D = Diameter of the Propeller in feet. P = Pitch. L = Length of the Propeller in feet. = = Area in square feet. Slip in decimal fractions. Number of Blades in the Propeller. Extreme Breadth of the Propeller Blade over its extremity in feet. Displacement in tons. Ow, 1w, w, v, 1v, and V M H α d = = = Statute miles per hour. Actual horse power. Angles in degrees of a Circle. Area of the Piston in square inches. Diameter of Cylinders in inches, when there are two direct action Cylinders, and stroke of Piston d = 1.5s. p = Effectual Pressure on the Piston per square inch, including the P S C = = = vacuum. Stroke of Piston in feet. Cubic feet of Steam for each revolution. Volume of Steam compared with Water. Coefficient of Water evaporated per pound of coal per hour. f Friction and working the Pumps, per cent. Tuble for Finding the Pitch of Propellers. By this table (plate xiv.) the pitch of a propeller can be found in an instant, with any diameter and angle of the propeller blades, between 45° and 70°. The degrees will be found on the circular arc; the diagonals. show every tenth minute where the pitch arm crosses the same at a. The pitch will be found on the pitch-arm, where the diameter lines cross the same, when set on a given angle at a. If the angle is to be found, when the diameter and pitch are given, move the pitch-arm until the given pitch crosses the given diameter line; the corresponding angle will be found at a. Propeller makers not fully acquainted with the theory of the propeller, will sometimes find it difficult to lay out the propeller with a certain pitch, or when a few dimensions are given, to ascertain the pitch; for that purpose there are some tables, which will probably meet with their approbation, for convenience in practical use. When the centripetal propeller has been fully tested by experiment, it is intended to publish a small work, containing those Tables, and directions relating to propellers in general. As the screw propeller is the most valuable instrument for the propelling of vessels, and thus far, behind the paddle wheels in the employment of steam for propelling, its friends therefore, ask for the screw propellers and their engines, that they should be submitted to a series of patient and impartial experiments; extending through two or more different vessels, and six or more different propellers. These experiments I feel confident, will result in proving its equality to paddle wheels, and as the screw propeller (if rightly made,) is a direct action propelling instrument, as well as the paddle wheels, why should the preeminence be justly on the side of the latter? Experiments, we know, have been made again and again, but the principal feature has been too often overlooked. In reference to propeller engines with gearings, it is worthy of inquiry what is gained by them? If they are intended to increase the number of revolutions of the propeller, we see where ever they are applied, that they do not. They only increase the size, weight, and expense, and occupy more room in the vessel. Propellers with equal diameter and pitch, will make rather more revolutions from a direct action steam engine with the same quantity of steam. The mechanical law says, "Apply the power as near the object as possible." And the words stationary and DIRECT ACTION, (no trunks,) are perfect teachers on the subject. Philadelphia, September, 1851. Translated for the Journal of the Franklin Institute. Experiments on the Application of Electro-Magnetism as a Motive Force. By M. ARISTIDe Dumont. The author announces in the following terms, the consequences to be deduced from the experiments reported in his memoir: 1. The electro-magnetic force, although it cannot yet be compared to the force of steam in the production of great power, either as it regards the absolute amount of power produced, or the expense, may nevertheless, in certain circumstances, be usefully and practically applied. 2. While in the development of great power, the electro-magnetic force is very far inferior to that of steam, it becomes equal and even superior to it in the production of small forces, which may be thus subdivided, varied, and introduced into trades and occupations using but small capitals, where the absolute amount of mechanical power is less exerted than the facility of producing it instantaneously and at will. 3. In this point of view, the electro-magnetic force assists, as it were, the usefulness of steam, in place of uselessly competing with it. 4. Other things being proportional, electro-magnetic machines with direct alternating movement, present a great superiority of the power developed over rotating machines; since in the first there are no compo |