The wheel and spindle are turned by the revolutions of a second spindle placed at right angles with the first, carrying a worm or endless screw, the threads of which mesh with the teeth of the wheel. The second spindle carries also a series of fans, arranged like the blades of a screw propeller, or mike the vanes of the common anemometer.
By means of an air current, which flows in one direction when the ship's engines are going ahead, and in the opposite direction when they are backing, the fans and their spindle are rapidly revolved, and the proper motion transmitted through the spiral gearing to the index. The movement of the index is moderate in speed; but the speed is variable with the speed of the engine, and incidentally affords a means of estimating, by the eye, the speed as well as the direction of the movement of the engines and the ship.
The air current is derived from a small rotary blower, placed near the engine shaft, and turned by it through the operation of belts. When turned in one direction, the blower draws the air from the vanes of the annunciator through a pipe, in one enlarged extremity of which, forming a mouth, the vanes revolve. When turned in the opposite direction, the air is driven through the connecting pipe to the vanes, and the direction of the movement of the latter, upon the instant, reversed.
It is a very great advantage to the person manceuvering the ship, to know, without the delay attending inquiry, or observation of the movement of the ship herself, exactly what the latter is to be. Should mistake be made, it will be apparent before it is too late to correct it.
The apparatus is simple and elegant, the power consumed by it is inconsiderable, and it is not at all likely to get out of order.
Its first cost needs never to be great, and the cost of maintenance trifling. Drawings of it are hereto appended.
The Board recommends it for purchase and use for purposes under cognizance of the Bureau of Steam Engineering.
We are, Sir, very respectfully, Your obd't serv'ts,