5l6 REPORT OF THE COMMISSIONERS OF 



proposition that there is a direct relation between mean temperature and run-off is 

 correct, so marked a relation indeed that a difference of one de.ijree in mean 

 temperature will make five per cent, difference in the annual run-off for a given 

 drainage area, it follows, with the certainty of a proposition in geometry, that forests 

 must have a marked effect on the run-off. Moreover, the forest meteorological 

 observations have further shown that evaporation, not only from water surfaces but 

 from bare as well as littered soils, is very much less in thick forests than in the open. 

 The effect of the forest is here exceedingly marked. Indeed, it has been shown that as 

 a mean of the months from April to October, inclusive, evaporation from soil under 

 forest litter within the forest is only I 3 per cent, of what it is from a water surface in 

 the open, whereas evaporation from the bare soil in the open is 93 per cent, of what it 

 is from a water surface in the open. Again, a water surface in the woods, in localities 

 where the water is fairly surrounded by heavy timber, gives only 36 per cent, of the 

 evaporation, from April to (3ctober, inclusix'e, that occurs on a similar water surface in 

 the open. Mr. Vermeule and the gentlemen championing the other view may ignore 

 data of this character if they w'ish to, but I frankly say I cannot afford to ignore 

 such data. 



As further data which cannot be safely ignored we may refer to some of the 

 results obtained by Mr. FitzGerald in his evaporation experiments at Boston. In 

 determining winter evaporation Mr. FitzGerald found by experimenting upon blocks 

 of ice, that, when the blocks were so placed as to be subject to air currents, the loss 

 was much greater than when exposed to the same temperature in still air. These 

 results appeared even in zero temperature. We may apply them directly to a 

 drainage area, where, when the same is covered with primeval forest, the force 

 of the wind is so much broken as to make a distinct difference in the wasting away of 

 snow during the winter. As illustrating the possible magnitude of such loss, we may 

 cite that on one occasion Mr. F"itzGerald found evaporation from an ice surface, with 

 the wind at twelve miles an hour, proceeding at the rate of 0.2 inches per day. A 

 number of experiments were made as to the evaporation from ice exposed to wind in 

 comparison with evaporation from the same substance when protected in a covered 

 and cold shed through and into which the wind could not enter, with the result of 

 showing that the wind was an exceedingly important factor in winter evaporation, 

 and that, therefore, whenever we do anything which tends to give the wind free 

 access to the snow covering on a drainage area, we have done that which will lead to 

 a large loss of water from the area. It is facts of this character which emphasize the 

 injury done to streams by extensive deforestation. 



As to the relation of the foregoing facts to flood flows we may point out that there 

 is a certain balancing of conditions. Thus, on a deforested area we may expect the 



