located where the tail is attached to the body. The compression spring and hydrostatic valve actuate the horizontal rudder. By regulating the compression spring, the depth at which the hydrostatic valve takes charge is controlled. When the para

vane has reached the depth for which the compression spring has been set, the pressure on the valve equalizes the pressure of the spring, resulting in a horizontal rudder. Should the paravane rise above the set depth, the compression spring takes charge, resulting in down rudder and deeper submergence. The opposite action takes place when the paravane has reached too great a depth, causing the hydrostatic valve to take charge.

Oscillator.-Types "B" and "C," designed for higher speed vessels, are fitted with a stabilizer, called the mercury oscillator, Fig. 3, which makes the depth control more sensitive. The hydrostatic valve, being dependent on water pressure for its action, does not respond quickly enough at speeds above 16 knots to prevent large changes in depth above and below the desired running depth. The purpose of the mercury oscillator is to prevent this porpoising action. The oscillator consists of a tube. of mercury about 6 feet long, fitted with a valve or diaphragm at each end. Each oscillator contains about 45 pounds of mercury, which exerts a considerable pressure against either diaphragm. A shaft extends from the compression spring in the forward end of the paravane, through the mercury tube to the rudder shaft, and to this main shaft are attached the mercury tube with a valve at either end, and the compression spring directly forward of the forward valve. Just aft of the after valve lies the hydrostatic valve at the point where the tail is attached to the body. It will be noted that the holes in the tail allow the water to come in contact with the hydrostatic valve.

When the paravane is level, the two mercury valves are subjected to the same pressure, and, therefore, no rudder action takes place. The instant the paravane starts to dive, the pressure is increased on the forward valve, resulting in up rudder before the hydrostatic valve has begun to act. In the same way, when the paravane starts to rise, the pressure on the after valve is increased, resulting in down rudder. In this way, the mercury oscillator anticipates the action of the hydrostatic valve, which results in a much more sensitive depth control.

Because of the heavy stress, about 6000 pounds in the case of a vessel moving at a speed of 18 knots, and the severe vibration to which the towing ropes are subjected, the British Admiralty found it necessary to use wire rope of special construction for this purpose. The production of suitable rope in the United States presented some difficulties at first, but these difficulties were satisfactorily overcome by John A. Roebling Sons Company.

The similarity of paravanes to torpedoes and the urgent necessity for early production practically limited the manufacturing field to concerns experienced in the manufacture of torpedoes. The maintenance of secrecy prevented advertising for bids in the usual manner. Two bids were secured, however, the bid of the E. W. Bliss Company, Brooklyn, the principal torpedo manufacturers in this country, submitting the lower bid. A contract for several hundred of each type of paravane was accordingly awarded to this concern.

In the meantime, such information as had been received from England regarding deck fittings and under-water fittings was forwarded to the various navy yards, and the yards were requested to prepare designs for the installation of paravanes on battleships and cruisers.

In October, 1917, the writer having already been assigned to duty in connection with paravanes, under the Bureau of Construction and Repair, was ordered abroad to make a study of the subject, with the view of starting the Bureau's organization for this work.

British Organization.-The British Admiralty at this time had established a separate department at Portsmouth Dockyard, called the paravane department, consisting of a total personnel of over 300, including 71 officers. This organization had charge of the design and installation of the special fittings required in connection with paravanes, the manufacture, inspection and testing of paravanes, and the training of personnel in the use of paravanes. Before the writer returned from this trip, the Admiralty stated that one battleship, five cruisers, and twenty merchant ships had cut German mines with their paravanes. addition to saving these ships, the cutting of these mines had led to the discovery of mine fields hitherto unknown. In consequence, the Admiralty had ordered all British naval vessels, and

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all British merchant ships of 12 feet draft to be equipped with this device as soon as possible.

When the writer returned to the Bureau in January, 1918, it was decided to establish a section called "Protective Devices under the Maintenance Division of the Bureau, to organize and direct all activities in connection with paravanes. Briefly, the work to be done consisted of the following:

1. Preparing instructions and drawings for fitting paravanes to naval vessels.

2. Preparing instructions and drawings for fitting paravanes to Emergency Fleet Corporation vessels.

3. Selecting and equipping a vessel for testing paravanes. 4. Establishing and equipping a shore station for testing paravanes.

5. Organizing and instructing the testing ship personnel. 6. Preparing a confidential handbook.

7. Selecting and fitting out a PV. instruction ship.

8. Organizing the instruction personnel and laying out a course of instruction.

9. Organizing an inspection force for the inspection of PVs in course of manufacture.

10. Organizing an inspection force to inspect Emergency Fleet vessels being equipped with PVs.

Plans for Naval Vessels.-The various naval vessels were listed, and the scheduled dockings charted so that the installation on each vessel could be anticipated by the Bureau, and the necessary instructions issued in advance to the navy yard at which the vessel was to dock. The system adopted was to have each navy yard prepare a design of the under-water fittings and general arrangement for each vessel in general accordance with the type plans sent out by the Bureau. Blue prints of the plans for each vessel were forwarded to the Bureau for examination and approval, but the navy yards were authorized to proceed with the work in advance of this approval, in order to save delay. The Bureau arranged for the purchase, in quantities, of certain special fittings, common to all naval vessels, and allotted a certain number to each yard to be reserved for paravane gear. Special forms were prepared and sent to the navy yards on which they reported every two weeks the status of the paravane work on each ship. This information as received was entered on a

progress card for each vessel. Also record books were started in connection with paravanes and towing ropes and a policy established of controlling the assignment of paravanes direct from the Bureau. No vessel was allowed to receive paravanes until the Bureau was satisfied that her installation of fittings was complete and in working order.

Plans for Merchant Ships.-The Secretary of the Navy having advised the Emergency Fleet Corporation to equip their vessels with paravanes, requested the Bureau to furnish the Corporation with information as to how they should proceed. Several conferences were held for this purpose, and all the drawings and necessary directions were prepared and furnished to the Fleet Corporation.

In this connection, the use of the word "otter" should be explained. The British Admiralty, at the time the fitting of British merchant ships was started, decided that it would be inadvisable to use the technical word paravane in the merchant marine, and chose the word "otter," to be used as the name of the device in connection with merchant ships. As merchant ships usually use type "M" paravanes, these were commonly called otters, while the "B" and "C" types were called PVs. This policy was followed out rigidly in the British Paravane organization, in fact very few captains of merchant ships ever heard of the word paravane, always referring to the device by the name of otter," or "otter gear."

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In accordance with this policy, the United States Navy has used the words otter," and "otter gear " in connection with merchant ships, and in naval work the abbreviation PV.

The Fleet Corporation, therefore, under advice of the Navy Department, organized an otter gear department to take charge of the fitting of their vessels. The Bureau, at the same time, arranged for the purchase of an additional supply of otters, towing-ropes and inhaul-ropes to be used in the equipment of the Emergency Fleet Corporation ships.

Selection of Testing Ship.-Although torpedo boats were used by the British Navy for paravane testing vessels, it was believed by the Bureau, that some fast yacht might do the work, and the U. S. S. Winchester (Fig. 4), a converted yacht of 30 knots speed, was assigned to this work and ordered to the Norfolk Navy Yard, where she was fitted with the necessary gear.