GENEEAL EEPOET. 37 



Of course, the question as to how long we must wait for these trees to 

 develop into timber is another element of the problem, and this will vary 

 with the particular species of tree. In California, we might expect the blue 

 gum would in five years be large enough to use as fuel and as fencing, but 

 we could hardly expect it to have sufficient girth to answer most commer- 

 cial purposes inside of thirty years. Emerson, in his " Trees and Shrubs 

 Growing Naturally in the Forests of Massachusetts", instances a white pine 

 planted near Paris thirty years before, that had attained a diameter of 3 

 feet and was 80 feet high. Marsh, in "Man and Nature", p. 274, tells of 

 another that in thirty-six years had grown to 25 inches in diameter. The 

 Ailanthus, American elm, and chestnut may be given as illustrations of 

 rapid growth. Either of these would in thirty years produce good timber. 



The Nevada and Utah districts have been so thoroughly elaborated by 

 Mr. Watson in Vol. V. of King's Reports, that no further statements con- 

 cerning them are requisite. There is also an able article by Dr. W. J. 

 Hoffman on the distribution of vegetation in portions of Nevada and Ari- 

 zona, in the American Naturalist for June, 1877. Dr. Hoffman's former 

 connection with this Survey, his opportunities for observation, and his zeal 

 in botany, confer a special value on his paper. 



I am indebted to my friend Dr. George Martin, of West Chester, Pa , 

 for the following important note : 



Mean annual rainfall in Philadelphia for 43 years, i. e., from 1825 to 1867, 



inclusive 44.02 inches. 



Thus, mean for the first series, 21J years, of this period 42.12 inches. 



For the second series of the above period 45.86 inches. 



Taken from Smithsonian Tables (May, 1872). 



Mean annual rainfall in West Chester, Pa., for 18 years, i. e., from 1860 to 



1877, inclusive 51.18 inches. 



Thus, mean for the first series of 9 years of above period 50.16 inches. 



For the second series of same period 52.20 inches. 



Taken from Eegister of Dr. Jesse C. Green. 



All will admit that a large body of woodland has been cleared during these periods 

 in the vicinity of both these stations, and yet there has been an increase in precipitation. 



It will not do to compare the observations taken at West Chester with those of 

 Philadelphia unless the whole series could be taken, as the local influences and errors 

 of instruments have not been eliminated. Each, however, is complete and conclusive 

 in itself. 



