125. Q.—­What do you mean by the pitch of the screw?

A.—­A screw propeller may be supposed to be a short piece cut off a screw of large diameter like a spiral stair, and the pitch of a spiral stair is the vertical height from any given step to the step immediately overhead.

126. Q.—­What is the usual number of arms?

A.—­Generally a screw has two arms, but sometimes it has three or more.  The Great Britain had three arms or twisted blades resembling the vanes of a windmill.  The multiple of the gearing in the Great Britain is 3 to 1, and there are 17-1/2 square feet of heating surface in the boiler for each nominal horse power.  The crank shaft being put into motion by the engine, carries round with it the great cog wheel, or aggregation of cog wheels, affixed to its extremity; and these wheels acting on suitable pinions on the screw shaft, cause the screw to make three revolutions for every revolution made by the engine.

127. Q.—­What are the principal varieties of direct acting screw engines?

A.—­In some cases four engines have been employed instead of two, and the cylinders have been laid on their sides on each side of the screw shaft.  This multiplication of engines, however, introduces needless complication, and is now but little used.  In other cases two inverted cylinders are set above the screw shaft on appropriate framing; and connecting rods attached to the ends of the piston rods turn round cranks in the screw shaft.

128. Q.—­What is the kind of direct acting screw engine employed by Messrs. Penn.

A.—­It is a horizontal trunk engine.  In this engine a round pipe called a trunk penetrates the piston, to which it is fixed, being in fact cast in one piece with it; and the trunk also penetrates the top and bottom of the cylinder, through which it moves, and is made tight therein by means of stuffing boxes.  The connecting rod is attached at one end to a pin fixed in the middle of the trunk, while the other end engages the crank in the usual manner.  The air pump is set within the condenser, and is wrought by a rod which is fixed to the piston and derives its motion therefrom.  The air pump is of that species which is called double-acting.  The piston or bucket is formed without valves in it, but an inlet and outlet valve is fixed to each end of the pump, through the one of which the water is drawn into the pump barrel, and through the other of which it is expelled into the hot well.

THE LOCOMOTIVE ENGINE.

129. Q.—­Will you describe the more important features of the locomotive engine?