contrivance is used for the circulation, the tubes of the condenser are provided at both ends with a conical nozzle. In the engine of the Hirondelle the packing of the tubes consists of a sheet of india-rubber, and the nozzles are very simply made by counter-sinking the holes of the compressing brass plates. By these means the velocity of the stream may be made to increase gradually at the ends of the tubes, instead of changing suddenly from 1 to 3 or 4, and vice versa. As will further be seen, the arrangement has answered exceedingly well, and the quantity of water running through the condenser is proportionately much greater than is generally the case. A simple arrangement has been provided for causing during stoppages a circulation of water sufficient to prevent the vacuum from disappearing. A steam pipe is made to open in the after canal and in the direction of the ordinary flow of the circulating water. It has been impossible to measure the quantity of steam thus expended, but when at moorings with the engines running at full speed a small jet has been found sufficient to ensure a vacuum of 26 inches. The pipe can be made, as in the Hirondelle, much larger than necessary for that purpose only, so as to be available for the escape of steam when the pressure is too high-a good plan especially on board passenger steamers. On account of the small expansion of steam (about 4 times), and in order to reduce the weight of the machinery, the cylinders are not steam-jacketed but covered with felt and wood. The steam is slightly super-heated in the smoke-box. If a greater expansion were used it would be impossible to dispense with steam jackets, especially for the lowpressure cylinder. The following are the principal dimensions of the hull and engines of the Hirondelle : Admission of steam in high-pressure and low-pressure cylinders in cents. of stroke Diameter of air pump (single acting) ... ... ... Diameter of paddle-wheels at centre of rotation of floats ... ... ... 373 in. 29" 471, 68 193 in. The trials made in September last gave the following results : At full speed (13 knots, 400 horse-power and 47 revolutions) the temperature of circulating water was found to be at entrance 68°, and at exit 77° Fahrenheit, the feed varying from 95° to 100°. |