216 SPARKS FROM A GEOLOGIST'S HAMMER. 



ological tables, since at Manitowoc the easterly winds in 

 May are to the westerly as 37 to 26 ; at Milwaukee as 

 62 to 24, and in April as 52 to 33; at Chicago, including 

 north winds, which are here lake winds, the ratio of lake 

 and land winds is in May as 44 to 40. Now, in May, a 

 lake wind is a chilling influence, except when the ther- 

 mometer is sinking below the growing temperature for 

 vegetation. It is then an influence which prevents frost. 

 It follows, therefore, that during the mild days of May 

 the eastern shore of the lake is exempt from the chilling 

 and retarding influence of westerly winds; while, during 

 a cold period, when, as a rule, the wind is westerly, the 

 eastern shore receives the benefit of protection from frost. 

 Thus, on the 16th of May, 1868, a destructive frost oc- 

 curred throughout Illinois, Indiana and Ohio, but did no 

 damage in the Grand Traverse region. It is a frequent 

 occurrence to read of killing autumnal or vernal frosts 

 in any of the states south or west of Michigan, while the 

 Michigan peninsula remains completely exempt. This 

 unique arrangement of 'the prevailing winds seems 

 prompted by a beneficent regard for- the interests of 

 early vegetation on the eastern side of Lake Michigan. 

 Westerly winds cease to predominate only in that month 

 when they cease to be beneficial to Michigan. And- yet 

 even in that month they exist whenever the interests of 

 vegetation demand. Not only do westerly winds cease to 

 predominate at the juncture when they cease to be bene- 

 ficial, but at the same juncture, the warmer land winds 

 from the east of the meridian become predominant. Both 

 causes accelerate vegetation on the east side of the lake. 

 A study of the means for a series of years, at places on 

 opposite sides of the lake, shows that the temperature of 



