214 SPARKS FROM A GEOLOGIST'S I^MMER. 



tion. Thus in January, at Chicago, according to eleven 

 years' observations, the winds from the west of the me- 

 ridian are to the winds from the east of the meridian as 

 72 to 5 ; at Milwaukee, for thirteen years, as 60 to 18; 

 at Manitowoc, for eleven years, as 67 to 11; at Grand 

 Haven, for one and a half years, as 34 to 16. A similar 

 excess of westerly winds is shown for all the months of 

 the year except April and May, and especially the month 

 of May. 



In consequence of this prevalence of westerly winds 

 the east side of Lake Michigan is warmed in winter and 

 cooled in summer. While, therefore, the winter mean at 

 Chicago is 24^, that of New Buffalo, in the same latitude, 

 is 28. While that of Milwaukee is 22, that of Grand 

 Haven is 26. While the winter mean of Fort Howard 

 is 20, and that of Appleton 19, the winter mean of 

 Traverse City, farther north than either, is 23|-. In 

 autumn, also, the preponderance of westerly winds raises 

 the mean temperature one or two degrees along the south 

 half of the lake shore, and three to four degrees along 

 the northern half of the shore. This is strikingly shown 

 on an isotherm^al chart where continuous lines are drawn 

 from east to west through places having the same au- 

 tumnal means. To the west of the lake region the lines 

 conform approximately to the parallels of latitude, but 

 over and east of Lake Michigan they bend abruptl}^ north- 

 ward. The autumnal isotherm of 46, which passes 

 through Fort Winnebago, bends northward nearly to the 

 extreme point of Lake Michigan, a difference of latitude 

 of about 185 miles. The isotherm of 47, which passes 

 through Fort Atkinson, bends northward to the Beaver 

 islands 192 miles. The isotherm of 48 is deflected north- 



