47o POPULAR SCIENCE MONTHLY 



mical relations of earth and sun ; in the distribution of land and water, 

 in the position of the earth's axis; in the altitude of the land; in the 

 presence of volcanic dust — changes now in cosmic, now in terrestrial 

 conditions — have been suggested, combated, put forward again. None 

 of these hypotheses has prevailed in preference to others. No actual 

 proof of the correctness of this or that theory has been brought forward. 

 No general agreement has been reached. Under these conditions, and 

 in view of the fact that practical climatology is concerned with climatic 

 changes, not of the geological past but of the historical present, this 

 portion of our subject may be dismissed with this brief mention. 



Conclusion. — There is a wide-spread popular belief in permanent, 

 progressive changes of climate during a generation or two. This belief 

 is not supported by the facts of meteorological record. Abundant evi- 

 dence has been adduced in favor of secular changes -of climate in his- 

 torical times. Much of this is unreliable, contradictory, and has been 

 interpreted without sufficient regard to possible controls other than 

 climatic change. Without denying the possibility, or even the prob- 

 ability, of the establishment of the fact of secular changes, there is as 

 yet no sufficient warrant for believing in considerable permanent 

 changes over large areas. Dufour, after a thorough study of all avail- 

 able evidence, has concluded that a change of climate has not been 

 proved. There are periodic oscillations of slight amount. An eleven- 

 year period has been made out, with more or less certainty, for some 

 of the meteorological elements, but it has been of no practical im- 

 portance as yet. A thirty- five year .period is less uncertain, but is 

 nevertheless of considerable irregularity, and can not as yet be prac- 

 tically applied in forecasting. Longer periods are suggested, but not 

 made out. As to causes, variations in solar activity ' are naturally 

 receiving attention, and the results thus far are promising. But climate 

 is a great complex, and complete and satisfactory explanations of all 

 the facts will be difficult, perhaps impossible, to reach. At present, 

 indeed, the facts which call for explanation are still in most cases but 

 poorly determined, and the processes at work are insufficiently under- 

 stood. Climate is not absolutely a constant. The pendulum swings 

 to the right, and to the left. And its swing is as far to the right as 

 to the left. Each generation lives through a part of one, or two, or 

 even three, oscillations. A snap-shot view of these oscillations makes 

 them seem permanent. As Supan has well said, it was formerly 

 believed that climate changes locally, but progressively and perma- 

 nently. It is now believed that oscillations of climate are limited in 

 time, but occur over wide areas. Finally, it is clear that man, whether 

 by reforestation or deforestation, by flooding a desert or by draining a 

 swamp, can produce no important or extended modifications of natural 

 climate, which is governed by factors beyond human control. 



