302 



NA TURE 



[January 25, 1894 



like Japan. Near the origin of the dislocation the shaking 

 brings down forests from the mountain-sides, and the neighbour- 

 ing district is devastated. As the waves spread they become 

 less and less violent until, after radiating a few hundred miles, 

 they are no longer appreciable to our senses. But the earth- 

 quake has not ended. As long, flat, easy undulations it con- 

 tinues on until it has spread over the whole surface of our globe. 

 The waves passing under Asia and Europe reach England first, 

 while those crossing the meridian of our Antipodes and North 

 America arrive somewhat later. At Potsdam, Wilhelmshaven, 

 and in Japan, waves of this order have often been recorded, but 

 for the rest of the world they are thus far unrecognised. Great 

 cities like London and New York are often rocked gently to 

 and fro ; but these world-wide movements, which may be 

 utilised in connection with the determination of physical con- 

 stants relating to the rigidity of our planet's crust, because they 

 are so gentle, have escaped attention. 



That the earth is breathing, that the tall buildings upon its 

 surface are continually being moved to and fro, like the 

 masts of ships upon an ocean, are at present facts which have 

 received but little recognition. Spasmodic movements which 

 ruin cities attract attention for the moment, but when the dead 

 are buried, and the survivors have rebuilt their homes, all is 

 soon forgotten. It seems desirable that more should be done 

 to advance our knowledge of the exact nature of all earth-move- 

 ments, by establishing seismological observatories, or at least 

 •preventing those in existence from sinking to decay. 



J. Milne. 



THE CLIMATIC AND NATIONAL-ECONOMIC 



INFLUENCE OF FORESTS. i 



TT is to German scientific men that we owe the first steps taken 1 



in order to ascertain data concerning the actual climatic 

 effects of forests. Since then, however, most civilised countries, j 

 except Britain, have been actively engaged in the collection of j 

 accurate data concerning this very important subject. So far as . 

 those data have yet been collated and compared they lead to 

 the following results. 



It was not until the year 1867 that exact scientific observa- 

 vations were undertaken on an extensive scale to determine the 

 actual influence which forests have in modifying the temperature 

 of the air and of the soil within their own areas and over the sur- 

 rounding tracts of country, and the first results were published in 

 Ebermayers celebrated work. Die pliysikalischeti Eimvirkitngen 

 des IValdes aiif Liiftc mid Boden, 1873. 



I. As regJ.rds Atmospheric Temperature. — The average results 

 of observations made during ten years (1876-85) throughout 

 nearly the whole of Germany, and in parts of France and 

 Switzerland, in different kinds of forest, at heights above the 

 sea-level varying from 10 to 3C00 feet, and at latitudes varying 

 from 472" to 55i^ prove conclusively that in general the annual 

 average temperature within forests growing in closed canopy is 

 lower than in the open, although the crowns of the trees are on 

 the whole a little warmer in winter. The difference is greatest 

 in summer, least in winter, and about midway between these 

 extremes in spring and autumn ; the mean annual difference, 

 however, seldom amounts to over i°Fahr. near the ground, and 

 is scarcely V in the crowns. The prevention of insolation of 

 the soil during the long hot days of summer, and the rapid 

 transpiration taking place through the foliage, exert a greater 

 influence on the atmospheric temperature than can be ascribed 

 to shelter from wind and to decrease of nocturnal radiation. 



The ohservatioas recorded prove (i) that the variations 

 between the temperatures of the trees themselves and the air in 

 the open exceed those between the woodland air and the latter 

 except during winter, (2) that they are largest during the most 

 active period of vegetation in summer, and (3) that they are 

 greater in spring, when the circulation of sap begins, than 

 during the autumn months, when vitality becomes sluggish and 

 dormant. 



In the crown of the trees, where insolation by day and 

 radiation by night make their full influence felt, the difference 

 in the daily average over the whole year is less than it is near 

 the ground. In winter it averages little either above or below 

 o' , and in summer usually about the half of the reading at 5 ft. 

 above the ground. 



Observations made in Southern Germany establish the fact 

 that in the forests it is cooler during the day and warmer 

 during the night than in the open. 



