mercury. Now, when the ship is at rest, if the small air valves C, and D, be opened, the water will rise to these points in the two airvessels C, and D, compressing the air above; if now the air valves be closed, the pressure in these air-vessels will be always equal each to each; and consequently the pressure conveyed along the air pipes, E, and F, will affect the two surfaces of the mercury equally, whatever be the immersion. Let us call this pressure upward the statical force, and the pressure resulting from resistance, the dynamical force. Then, whatever be the direction of the aperture of the tube, the statical forces are always equal in the two air vessels, whether in motion or at rest, but not so the dynamical force, which acts at A, but not at B, because the aperture of B is at the zero point.

U

The consequence is that the pressure is increased in the air-vessel C, but not in D, which depresses one surface of the mercury in the tube, and raises the other to heights, varying as the squares of the velocities; and thus all vertical forces are compensated or eliminated, and the dynamical one alone is registered.

The annexed figure represents the glass and frame of an improved indicator. The only inconvenience found in the use of the simple column of mercury is, that at very low speeds the graduations are very small indeed. We seem to want a fluid of a medium density between mercury and water, for while water, or even spirit is most desirable to show low speeds, it is inadmissable in high speeds on account of the great length of column. Sulphuric acid might be used, but it is dangerous, and also inconstant in specific gravity,

The improved indicator with two conical bulbs supplies the want completely, combining the advantages of a water range of scale for low rate of sailing with the shortness of the mercurial one for high velocities. A and B are two conical bulbs; A nearly full of mercury which rises also in B to the level b when the vessel is at rest. Super-imposed upon the mercury in B, is coloured water W, up to the point c which is the zero point of the scale. Now the pressure due to velocity acting in A forces down the mercury in that bulb, and up in B, forcing upwards the water into the index-tube, where, as the sectional area is only about eth of the largest part of the bulb it rises ten times as high. But as the speed increases the mercury falls lower in A and rises higher in B,

the surfaces of the mercury constantly decrease, while that of the water in the index-tube remains the same. Thus we have two columns, one of mercury, the other of water; the latter rising the most rapidly at low speeds, and gradually diminishing its proportional ascent till the mercury reaches the point c, when its further rise will be equal to that of the mercury only.

In practice the two tubes are conducted separately into one pipe below, which is divided into an upper and lower chamber, for the two forces; and this is a very advantageous arrangement, because it not only makes one hole enough in the ship's bottom, but the ports of the two chambers are always in the same relative position. It is also found more convenient to use a zero chamber instead of a negative one.

The Leeway, or rather the true course of the vessel is found in two

ways.

1. Hydrostatically.-It is clear that the maximum elevation of the mercury can only be when the plane of the aperture is exactly at right angles to the line of motion. So if the ship be moving right ahead the axis of it should be parallel to the keel; but if she goes at all to leeward this will not be the line of the greatest resistance, and the tube is turned round to the lee bow, by the action of a vane (described hereafter), in the direction of the true line of motion; at the same time an index finger connected with the vane, by which the tube is turned, moves over a horizontal circle and points cut the angle between the real and apparent course of the ship.

2. The leeway is thus shewn mechanically, and is self-acting, for whilst the vane always keeps the ports in the right direction it moves the rod and index needle and points out the true course of the ship.

The first experiments with the double acting indicator were made with the tubes full of water which rested on the mercury. This was found to soil the surface and the glass, so that now the water is never allowed to rise above the water line in the two air-vessels; and from these the pressure of the water is communicated to the mercury through an intervening column of air, which though expansive, does not affect the result, because any contraction or expansion acts equally upon both limbs.

We now annex a description of the Perpetual Log represented complete in the figure attached.

AA, a hollow brass socket nine inches long, screwed into the bottom plate if the ship be of iron. If the ship be of wood it should pass through the kelson to the water as shown in the figure. This socket is bored from end to end and has a stuffing box on the top, and a stop-cock a near the middle.

B, a brass tube ths of an inch in diameter, which passes through the socket into the water. It is divided into two chambers at the lower end, and from these proceed the two pipes with the apertures bb'; C'C are flexible tubes uniting the pipes with the fixed pipes EE and E'E' and conveying the water from the chambers to the air vessels FF', where it reaches

about the middle, the upper part containing air. GG, G'G' are air pipes which transmit the forces received in the air vessels through the flexible tubes HH' to the indicator.

KK, a rod with a shackle bb below to grasp the head of the tube B, and a handle L above to move the tube up and down, as well as to turn it round. Below the handle is an index finger moving over a horizontal segment N to show the leeway.

Attached to the bottom of the tube

12

H

with a joint is a vane about eight inches long, which acted upon by the water always keeps the tube in the right position. It is made of the same size as the tube, so that the whole is drawn up or let down at pleasure.

We have added to the annexed figure, shewing the whole log, an enlarged sketch of the indicator carrying the scale graduated to miles and parts; the velocity being indicated by the height to which the fluid rises.

In the following page is a section of the lower part of the tube enlarged. A, is the loweror negative chamber, communicating with the tube E', by the internal tube a, B the upper or positive chamber communicating with E. The vane ccc is composed of two slips of hard brass about nine inches long, rivetted together at one end, and when not compressed spring apart at the other, and extend a piece of vulcanized rubber like the web of a duck's foot.

When these two slips are forced into the water through the large lower tube they open and extend the web which by the action of the water turns the

The recent trial of H.M.S. Dauntless was the first occasion of placing the perpetual log in a screw steamer. The inventor anticipated that the propeller would affect the instrument in excess, but could not predict the amount. The distance of the tube from the stern was about 45 feet, but even at that distance the excess of effect was nearly equal to the slip of the screw being 13 per cent. more than the actual speed of the ship, while the slip of the Screw was 14 per cent. Thus the indicator gave the speed of the screw very nearly and not that of the ship. This clearly proves that there is a strong back current drawn aft by the screw under the ship's bottom : how far this current extends is not yet proved, but it is expected not beyond the midship body. A second instrument complete is being fitted at the step of the foremast, where, without doubt, the true resistance will be obtained; and by comparing this second indicator with the other at the same time, and at various speeds some interesting results are likely to be obtained respecting the screw propeller.

The instrument has recently been placed in the little Bee, steam tender to H.M.S. Excellent. All the trials in her over the measured mile have been very satisfactory; as also a set of 12 experiments in the successive reduction of the steam pressure, taking a pound off the boiler at each experiment; in every case the reduction of speed shown by the indicator was

correct.

The compound indicator, having, conical bulbs, and using mercury and water balanced against each other, is found in the Bee admirably adapted to the purpose of showing minute changes of speed. The 100th part of a knot can be readily perceived. The inventor recommends this form of indicator for all vessels; it is longer than the other, but not inconveniently long, about two feet. He has also applied with complete success a moderator to the instrument, a very simple contrivance by which the indicator is made to denote the mean speed of the ship for any desired interval of time.

TURKISH FOR TARS, No. V.-By Mahmouz Effendi.

(Continued from page 259.)

In the May number of the Nautical there was merely space enough for the first half of the alphabetical vocabulary we had penned for our fourth number of Turkish for Tars, the second half of which we now subjoin without further observation in this the number for August.