48 MICHIGAN SURVEY,. 1908. 



winds, the rotation of /the earth, the form and contour of the basin, 

 and the position of the outlet. A detailed investigation of these cur- 

 rents was made by Harrin^on and Conger (Harrington, '95) who 

 paid particular attention to the currents about Isle Eoyale. As these 

 investigations were made during the season of navigation, they are of 

 particular interest from the standpoint of the biota, because it is dur- 

 ing this same period that we must in general expect the most advantage- 

 ous dispersal of plants and animals to take place. 



The simplest of these factors influencing currents are: the general 

 movement toward the outlet of a lake, the prevailing westerly winds, 

 the deflection to the right (or southward) of the current on account of 

 the rotation of the earth. But the general form of the lake and its shore 

 line, the contour of the bottom and the location of islands, introduce 

 important complexities into the problem. As may be seen in Fig. 53 

 Lake Superior well illustrates the influences of all these conditions. The 

 small size of the outlet does not allow the escape of this vast current, 

 so that there is a return along the north shore, where islands are 

 encountered which produce eddies ; and in their shallow water and along 

 their coasts breakers are encountered which tend to carry shoreward and 

 lodge drift. 



When the return swirl reaches Isle Eoyale the problem becomes com- 

 plex and is of such importance that these currents were made the sub- 

 ject of a special investigation by Harrington and Conger. In their study 

 of the lake currents, bottles containing instructions were sent adrift and 

 the finder was requested to communicate their recovery to the Weather 

 Bureau. In this manner, supplemented by other sources of information, 

 these currents were determined. The results of the investigations 

 about Isle Royale are as follows: 



"Not a single bottle has been recovered on the northwest coast of 

 Lake Superior. This is not due to no bottles having been floated in that 

 vicinity, as during the season of 1893 alone Mr. Conger floated 250 bot- 

 tles between Duluth, Minn., and Thunder Bay, Out. 



"This fact was deemed of such importance that the Chief of the 

 Bui'eau, accompanied by the inspector in charge of the Lake Marine 

 Service, made a special trip from Duluth, Minn., along the northwest 

 coast around Isle Royale to Port Arthur, Out. Careful note was made 

 of the entire coast, all beaches examined, and observations of water 

 temperature made to assist in solving the direction of the current flow 

 in this region. At French River, observations were made with special 

 current floats, and it was discovered that the main current was to the 

 northeast from 1 to 2 miles from shore. Inside this line was found a 

 ■current flowing to the westward. This shore current evidently^ begins 

 farther to the east, and continues to the west end of the lake, and is 

 positive at or near Duluth, as is conflrmed by investigation of the offi- 

 cials of the city of Duluth, however, narrow and does not extend far 

 into the lake. 



"Around Isle Eoyale there was found abundant evidence that the cur- 

 rent flows to the west along the north shore of this island. Observa- 

 tions of water temperature at this point are very interesting and indi- 

 cate a deep stream flowing from the eastward. There appears but little 

 ■difference in the temperature of the water at the surface and at the 



