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back, it is probable that they would be available at sea in rough weather when the boat built only for speed would hardly be safe.
There are two drawbacks to the use of existing torpedo boats for which various remedies have been proposed. The first and most serious is the discharge of smoke and sparks through the funnel which make known the vessel's presence to the object of attack. The second is the difficulty of steering with the ordinary rudder at the very high speeds obtained. In a recent boat built for a foreign Government, Messrs. Yarrow attempted to remedy the first defect by discharging the smoke through side ports, the side being used which was farthest from the enemy, and the port having valves which are kept open by the blast and closed by any large wave. The arrangement was only available in fine weather, and a temporary funnel had to be rigged on other occasions. The same boat has also, in addition to the rudder aft, a rudder placed about 10 feet from the bow, both being worked simultaneously by connection with the same steering-gear. The forward rudder can be raised within the vessel when she is going at her low speed and when it is not required. It is said that on the trial of the boat, it was found that at high speeds the forward rudder was more useful in steering the boat than the after one. Another novel design was the Herreshoff torpedo boat, built last year for our Government by a company of that name at Rhode Island, U.S.A. Her engines are in the fore part of the vessel and by means of an inclined shaft, which for most of its length is below the keel of the boat, turn a propeller which revolves entirely below the keel, and in the midship part of the vessel. The rudder is aft and is also entirely below the keel. The boat can be propelled a-head or a-stern with equal speed and steers well. With two torpedoes on board, the total weight including fuel and crew of four men is 7 tons. attained on trial a speed of 16 knots. The Herreshoff boat is 591 feet long, 7ų feet beam, and 5} feet depth, of which more than 4 feet is above water. She is covered in above with to-inch steel plates, but her bottom is planked with wood on steel frames and with five steel watertight bulkheads. This boat has a special kind of boiler, consisting of two continuous coils of pipes enclosing the combustion chamber. It is said that steam can be got up in five
earlier boats in the French Navy were designed to be armed with
Our own Government has been blamed for not having gone in
Large ironclads take years to build, and no amount of money can in an emergency make up for lost time, as far as they are concerned, but, as regards the small torpedo craft, it would appear to be the wisest course to increase their numbers gradually, thus acquiring experience as to the respective capabilities of various classes of boats. It must also be remembered that these vessels are of necessity comparatively short-lived. They trust to their high speed for safety in presence of a heavily-armed enemy, who has difficulty in hitting a rapidly moving object, and high speed is obtained by cutting down the weight carried to a minimum-hence the plating is of necessity so thin that a very small amount of deterioration tells heavily upon it.
It is difficult, in the present state of our knowledge, to estimate the relative importance of the torpedo boat in naval wars of the future. In the defence of harbours it, in combination with submarine mines, will doubtless be largely used, but in a great battle on the high seas our present experience would appear to indicate that the big gun and the ram will both be more important weapons than the torpedo.
ON COMPASSES, AND THEIR ADJUSTMENT IN
HE HEELING ERROR. All that has been said so
far relates to a ship when upright, that is, to a ship on an even beam and keel, which should be her
position when swang for the deviation of the compass, or for the adjustment of any of them; and be it remembered that though the compasses have been adjusted in respect to the semicircular and quadrantal co-efficients, the effect of the correction is not such as to efface, though it may mitigate, the heeling deviation.
The following diagrams, 32 and 33, in respect to a ship built head towards North in the northern hemisphere, illustrate the effect of the magnetism as she heels over; and the worst possible position for a compass in such a ship is near the stern, for there all the forces conspire to magnify the error.
It is easily seen that, as the ship heels over, the transverse iron, such as the deck beams, becomes magnetic as it inclines, and the upper or weather end, s, (with blue magnetism) attracts the N. (red) point of the needle ; farther, the upper end of the soft iron, which, before heeling, acted vertically below the compass and did not disturb the horizontal needle, is now with its s, or blue magnetism, brought out to windward, and is consequently an additional force pulling the N. point of the needle to windward; to this must be added the effect of the vertical force of the subpermanent magnetism, which, according to circumstances, may act
apwards or downwards. The heeling deviation is, in this case, to the high or weather side, whether heeling to starboard or port.
Since, however, the effect of induction will be changed at the magnetic equator, and will be reversed, as the upper end of the iron acquires magnetism south of the equator, the heeling error, to state a general case, arises from the following causes :
1. Vertical induction in transverse iron, which draws the N. end of the needle to windward in N. latitudes, and to leeward in S. latitudes.
2. Vertical force arising from sub-permanent magnetism; and vertical induction in vertical iron; which, in the usual position of the steering (aft) compass, draws the N. end of the needle to windward in ships built head North, to leeward in ships built head South.
Hence, an iron ship, built head North, will generally have a large heeling error to windward in N. latitude; and a small heeling error, which may be to windward or to leeward, in S. latitude.
Also, an iron ship, built head South, may be expected to have a small heeling error to windward or to leeward in N. latitude, and a considerable heeling error to leeward in S. latitude.
The heeling error being a maximum on the North and South points by compass, and nil at East and West, it is evident that the coefficient C is that which must be most affected, and if the deviation for an upright ship be,
d = B sin u + C cos z + D sin 2 % we shall have, for a ship heeling n degrees, with c taken to be the change in C for one degree of heel, the following formula
= B sin z + C + c no) cos 2 + D sin 2 % or, dn = d+ c no cos z that is, the heeling error alters proportionally to the number of degrees of heel and the cosine of the azimuth of the ship's head.
It is possible to put this, for practical purposes, in a more simple form. Since, in the northern hemisphere, in the majority of iron ships, the North end of the compass needle is drawn to windward (to the weather, or high, side) when the binnacles are above