever may have been the cause of that absence, we recognize that the ring is caused primarily by changes in temperature and moisture. Now if successive years were exactly alike, the rings would be all of the same size with some alteration with age and injury. But successive years are not alike and in that difference there may be some factor which appeals strongly to the tree. In northern Arizona, with its limited moisture and great freedom from pests and with no dense vegetable population, this controlling factor may reasonably be identified as the rainfall. If the trees have all the moisture they can use, as in north Europe about the Baltic Sea and other wet climates, we look for it in something else. It could be-I do not say that it is-some direct form of solar radiation. It could be some special combination of the ordinary weather elements with which we are familiar. Shreve has studied this phase in the Catalinas. If the abundance of moisture is so great as actually to drown the tree, then decrease in rainfall which lowers the water table below ground will be favorable. A fact often forgotten is that more than one factor may enter into the tree rings at the same time, for example, rainfall, temperature and length of growing season. These may be isolated in two ways. We may select a special region, as northern Arizona, where nature has standardized the conditions, leaving one of them, the rainfall, of especial importance. Or we may isolate certain relationships. as in any other investigation, by using large numbers of observations, that is, many trees, and averaging them with respect to one or another characteristic. For example, I can determine the mean growth curve of the Vermont hemlocks and then compare it separately with rainfall and solar activity, and I may, and do, find a response to each. For that reason, I have felt quite justified in seeking first the correlation with moisture. A temperature cor

relation doubtless exists and in fact has been noted, but its less minute observance does not lessen the value of the rainfall relationship.

The first real result obtained in this study was in 1906 when it became apparent that a smoothed curve of tree growth in northern Arizona matched a smoothed curve of precipitation in southern California since 1860. That degree of correlation is now extensively used in the Forest Service. This was followed almost at once by noting a strikingly close agreement between the size of individual rings and the rainfall for the corresponding years since 1898, when the Flagstaff weather station was established. The more detailed comparison between rainfall and ring growth was made with Prescott trees in 1911. Some 67 trees in five groups within ten miles of Prescott were compared with the rainfall at Whipple Barracks and Prescott which had been kept on record since 1867. The result was very interesting. For most years the tree variations agree almost exactly with the rainfall but here and there is a year or two of disagreement. The cause of these variable years will sometime be an interesting matter of study. Taken altogether the accuracy of the tree as a rain-gauge was 70 per cent. But a little allowance for conservation of moisture raised the accuracy to 85 per cent., which is remarkably good. The actual character of this conservation is not evident. At first thought it might be persistence of moisture in the ground, but the character of the mathematical formula which evaluated it allowed a different interpretation, namely, that in a series of poor years the vital activity of the tree is lessened. During the dry period from about 1870 to 1905 or so, the trees responded each year to the fluctuations in rainfall but with less and less spirit. This lessening activity took place at a certain rate which the meteorologists call the "accumulated moisture" curve. This suggested that the conservation was in the tree itself. There is much to be done in this comparison between tree growth and rainfall, but the obstacle everywhere is the lack of rainfall records near the trees and over adequate periods of time. The five Prescott groups showed that in a mountainous country nearness was very important. But the nearest records to the sequoias are 65 miles away and at 5,000 feet lower elevation. The best comparison records for the Oregon Douglas spruce are 25 miles away. It is so nearly everywhere. The real tests must be made with records nearby.

In 1912 while attempting to test this relationship of tree growth to rainfall in north Europe, I found that the Scotch pines south of the Baltic Sea showed a very strong and beautiful rhythm matching exactly the sunspot cycle as far back as the trees ex

tended, which was close to a century. The same rhythm was evident in the trees of Sweden, and perhaps more conspicuous in spruce than pine. Near Christiania the pines were too variable to show it, but it reappeared on the outer Norwegian coast. the south near the Alps it disappeared, and in the south of England it was uncertain but probably there. In this country it shows prominently in Vermont and Oregon, but the two American maxima come one to three years in advance of the sunspot maxima. There is evidently an important astronomical relationship whose meaning is not yet clear. It is to be noted that it appears in regions whose trees have an abundance of moisture and it thus appears to be a wet climate phenomenon.

But the correlations do not stop at rain and sunspot periodicity. The pines of northern Arizona which are so sensitive to rainfall show a strong half sunspot period. And on testing it one finds that the rainfall does the same and that these variations are almost certainly related to corresponding temperature variations and to the solar period. Thus, the Arizona trees are related to the weather and the weather is related in a degree at least to the sun. Thus we find evidence in forest trees that the 11-year sunspot period prevails in widely different localities and in many places constitutes the major variation. This introduces us to the study of periodic effects in general.

VII. CYCLES

Considering first that cycles as we have just shown are revealed in tree growth, second, that the trees give us accurate historic records for hundreds and even thousands of years, and third, that simple cycles or even some more complex function could give a basis for long range weather forecasting, we recognize the vital importance of this elemental part of the story told by the trees. It was exactly for this purpose that the periodograph was designed and constructed and some ten score curves have been cut out for analysis, after minute preparation of the very best yearly values. In fact the major time for two years has been given to this preparation of material. It is hardly done yet, but it is far enough along to anticipate its careful study in the near future. Our present view may be profoundly modified, but it is safe to say that the sunspot cycle and its double and triple value are very general. The double value, about 22 years, has persisted in Arizona for 500 years, and in some north European localities for the century and a half covered by our tree groups. The triple period, essentially Brückner's cycle, has operated in Arizona for the last 200 years and in Norway for nearly 400 at least. A one-hundred-year cycle is very prominent throughout the 3,000 years of sequoia record and

VOL. XV-2

also in the 500 years of yellow pine. An hypothesis covering all these sunspot multiples will be tested out in the coming months. Should a real explanation be found a step will have been made toward long-range prediction and an understanding of the relationship of the weather and the sun. Other periods, however, than the multiples of the sunspot period do occur and general analysis shows that different centuries are characterized by different combinations of climatic cycles. This suggests to us a great and interesting problem. If we can establish the way in which different regions act and react at the same time, then it may become possible to determine the age of an ancient buried tree by finding the combination of short cycles its rings display and then determining when this combination or its regional equivalent existed in our historic measuring tape, the great sequoia.

VIII. PREHISTORIC RECORDS IN TREES

A new method of investigating the relative age of prehistoric ruins has been developed in connection with this study of climate by the growth of trees, and is being applied to the remarkable ruins at Aztec, in northwestern New Mexico, with its 450 rooms, now in process of excavation by the American Museum of New York City. The ceilings were built of tree trunks placed across the width of the rooms. Smaller poles were laid across these beams and covered with some kind of brush and a thick layer of earth. The beams used in this ceiling construction are almost entirely of yellow pine or spruce and for the most part are in good condition. Many of the rooms have been hermetically sealed for centuries. The beams which have been buried in dust or adobe or in sealed rooms are well preserved. Only those which have been exposed to the air are decayed.

In 1915 Dr. Clark Wissler of the American Museum offered sections of such beams for special study of the rings, knowing the writer's work upon climatic effects in the rings of trees. This offer was gladly accepted, and some preliminary sections were sent at once from the Rio Grande region. These first sections showed that the pines and spruces were far better than cedars for determining climatic characteristics.

The next lot of sections came from Aztec and was cut from loose beams which had been cleared out of the rubbish heaps. Six of these sections cross-identified so perfectly that it was evident that they had been living trees at the same time. This success led to my visit to Aztec in 1919 and a close examination of this wonderful ruin. It was at once apparent that an instrument was necessary for boring into the beams to procure a complete sample of the rings from center to outside, and that the process must avoid