198 DIMINUTION OF THE WATER OF RIVERS AND STREAMS. 



above-mentioned cause. 3. In this cbauj?e in the surface-condition of 

 the earth, the amount of water precipitated, instead of being held in 

 reserve and slowly percolating, rushes suddenly into the stream, and 

 for a short time high water prevails, which is followed by a long period 

 of dryness. 



The question of the influence of forests upon the amount of precipi- 

 tation has for some time engaged the attention of naturalists. Such 

 an influence has been asserted, partly from theoretic consideration 

 and partly on account of the entire change presented by the climatic 

 relations of the countries in which the forests have disappeared. It 

 was at first supposed that the woods furnished for the rain-cloud an 

 obstacle similar to that presented by mountain-ridges to cloud-currents, 

 and condensed the watery vapor; also, that the lower temperature and 

 greater relative humidity of the woods promoted precipitation. In the 

 latter view of that subject, Dr. Berger, of Frankfort, especially made 

 some careful comparisons between the temperature and humidity of the 

 woods and that of the open fields. 



It is probable that such influence exists; but while, on the one hand, 

 its consequence may be overestimated, on the otber hand there is want 

 of direct proof, inasmuch as the rain-measurements have been continued 

 too short a time, both at stations situated within the woods and outside 

 of them in the open fields. Besides, the observations of the rain-measures 

 cannot well be compared on account of the difference in the conditions, 

 (variations in altitude, inclination of the surface toward different points 

 of the compass, &c.*) 



On account of the great practical importance of the alleged decrease 

 of the quantity of rain to the interests of national economy, the Me- 

 teorological Society of Edinburgh, in the year 1859, at the suggestion of 

 its president, the Marquis of Tweedale, offered a prize for the best in- 

 vestigation of the question whether the amount of rain in Western 

 Europe had undergone any alteration. The author of the article which 

 gained the reward, Mr. T. F. Jamieson, found in the yearly measure- 

 ments of rain-falls, to which he had access, no grounds for the appre- 

 hension of a progressive decrease in the quantity. There ai)peare(l to 

 be only local oscillations, to which it was difiicult to assign a cause. 



* Mollendoif compares, iu bis work, The Proportiou of tbo Kain-fall in Germany, 

 page 95, the amount of rain for 1856-1858 of Neidcr-Bielan, with that of Tiefenfurt, 

 (situated in the woods,) and found for the first 17 8 and for the last 21.8 par. inches; 

 but he did not himself consider this result decisive. Professor Hoffman, iu Giessen, 

 found that the rain-curves of three difi^erent situations in the woods were, for the 

 entire year, parallel with each other, which says notbiug as to tbo alleged inUuence of 

 the woods. — Journal of MeteoroJogy, vol. ii, p. 2151. 



As to the comparative observations of rain-deposits in the department of the Loire, 

 which it must be admitted included only twelve months, Becqnerel found that the 

 amount of rain precipitated in the neighborhood of forests was greater than at a dis- 

 tance from them, in the proportion of 5 ; 4, (Comptes Rendus, t. Ixiv, ji. 16.) — Journal 

 of Meteorology, vol. ii, p. 231. 



