FOREST INFLUENCES. 313 



arrived at the following results: A number of the most approved instruments placed side by side 

 and read at the same time of day gave readings differing by as high as 1.6° F., and usually by as 

 much as 0.7° F., thus indicating clearly that such differences of temperature as had hitherto been 

 considered real or valid differences were possibly nothing more than inaccuracies or insufficiencies 

 of observation or due to nonuniformity of conditions. Nevertheless, having thus ascertained the 

 difficulties and errors of instruments, Hoppe proceeded to investigate the influence of soil covers 

 and found that even over the sod of a poor meadow the temperature is lower and the humidity 

 greater than over a piece of rocky bare land, temperature and humidity being measured by the 

 same instruments in both cases. He finds that this is still more constant and pronounced when 

 forest and bare land are compared. The differences were small, however, the average of his results 

 for sixty-six days being a difference in temperature of 3.2° F., and in relative humidity of 7 per 

 cent. His results would seem to indicate a great uniformity of difference, and that the differences 

 in temperature and humidity are nearly as great at night as during the day. A point of great 

 interest is also brought out prominently by these experiments, namely, the need of a large 

 number of observations. Thus, Hoppe found that the same instrument (an Assmann aspiration 

 psychrometer) varied from minute to minute often with the slightest changes in cloudiness, so 

 that during noonday and in one minute the relative humidity fell from 47.4 per cent to 41.2 per 

 cent, and the temperature rose from 73.5 to 75° F., and within five minutes the humidity rose from 

 43.8 to 50.9, fell to 48.8 and rose again to 52.2. 



WIND-BREAK EFFECTS. 



Prof. F. W. King, of the University of Wisconsin, has made an investigation into the protec- 

 tion afforded by wind breaks, and records his observations in Bulletin 42 of that institution. The 

 following extracts show the general character of his observations : 



Lying to the eastward of a field of clover, seeded last year, is a piece of oats seeded to clover, and here the 

 catch is very much better close to the grass, and is evidently so as far out in the field as 2 rods. 



A north-and-south load, fenced with wire and 2 rods wide, has scattering trees from 10 to 18 feet high, together 

 with a scanty growth of ha/el on both sides. To the east of this is a field of oats badly damaged by the winds at a 

 distance from the shelter, but a strip 2 rods wide adjoining that seems wholly to have escaped injury. 



A le\el field seeded to clover and timothy last year is bounded on the north by a road and a stiip of woods. 

 Here the cIoa er has a much thicker stand and ranker growth in a belt alongside than it has to the southward. 



Coming next to a field of oats some 60 rods from east to west and 30 rods north to south, lying east of a piece of 

 woods, we find its whole eastern two-thirds so completely ruined that it is scarcely more than a naked field, while 

 the western third is fresh and green. 



Another piece of oats seeded to clover, and lying on the south side of a wooded pasture, has a length of 80 rods 

 from east to west, but a width of only 15 rods. This field is fresh and green throughout its whole extent and has a 

 good catch of clover, but the patch is best and thickest in the strip 3 rods wide along the wooded pasture. 



Influence of woods on the rate of evaporation to the leeward. — To study the rate evaporation at different distances 

 from groves, six o vapor lmeters were used made after the plan of the Piche evaporimeter, but with the evaporating 

 surlace much larger, while the giaduated tubes were the same size, the object being to make the instruments 

 more sensitive. 



Sheets of chemical filter paper were used as the evaporating surfaces, all fiom the same packages and having 

 a diameter of 5.9 inches,* this gives an area, after deducting that covered by the graduated tube, of 27.06 square 

 inches. 



The first experiment was made to the northwest of Plainfield on a piece of ground planted to corn, lying to the 



south of a grove of black oaks having a mean height not far fiom 12 to 15 feet. At the time there was a gentle 



breeze from a little west of north. The instruments were all suspended at a height of 1 foot above the surface of 



the ground and unsheltered in any way from the wind or sun, and in the first tiial they were placed at intervals 



of 20 feet along a line at right angles to the south margin of the woods. The amount of evaporation at the six 



stations between 11.30 a. m. and 12.30 p. in., is given in the following tables: 



Cubic 



centimeters 



At Station A, 20 feet from woods, the evaporation was * 11.5 



At Station B, 40 feet from woods, the evaporation was < 11. 6 



At Station C, 60 feet from woods, the evaporation was 11.9 



Sum 35.0 



At Station D, 280 feet fiom woods, the evaporation was - 15.5 



At Station E, 300 feet from woods, the evaporation was * 14.2 



At Station F, 320 feet from woods, the evaporation was * , 14.7 



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