As the saving of the fuel is the only string on which the advocates of the hot air engine harp, we must say, they exhibit an amount ot ignorance on the subject worthy of a native of the interior of Africa. The Arctic burns 84 tons of coal per day, and we assert that the Ericsson cannot go as fast a.nd use 100 tons; When our north river steamboats increase their speed to but a few miles beyond their average rate per hour, they consume four and five times the usual amount of fuel. Dr. Lardner himself, who presents in his " Railway Economy" the Iron Witch, of Capt. Ericsson, which turned out a complete failure, as a favorable specimen of a north river steamboat, admits this to be true. When the Oregon and Vanderbilt had their famous race on the 2d of June, 1847, the former consumed IS tons of the very best picked coal in three hours, running at the rate of 24 miles per hour. She will run to Albany in 10 hours at the rate of 15 miles per hour, and use only 12 tons of common coal. Thus with an increase of only three-fifths the speed, she consumed more than 6 times the quantity of fuel—the increase was as 6 tons to 0'83 of a ton. With this data of the quantity of fuel necessary to run a steamboat according to a certain speed, the Oregon would only use li tons of coal in running to Albany at the rate of 7 miles per hour, that is allowing a double speed to require 8 times the amount of fuel1 which appears to be about the quantity.