benefited much, for two reasons: first, by the time that the gun can be gotten ready in accordance with any range and direction signaled, the range and direction will have changed; second, the people at the guns want to know the distance and direction of the target from their guns and not from the position finder. To remedy the first difficulty the people at the position finder do not signal to the guns what the range and direction are at that instant, but they predict what they will be thirty seconds later, so that the gun people have thirty seconds in which to lay the gun. To enable the position-finder people to predict they take observations every twenty seconds, plot the exact position of the target each time on the plotting table of the position finder, and connect the various points by a line, more or less broken. A little practice enables them to thus lay down on the chart the exact track a ship is making. If the track shows that the ship has gone a certain distance in a certain direction in a certain time, it is not hard to prolong the track line so as to show where she will be two minutes later; in other words, to "predict her position." It is this predicted position that is signaled to the guns. When the ship arrives at or sufficiently near the predicted position, the signal is sent to the guns to fire. Of course if during the interval the ship suddenly changes her course and speed very greatly, she will not reach the predicted position at exactly the end of two minutes. But a heavy ship cannot alter her course and speed so much in two minutes as to throw out the predicted position much, as even a rough calculation will show, especially if she is in company with other ships in a channel; and even if she could, it would be simply necessary to hold the fire until a new position were determined, which would be a matter of a few seconds only. The manner of using the position finder can be best shown by an extract from the official report on one placed at Spezia, Italy.

The disposition of the Fiske position finder renders it possible to make rapidly a series of observations upon a target in motion, and to solve the problem of how she is going; to determine the route, the radius of the circle of turning, the speed, etc. During the recent trial at Spezia, the base line of the position finder being 104 meters long, there was determined the velocity of a torpedo-boat which was going at a speed of 8 knots, of 10 knots, of 12 knots, maintaining a distance from the position finder ranging from 2000 meters to 5000 meters. There was determined at regular intervals the different points of her track, and the

resulting speed deduced varied from the speed obtained on board the vessel itself by only 3 per cent. In order to give an idea of the quickness with which it is possible to fix the ship's position, and after the observers have had a certain amount of practice, it is sufficient to cite the fact that while the torpedo-boat was making a complete circle of 250 meters diameter at a speed of 10.5 knots, the average distance from the position finder being 1500 meters, there were plotted II successive positions. In another trial, while the boat was making a circle of 350 meters diameter, at a distance of 2500 meters from the position finder, at the same speed her position was plotted 17 times.

The position finder having determined the range and direction of the target from itself, it remains for the people at each gun to determine what are the range and direction from that gun. This may be done by means of tables of figures, which show for each gun what are the directions and ranges from that gun of every position that can be signaled from the position finder. But a speedier plan is offered by the instrument invented by Lieutenant Rafferty, United States Artillery, called a relocator," in which the conversion from one system of co-ordinates to the other is done mechanically. The instrument is extremely simple in construction, and (which is more important) is simple in operation, but not being electrical, its description hardly belongs here.

DEPRESSION POSITION FINDER.

When the ground in the vicinity of the water is high, the distance of a vessel on the water can be found very simply by mounting a telescope on an eminence, pointing it at the vessel's water line, and noting the angle of depression of the telescope below the horizontal. The height being known, the distance is inversely proportional to the sine of this angle, so that the instrument may be graduated at once in yards. The direction is obtained with equal simplicity, by noting the azimuth of the telescope, as shown on a horizontal circle. To make these measurements and send them to the guns automatically is the office of the instrument hereinafter described. Corrections are, of course, needed for the rise and fall of the tide and for changes in refraction. The special limitation of this class of apparatus is the fact that the smoke of guns frequently hides a vessel's water line altogether, as was conspicuously shown at the battle of the Yalu.

The apparatus consists in two principal parts, namely, a device for determining the distance and a device for determining the direction or bearing of the object. The said two parts are used conjointly, and thereby the location of the object may be recognized upon a chart representing the area of the harbor, for example, drawn on a reduced scale.

In the accompanying drawings, Fig. 38 is a diagram illustrating the operation of the distance or range finder. Fig. 37 is a diagram illustrating the operation both of the range finder and of that part of the apparatus which shows the bearing of the distant object. Fig. 39 is a side elevation of the observer's instrument and shows the mechanism thereof on a larger scale and in detail.

Similar letters of reference indicate like parts.

Referring first to Fig. 38, A is a telescope, sight bar, or other like means of directing the line of sight, indicated by the dotted line upon the object B. This telescope is to be located upon an elevation adjacent to the waterway to be protected. The telescope is pivoted at its outer end, so that it can be depressed through any desired angle in order to bring it to bear upon the object. The telescope is provided near its sight end with a contact piece or wiper which always bears upon a body of conducting material, represented symbolically at C. Connected with the ends of the body C is a voltaic battery, D, and connected in circuit with one end, E, of the body, C, and with the movable wiper or contact piece carried by the telescope A is a galvanometer, F. It will be apparent that as the telescope A is moved on its pivot its contact piece or wiper will be carried along the body C, and, as a consequence, a greater or less amount of the body C will be brought into the circuit which includes the galvanometer F. Inasmuch as the body C is to be constructed of, for example, a wire of uniform resistance per unit of length, it is obvious that as the telescope A is moved and a greater or less length of said wire is brought into the galvanometer circuit, the resistance thus interposed in said circuit will be increased or diminished; and as this length, and hence this resistance, depend upon the angle of depression of the telescope, it becomes a function of the angle of depression; and, equally, the deflection of the galvanometer F, due to this change, is also a function of the angle of depression. Therefore, knowing the

height of the telescope above the level of the object, the galvanometer deflection will indicate the distance of the object from the telescope, for which purpose the galvanometer may be once for all graduated in any suitable unit, such as meters or yards. Hence, if the galvanometer be located at a station distant from that telescope, an observer at that distant station, by reading the galvanometer, can recognize at once the distance of the object, while the person stationed at the telescope has nothing to do but to keep it properly directed upon the object.

The telescope pivot is carried upon a bar G, Fig. 39, which is pivoted upon a circular table, H. Placed in a groove around the periphery of this table is a wire I, Fig. 37, of conducting material, having a uniform resistance per unit of length. Upon the bar G is supported a contact piece or wiper, as will be more particularly explained hereinafter with reference to Fig. 39, which contact piece or wiper always bears upon the wire I. At the distant station, Fig. 37, there may be arranged a circular table, J, having around its periphery a wire, K, similar in all respects to the wire I. Upon the table J is pivoted a bar, L, which bar L carries a wiper or contact point which constantly presses upon the wire K. The contact point on the bar G and the contact point on the bar L are connected by a wire, N, which also includes the battery D. The ends of the wires K and I are connected by wires O and P, and said wires O and P are respectively connected to wires Q and R, which lead to the terminals of a galvanometer, S. It will be obvious, by a simple inspection of the drawing, Fig. 37, that the wires I and K at the separated stations and the pivoted bars G and L, together with the battery and the galvanometer S, are connected in Wheatstone bridge circuit, and that a movement of either the bar L or the bar G, displacing the contact pieces over the wires K or I, will vary the resistance of the bridge arms so that the bridge may be brought into or out of equilibrium by the movement of these bars upon their pivots; and further, it will be obvious that the fact when equilibrium is produced in the bridge will be made manifest by the movement of the pointer of the galvanometer S. The construction is to be such, therefore, and the instruments at the separated stations are to be placed with reference to one another, so that when the bar L makes the same azimuth angle with reference to one end of its wire K as does the bar G, then the