The only ship of her kind ever built with a hot-air engine was the _Ericsson_, named after her inventor and generally called the _Caloric_, because of her peculiar engines. These had four immense cylinders which drove paddle-wheels 32 feet in diameter, the energy being transmitted by a contrivance Ericsson invented and termed the “regenerator.” The shape of the furnaces and the small amount of fuel they required, together with the absence of boilers, enabled a greater amount of space to be devoted to the accommodation of merchandise and passengers. The vessel was 250 feet long, 40 feet broad, 31 feet deep, and had a gross tonnage of 1920. She was built in 1852, of wood, and was asserted to have made a speed of 12 knots an hour on her trial trip, but she never came anywhere near this subsequently. The absence of funnels and the presence of two large paddle-boxes made her one of the most extraordinary vessels ever seen. She made one slow journey across the Atlantic to Liverpool and back to America, and after another set of caloric engines had been tried in her with about as much success, in regard to her speed, as the first, she was fitted with engines of the ordinary type. Three other inventions which have not yet passed the experimental stage are the Hydrocurve, the Hydroplan, and the Hydroplane. The hydroplan is a motor-boat carrying two enormous propellers, one above the stem and the other above the stern, which revolve in the air and not in the water. The vessel is said to have been invented by a gentleman named Fortanini, and with a 70-horse-power motor is claimed to have attained, on Lake Maggiore, two or three years ago, a speed of 40 miles an hour. For all practical purposes the hydroplan may be described as a “skimming dish” hull gliding on the surface of the water, its draught being a few inches only. For some time past some attention has been directed to the trials, on the Illinois River, of a curious type of aquatic motor, named the hydrocurve. Instead of ploughing through the water, the hull of the hydrocurve displaces the water, not sideways as with an ordinary type of vessel, but downwards from the surface, each particle of water being moved in one direction only. According to a report published in the _Popular Mechanic_ of Chicago, this curious vessel on her first trial made a speed of 35 miles an hour. In a further test she achieved 1¹⁄₈ mile in 1 minute 30 seconds, or, roughly speaking, 45 miles an hour. She is 40 feet in length and carries an 80-horse-power motor. The bottom of the boat is concave, lengthways and across.