LAYING OUT THE PITCH.--First, we must get the circumference of the propeller, that is, the distance the tip of the propeller will travel in making one complete turn. This is done by multiplying 7 by 3.1416. This equals 21.99, or, practically, 22 feet. A line B is drawn, extending out horizontally along one side of the blank A, this line being made on a scale, to represent 22 feet. Secondly, at the end of this line drawn a perpendicular line C, 6 feet long. A perpendicular line is always one which is at right angles to a base line. In this case B is the base line. Line C is made 6 feet long, because we are trying to find the angle of a 6 foot pitch. If, now, a line D is drawn from the ends of the two lines B, C, it will represent the pitch which, marked across the end of the blank A, will indicate the line to cut the blade. PITCH RULE.--The rule may, therefore, be stated as follows: Multiply the diameter (in feet) of the propeller by 3.1416, and draw a line the length indicated by the product. At one end of this line draw a perpendicular line the length of the pitch requirement (in feet), and join the ends of the two lines by a diagonal line, and this line will represent the pitch angle. Propellers may be made of wood or metal, the former being preferred for the reason that this material makes a lighter article, and is stronger, in some respects, than any metal yet suggested. LAMINATED CONSTRUCTION.--All propellers should be laminated,--that is, built up of layers of wood, glued together and thoroughly dried, from which the propeller is cut. A product thus made is much more serviceable than if made of one piece, even though the laminated parts are of the same wood, because the different strips used will have their fibers overlapping each other, and thus greatly augment the strength of the whole. Generally the alternate strips are of different materials, black walnut, mahogany, birch, spruce, and maple being the most largely used, but mahogany and birch seem to be mostly favored. LAYING UP A PROPELLER FORM.--The first step necessary is to prepare thin strips, each, say, seven feet long, and five inches wide, and three- eighths of an inch thick. If seven such pieces are put together, as in Fig. 78, it will make an assemblage of two and five-eighth inches high. _Fig. 78. A Laminated Blank._ Bore a hole centrally through the assemblage, and place therein a pin B. The contact faces of these strips should be previously well painted over with hot glue liberally applied. When they are then placed in position and the pin is in place, the ends of the separate pieces are offset, one beyond the other, a half inch, as shown, for instance, in Fig. 79. This will provide ends which are eight and a half inches broad, and thus furnish sufficient material for the blades. The mass is then subjected to heavy pressure, and allowed to dry before the blades are pared down. _Fig. 79. Arranging the Strips._