CLIMATE OF THE LAKE REGION. 379 



of coincidence is reduced to harmony. It will also appear that the 

 rainfall extremes are not only followed invariably by corresponding 

 fluctuations in the water-levels, but that these succeed each other in 

 quite as marked and uniform periods. 



The rainfall maxima of 1836, '44, '55, '68, and '80 have their corre- 

 sponding extremes in the water maxima of 1838, '47, '58, '70, and '82 

 — the intervals or lag varying from two to three years. The rainfall 

 minima of 1839, '50, 'GO, '72, and '86 have corresponding lake minima 

 in 1841, '53, '65, and '75 — the intervals varying from two to five years. 

 The mean lag is 2*9 years. The true relation — dependence — of the 

 lake periodicities upon those of the rainfall is thus clearly shown. 



It will be observed that I have chosen to consider the Lake Erie 

 levels rather than those of Detroit River. I do so for the reason that 

 the relations of the former to the precipitation are more simple and di- 

 rect, and are not influenced by causes already pointed out (page 375), 

 which tend to create irregularities in the river. A marked illustration 

 is shown between the years 1859 and 1870 — where dotted lines repre- 

 sent the rainfall at Milwaukee, and the river-levels as compared with 

 those of Lake Erie — of the effect of excess of precipitation on the lakes 

 above, in keeping up the river to a disproportionate extent. 



I do not consider it necessary to examine the various theories which 

 have been broached from time to time, in explanation of the lake peri- 

 odical fluctuations. Nor will I undertake to explain all the irregulari- 

 ties of the river and lake, which would demand many factors that are 

 wanting to the present discussion. It will sufiice if I have succeeded 

 in making clear the relations which exist between the variations of the 

 water-levels and the rainfall, and in defining their periodicities. Prob- 

 ably few at this day would dispute the fact that the rise and fall, or 

 " secular " variations, in the waters are dependent upon the rainfall. 

 This is the first attempt, to my knowledge, at demonstration of their 

 true relations. 



Thus far I have not alluded to the important element of Tempera- 

 ture in its relation to rainfall. That an intimate relation exists is 

 an admitted fact ; it shall be my endeavor to show what this rela- 

 tion is. 



In the portion of the diagram devoted to the Detroit temperature 

 curve, the horizontal lines represent the degrees of mean annual tem- 

 perature, which varies from 42°, the lowest, to 52*^', the highest ex- 

 treme. Considering temperature as a controlling element, we should 

 expect to find a close correspondence between its curves and those of 

 the rainfall. And we do so find, as is shown by the diagram. But, 

 while the maxima and minima of the rainfall and the lake are directly 

 as each other, we discover that those of the rainfall and the tempera- 

 ture are inverse to each other. For a full discussion of the relation 

 between these two elements, no doubt we ought to take into account 



