40 FOREST INFLUENCES. 



The most elaborate series of observations of tbis kind were tbose taken 

 near Ried, in lower Austria, in 1887 and 1888. They were especially 

 devoted to the problems of liumidity, and to obtain tUorougbly satis- 

 factory results tbe ordinary wet and dry bulb tbermometer was rejected 

 and an exact chemical method substituted. This consisted in passing 

 a known volume of air through calcium chloride and phosi)horic acid 

 and ascertaining by weight the water taken up by the latter. The case 

 containing the chemicals was elevated, by means of a pulley on a mast, 

 to the height at which the observation was to be taken, and the air was 

 drawn through it by means of an aspirator below, connected with it by 

 a rubber tube. 



The observation occupied an honr and thirty minutes and the tem- 

 l^erature was taken at the beginning and end by means of an upsetting 

 thermometer, elevated by means of a pulley to the height required. 



The results were very exact for the absolute humidity, less so for the 

 relative. The heights at which the observations were taken were 4 

 inches (101.0"""), 16 (4.87'"), 36 (10.97'"), and 51 feet (15.5'"). The 

 second was under the tree crown, the third in it, and the fourth was 

 18 or 20 inches above it. The trees were red beeches, 60 to 70 years old, 

 with a few white beeches and firs. The station itself was in and over 

 the beeches. 



SOIL TEMPERATURES. 



The soil affords an excellent means of measuring the average tem- 

 perature effects of the forest. This is due in part to the fact that the 

 temperature of the air is not derived so much from the direct rays of 

 the sun as it is from the heat reflected or radiated from terrestrial 

 objects. These absorb the heat from the sun's rays more readily than 

 does the air and yield it again to the air. The soil serves in another 

 way also to average the effects of forests. The soil is more slowly 

 warmed by the sun than is the air, and the deeper the stratum the more 

 slowly it is warmed. The result is that rapid and passing changes of 

 air temperature do not affect it, or affect it only superficially. It tends 

 to show only the great periodic changes in temperature, and from it we 

 are able to ascertain Avhat influence tlie forest has on these changes. 



The results which follow are always given in terms of the difference 

 of temperature of the soil in woods and in open fields. If we put W for 

 woods or forests and for open fields, the data given hereafter* for 



*NOTR ON THE CONSTRUCTION AND READING OF THE DIAGKAMS IN THIS REVIEW. — 



The horizontal Hues (ordiuates) above or below the zero line represent values or 

 amounts, degrees of temperature, inches of precipitation or evaporation, percent- 

 ages, etc. The vertical lines (abscissae) represent time, dividing the field into 

 twelve seasonal divisions corresponding to the twelve mouths of the year, the outer 

 lines both standing for the month of December or commencement of winter. The 

 curve lines are constructed by noting on each monthly line the values found for the 

 month, and then connecting these points by either straight or rounded-oft' lines. 

 Unless otherwise noted, the values so plotted are the differouces between the read- 