During the night the trees interfere with the radiation of 

 heat, and in the day-time the shade afforded by the crowns 

 keeps the air from being rapidly warmed by the sun's rays. 

 These influences are naturally strongest during spring, summer, 

 and autumn, when foliage is most abundant, whilst in winter 

 the coniferous forests with evergreen foliage are milder than 

 deciduous forests. 



Owing to these differences in temperature, beneficial currents 

 of air are induced between the forests and the open country, 

 which follow the same law as obtains in regard to land and sea 

 breezes. During the day the cooler and moister air of the 

 forest sets outwards to take the place of the heated air ascending 

 in the open ; at night the current sets in from the open, cooled 

 by radiation, towards the forest. 



The statistics, upon which these deductions are based, prove 

 that the immediate action of forests is to modify the daily 

 maxima and minima of atmospheric temperature, whence 

 it may be deduced that a comparison of the absolute ex- 

 tremes of temperature during the year must exhibit definitely 

 the sum total of the influence exerted by forests on the 

 temperature of the atmosphere. This modification of the 

 extremes of temperature, which are bad alike for man and 

 beast, and also for agricultural operations, is of immense 

 importance from a national-economic point of view, since many 

 places that were once fertile are now little better than barren 

 wastes in consequence of the reckless denudation of forest. 



In registering the data, however, it was observed that the 

 geographical position, and the exposure of the forests to winds, 

 exerted a certain amount of modifying influence in lessening 

 the differences, and there are reasons to believe that towards the 

 crown the forest temperature in winter is considerably 

 higher than down nearer the ground. It was found, too, 

 that certain forest trees exerted greater influence than 

 others in consequence of the density of their foliage ; 

 for beech forests in summer exert, through their dense foliage 

 and complete canopy, a considerably greater influence in 

 diminishing the extremes of temperature than forests of spruce 

 or Scots pine, although after defoliation their influence is merely 

 similar to that of the pine forest, and only half so great as that 

 of the more densely foliaged spruce. 



2. As regards Soil-Temperatitre. — The influence exerted on 

 the soil temperature by forests growing in close canopy is of 

 considerable importance, especially with regard to the soil- 

 moisture. The observations made concerning this point seem 

 to make it clear that the mean annual temperature of the soil 

 in the forest is at all the above depths of observation cooler 

 than in the open, and that the differences are greatest in 

 summer, about the mean in spring and autumn, and very small 

 in winter. In countries with warm summers this reduction of 

 the soil-temperature over large areas by means of forest growth 

 has a decidedly beneficial result. . According to observations 

 made in Wiirtemberg, the difference between the maxima of 

 soil-temperature in forests and in the open can extend so far as 

 up to 14° Fahr. 



It was also found that the daily differences in soil-temperature 

 varied according to the season of the year, but that throughout 

 nearly the whole year the upper layers of soil in the open were 

 warmer in the afternoon than in the forenoon, whereas in the 

 forest the variations were inconsiderable. 



As with regard to the atmospheric temperature, the influence 

 of the forest trees in equalising the soil-temperature throughout 

 the year is greatest in the case of trees whose foliage is densest, 

 spruce heading the list. 



3. As regards the Degree of Atmospheric Humidity. — Ob- 

 servations recorded throughout Central Germany show that as 

 regards the absolute humidity of the air forests have no ap- 

 preciable climatic effect, for the annual averages showed merely 

 slight traces of differences at 5 feet above the soil. 



The differences between the relative humidity oj the air in forests 

 and in the open are, as might be expected, greatest in summer, 

 although very different results as regard variations are obtained 

 with changes of altitude and of other physical conditions. 



The results of the various series of observations, corrected so 

 as to eliminate, so far as possible, local differences due to 

 altitude and other physical dissimilarities in the various meteoro- 

 logical stations, show that the mean annual relative humidity 01 

 woodland air is from 32 to 10 per cent, greater than that of air 

 in the open, but that the difference varies greatly according to 

 the season of the year, being greatest in summer and autumn, 

 and least in winter and spring. They show, too, that large 



NO. 1265, VOL. 49] 



