evaporation requires the same amount of heat that would suffice to raise Niagara to boiling-point, if such a thing may be imagined; and imagining this, we may conceive of what is actually going on. The loss of this amount of heat, parted with by the lakes, leaves them and their region colder in the winter and summer.
"In salt lakes the water enters warmer than does fresh water, from this loss of heat to the latter, and in winter there is a further difference. When fresh water is exposed to a change of temperature, it grows denser until it reaches 30 degrees, or near freezing, when it expands, thus rising to the surface. These lakes in winter have their coldest water at the surface; with salt water the reverse is true, and the water, continuing to grow denser with a lower temperature, sinks and leaves the surface-water the warmest; were the lakes salt water, navigation would continue the entire year round.
"These preliminaries passed, the speaker connected them with the interests of the lake region, of which he proceeded to speak. Quoting fully, and with honourable mention of their author, the full and clear reports of Colonel Graham of this city, he reviewed fully the perils to which lake fleets are exposed.
"According to Colonel Graham's report for 1855 to the department at Washington, the value of the shipping and commerce interested in the construction of a single breakwater at Michigan city was $218,000,000.
"According to observations taken, the value of shipping and commerce passing the flats of the St. Clair, on an average of 230 days for the busy season, reached the daily average of $1,029,223. The entire value of the lake commerce was $200,000,000 to each lake, ranging more to Lakes Michigan and Huron, less to Lake Superior; and, as Erie is an outlet to the others, the amount on that lake was $300,000,000.
