THE ANNUAL MEETING. 159 



In fact, the mass of the water of the lake becomes an immense stove, holding 

 such a stove of surplus summer heat, that, during the winter it warms up "all 

 out doors." If the constant radiation of the lake is so perceptible in mean 

 winter weather, it becomes truly striking when the land temperature sinks to 

 its minimum point, and the difference between the land and the lake amounts 

 to 50° or G0°. 



Observations have shown that even the annual means of the regions con- 

 tiguous to the lakes are somewhat raised by the lake influence. The cooling 

 effect in summer is not equal to the warming effect in winter. In other words, 

 the mean temperature of the lake is a few degrees higher than that of the 

 land. As this fact cannot be attributed to an influx of river water from more 

 southern latitudes, and would seem to be only partially explicable from the 

 probably higher temperature of river-waters in the same latitudes, it remains 

 to seek an explanation of the higher mean temperature of the lake. Now, 

 let it be remembered that the waters of the lake penetrate 000 feet toward the 

 heated interior of the earth ; and that it has been ascertained that on the land 

 every 55 feet of descent beneath the plane of constant temperature brings us 

 one additional degree of heat. It will thus appear that if the depth of con- 

 stant temperature in the mean latitude of Lake Michigan is 60 feet, the water 

 of the lake reaches to a depth where the terrestial temperature should be 15° 

 higher than the constant temperature beneath the land, which would probably 

 be about the mean annual temperature of the locality. I have ventured to 

 suggest, in former papers, that though the cooling influence of the local an- 

 nual mean must have been felt by the earth, in the bottom of the lake, it must 

 be still true that the bottom of the lake has felt somewhat the warming influ- 

 ence of the normal terrestrial temperature at that depth. It seems to me en- 

 tirely reasonable, therefore, to maintain that the heat of the earth's internal 

 fires contributes something to the excess of the lake's mean warmth over the 

 mean warmth of the land. The great lake may, therefore, be conceived as 

 held in a vast natural dish, which is warmed over the imperishable fire which 

 we know to be imprisoned within the earth. When the temperature of the 

 land sinks to -20° or -30°, that of the lake is sixty or seventy degrees higher; 

 and the vapor which ascends from its surface is the literal similitude of the 

 steam rising from a kettle heated over a domestic fire. 



The other factor in our peninsular climate is the prevailing direction of the 

 wind. Were the atmosphere perpetually calm, the contiguous land and super- 

 incumbent atmosphere would only be very feebly warmed by direct radiation; 

 and this effect would be more than counterbalanced by a perpetual land breeze 

 as long as the lake should remain warmer than the land. But the general 

 atmosphere is always in motion. Warmed in winter, while passing over the 

 surface of the lake, it conveys some part of the lake-warmth to the land, and 

 the rigor of the cold becomes ameliorated, on the principle of a hot-air furnace. 

 As the wind, by turns, moves from all directions, the lake exerts some warming 

 influence on all the surrounding land. This is illustrated by the isothermal lines 

 for the cold months, which are bent southward, on approaching the lake from 

 either side. Evidently, that side of the lake which receives most wind from 

 the lake-surface, will be most impressed by the lake-influence. JSlow, it hap- 

 pens that the Michigan side of Lake Michigan receives most lake winds during 

 the cold season, because, as is well known, our cold winds approach from a 

 westerly direction. Thus, in January, at Chicago, according to eleven years' 

 observations, the winds from the west of the meridian are to the winds from 

 the east of the meridian as 72: 5—14.4; at Milwaukee, for thirteen years, as 



