374 THE POPULAR SCIENCE MONTHLY. 



1. A possible lunar tide ; but so small and so broken in upon 

 by greater causes as to be of very uncertain value. 



2. The -winds, which often cause a difference in level of many 

 feet ; strong westerly winds causing a rise at one place, and easterly 

 winds at another. These changes are irregular and transient, but 

 often considerable in amount, ranging from two to five feet. 



3. Annual variation attendant upon the seasons and confined to 

 the year. This kind of fluctuation is a winter and summer move- 

 ment. The supply from streams and rains being wholly or partially 

 checked in the cold season, the water is gradually drawn away, low- 

 ering the general level, which reaches its lowest ebb about January or 

 P'ebruary. As spring advances, with melting snows and increased 

 rainfall, the waters rise gradually, and attain their greatest height in 

 June or July. They then begin to fall again to their winter level. 

 The extreme of this variation is about 2"30 feet, and is about the same 

 in Lake Erie as in Detroit River. 



4. A rise and fall of the waters of the lakes and their connecting 

 channels, extending through several years, and amounting to an 

 extreme difference of five feet. Upon this kind of fluctuation Col- 

 onel Charles Whittlesey has bestowed the name of " secular vari- 

 ation." 



The causes of this variation were long involved in much mystery. 

 According to the old French tradition, it is independent of the seasons, 

 and follows periodical intervals of seven years. To what extent these 

 intervals of high and low water are regular in their recurrence, and 

 how far they are connected with meteorological or astronomical 

 causes, can be determined only after continuous and exact observations 

 for a long series of years. 



It is hardly more than a decade since the United States Signal Serv- 

 ice has given scientific exactness to observations, and not over thirty 

 years since thoroughly reliable statistics have been tabulated. Records 

 of independent observers often differ widely, and though the writer has 

 culled from different sources data sutficient to enable him to construct 

 a diagram for this region, covering the past fifty years, and even 

 more, many of these data are of uncertain value. For a period of 

 thirty-three years, beginning with 18.53, a record has been kept by the 

 Detroit Water Board of the daily fluctuations in the level of the river, 

 and partial records exist of other years since 183.5. 



In a comparison between the height of water in the river and the 

 rainfall at Detroit, no conclusions drawn from these data will apply 

 rigidly to the lakes above and below. The river-levels are influenced 

 not alone by the precipitation on its borders, but by the supply from 

 above. Other causes contribute to its irregularities — local rains, con- 

 fined channel, rapid current. While a sudden increase in the precipi- 

 tation will affect the broad surfaces of the lakes uniformly, a rise 

 would take place at such times in the confined straits to a dispropor- 





