Page:Physical Geography of the Sea and its Meteorology.djvu/240

ling by the same hidden ways, reach the frigid zone without losing more than the cold currents gained in temperature, viz., 7°? In 1840, Sir James C. Ross, being in the antarctic regions with the surface water at 32°, found the temperature in depth to be 38°.8 at 400 fathoms, and 39°.8 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 Rome, with the surface water at 77°, Barlocci reports.44° at the depth of 490 feet. The winter in Rome 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 somewhat influenced by pressure (§ 404), the law is uniform: as you descend in fresh-water lakes, the temperature decreases to that of maximum density. 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. We 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 contrary, 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 comparatively 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!