METEOROLOGY. 



535 



ally by stronger winds. The rain -tract is 

 nearly parallel to the line of progress of the 

 storm, as is also the narrow hail-tract. 



Rainfall. The results of investigations into 

 the influence of forests on rainfall are thus far 

 adverse to the supposition that, in respect to 

 the yearly average, it is material. Bat the 

 records are still too limited and imperfect, and 

 the conditions are too complicated, to allow a 

 final conclusion to be drawn. Prof. George F. 

 Swain describes the present condition of the 

 question as one in which the assumption is not 

 proved, but observes that practically, in consid- 

 ering the effects of a removal of the woods, we 

 Lave not to compare regions where they are 

 replaced by bare ground, but by growths of un- 

 derbrush, second growths, or fields of grain or 

 grass, the influence of which, as regards rainfall, 

 may be like that of the forest itself. A gradual 

 increase in the tillable quality of the soil and in 

 capacity to hold moisture in the plain regions 

 west of the Mississippi river, which has been 

 observed since the lands came under cultiva- 

 tion, has been assumed to indicate an increase 

 of rainfall. Thtf most direct evidence on the 

 subject should be sought in the meteorological 

 records, where they have been kept, at mili- 

 tary posts. Some of these extend for many 

 years back previous to the settlement of the 

 country they having been kept at Fort Leav- 

 en worth and Leaven worth City, for instance, 

 since 1837. Prof. Harrington has concluded, 

 from the comparison of the rainfall charts, 

 based on the recent Signal Service observa- 

 tions, with the charts contained in Blodgett's 

 " Climatology of the United States," that the 

 isohyetal lines have advanced westward over 

 the plains. Gen. Greely has expressed the 

 opinion that the rainfall has increased in this 

 region ; while, on the other hand, it is claimed 

 that the records at certain military posts within 

 it or near it, going back in some cases as far 

 as 1847, show that there has been no increase. 

 A comparison of the records kept at Fort 

 Leavenworth from 1837 to 1873, with those of 

 the Signal Service at Leavenworth City since 

 1873, shows an apparent average increase of 

 seven inches during the past twenty years ; 

 but the observations, having been made upon 

 different systems, are hardly commensurable. 

 Mr. Henry Gannett has compared the obser- 

 vations from twenty-six stations, covering a 

 large part of the region in question, for periods 

 ranging from six to twenty-six years, and giv- 

 ing a total of three hundred and ten years of 

 record. Cutting the series for each station in 

 the middle, he has added the earlier halves 

 and the later halves separately, assuming that 

 the totals would represent respectively the rain- 

 fall of an earlier and later term. The footings 

 thus made show an apparent increase of sixty 

 inches, or a mean of 0-4 inch per year for the 

 later term a difference that is hardly appre- 

 ciable. Other series of observations compared 

 by Mr. Gannett in the prairie regions of the 

 central States, where the forest area has been 



'.erably increased, running from ten to 

 forty years back; in Ohio, where most of the 

 forests have been removed, ten to forty-eight 

 years; and in New England, where the forests 

 having been removed, have been restored o\vr 

 nearly half of the territory concur in indi- 

 cating that if there be any difference in the 

 amount of rainfall, as affected by the forests, 

 it is too slight to be of material importance. 



H. F. Blanford's observations in India in- 

 dicate that the forests, particularly in the hot 

 zone, promote an increase of rainfall. 



The observations of Studinka upon the effects 

 of altitude and other conditions, made at seven 

 hundred stations in Bohemia, in which the 

 amount of rain that shonld be expected at 

 each step of altitude has been computed, ap- 

 parently point to an excess over the theoretical 

 amount in the neighborhood of densely wooded 

 regions. In Australia, where the soil in the 

 forests is bare and hard in dry weather, it has 

 been observed by Lendenfeld that the cutting 

 down of the trees is followed by a growth of 

 permanent grass that holds the water and 

 renders the soil permeable, and by an increase 

 of humidity in the air. 



"While the average yearly amount of rainfall 

 may not be visibly affected by the presence or 

 absence of forests, there is still room for the 

 inquiry whether the distribution of precipita- 

 tion through the year may not be affected by 

 it. Forests tend to equalize the temperature, 

 making the air cool and moist in summer, and 

 warding off extremes of cold in winter. "What 

 influence this fact may exert upon the relative 

 amounts of rain in summer and winter remains 

 to be investigated. Some light is thrown on 

 this subject by Hann's comparisons of the 

 mean temperature of the environs of Vienna, 

 in the open country, with that of the forest 

 station of Hadersdorf, in the Wiener WaM. 

 They indicate that the temperature in the 

 Wold is very sensibly lower than in the open 

 country around the forest. The difference is, 

 in January, 0< 5 C. ; in April, 0< 9; in July, 

 l-3; in October, 0'8; and for the whole 

 year, 0- 9. The influence of the forest is 

 therefore at its minimum in winter, and at its 

 maximum in summer. In the daily course, the 

 cooling effect of the forest is at the maximum 

 in the evening and early morning, and at the 

 minimum during the warmer hours. 



According to W. C. Dolberck's comparisons, 

 the rainfall on Victoria Peak, Hong-Kong, for 

 the past ten years, exceeds the record at the 

 observatory, about 1,700 feet below it, by 

 about one sixth. The fact seems to be the re- 

 sult of the mountain presenting an obstacle 

 to the wind from whatever side it blows, in 

 consequence of which the air is forced to rise, 

 and, being thereby cooled, precipitates more 

 moisture. Even when the air is moderately 

 dry at sea-level, its temperature may, in rising, 

 be brought below the dew-point. The com- 

 paratively greater rainfall in hilly districts 

 may be similarly accounted for. 



