214 PHYSICAL QEOGUAPHY OF THE SEA, AND ITS METEOIIOLOGT. 



ling by the t^aiuc liidtlen ways, reach the frigid zone without 

 losing more than tlie cold currents gained in temperature, viz., 7° ? 

 In 1840, Sir James C. Koss, being in the antarctic regions with 

 tlie surface water at 32°, found the temperature in depth to be 

 38°.8 at 400 fathoms, and SO^'.S at 600. At a greater depth there 

 is a greater pressure ; and there ought to be (§ 404) a certain 

 temperature, that after passing a certain depth in the deep sea 

 grows higher and higher as the depth increases. The thermal 

 laws of " deep-sea" temperatures for fresh and for salt water are 

 very different. In September, when the surface water of Loch 

 Lomond and Loch Katrine — Scottish lakes — which are between 

 500 and 600 feet deep, is 58°, that at the bottom is uniformly 

 41°, which is very near the point of maximum density for fresh 

 water. Saussure has shown the same for the Italian lakes : only, 

 at the depth of 1000 feet in the Lake of Geneva, it was a little 

 warmer, probably on account of pressure (§ 404), than it was at 

 less depth in Lakes Lucerne and Thun. In these it was 41°, or 

 1° colder than the bottom of Geneva, their surface water being 

 about 60°. In Lago Sabatino, near Eome, with the surface 

 water at 77°, Barlocci reports.44° at the depth of 490 feet. The 

 winter in Eome is not severe enough to cool such a mass of water 

 below 44°. But with the exception of the Lake of Geneva, 

 which is deep enough to have the temperature of its water some- 

 what influenced b}^ pressure (§ 404), the law is uniform : as you 

 descend in fresh-water lakes, the temperature decreases to that of 

 maximum densitj'. Saussure extended his experiments to the 

 Gulfs of Nice and Genoa — salt-water bays in the neighbourhood 

 of his fresh-water lakes. Here, with the surface temperature of 

 69^, he found even at the depth of 1720 feet, the water no cooler 

 than 55°. 8. This salt water might have been cooled 30° lower 

 before it would have reached the maximum density (25°. 6) of 

 average sea water. AYe see that the severest winters are not 

 sufficient to bridge our deep fresh-water lakes over with ice, 

 though their waters being cooled below 39°. 5, grow light, and 

 remain on the surface to be frozen. On the contrar}", sea water 

 contracts, grows heavy, and sinks, until the whole basin, from 

 the bottom to the top, be reduced to 27°. 2. Yet many confess 

 no surprise at the open water in fresh-water lakes that are com- 

 paratively shallow, while they can conceive of no such thing in 

 the Arctic Ocean, though it be very much deeper than the 

 deepest fresh-water lakes ! 



