CURRENTS.

A CURRENT is a progressive motion of the water, causing all floating bodies to move that way towards which the stream is directed. The set of a current is that point of the compass towards which the waters run, and its drift is the rate it runs per hour. The most usual way of discovering the set and drift of an unknown current, is the following, supposing the current at the surface to be much more powerful than at a great distance below the surface:

Take a boat a short distance from the ship, and, by a rope fastened to the boat's stern, lower down a heavy iron pot or loaded kettle to the depth of 80 or 100 fathoms; then heave the log, and the number of knots run out in half a minute will be the miles the current sets per hour, and the bearing of the log will show the set of it.

There is a very remarkable current, called the Gulf Stream, which sets in a north-east direction along the coast of America, from Cape Florida towards the Isle of Sables, at unequal distances from the land, being about 75 miles from the shore of the southern States, but more distant from the shore of the northern States. The width of the stream is about 40 or 50 miles, widening towards the north. The velocity is various, from one to three knots per hour, or more, being greatest in the channel between Florida and the Bahamas, and gradually decreasing in passing to the northward, but is greatly influenced by the winds, both in drift and set.

We are chiefly indebted to Doctor Franklin, Commodore Truxton, and Mr. Jonathan Williams, for the knowledge we possess of the direction and velocity of this stream. Its general course, as given by them, is marked on the chart affixed to this work. They all concur in recommending the use of the thermometer, as the best means of discovering when in, or near, the stream; for it appears, by their observations, that the water is warmer than the air when in the stream; and that at leaving it, and approaching towards the land, the water will be found six or eight degrees colder than in the stream, and six or eight degrees colder still when on soundings. Vessels coming from Europe to America, by the northern passage, should keep a little to the northward of the stream, where they may probably be assisted by a counter current. When bound from any southern port in the United States of America to Europe, a ship may generally shorten her passage by keeping in the Gulf Stream. By steering N. W. you will generally cross it in the shortest time, as its direction is nearly N. E.

In other parts of the Atlantic Ocean, the currents are variable, but are generally south-easterly along the coast of Spain, Portugal, and Africa, from the Bay of Biscay towards Madeira and the Cape de Verds. Between the tropics, there is generally a current setting to the westward.

There is also a remarkable current which sets through the Mozambique Channel, between the Island of Madagascar and the main continent of Africa, in a south-westerly direction. In proceeding towards Cape Lagullas, the current takes a more westerly course, and then trends round the cape towards St. Helena. Ships bound to the westward from India, may generally shorten their passage by taking advantage of this current. On the contrary, when bound to the eastward, round the Cape of Good Hope, they ought to keep far to the southward of it. However, there appears to be a great difference in the velocity of this current at different times; for some ships have been off this cape several days endeavoring to get to the westward, and have found no current; others have experienced it setting constantly to the westward, during their passage from the cape towards St. Helena, Ascension, and the West India Islands. Instances have however occurred, where an easterly current was experienced off the Cape of Good Hope.

All cases of sailing in a current are calculated upon the principle that the ship is affected by it in the same manner as if she had sailed in still water, with an additional course and distance exactly equal to its set and drift. On this principle the projection

EXAMPLE.

If a ship sail 98 miles N. E. by N., in a current which sets S. by W. 27 miles, in the same time, required her true course and distance.

OF THE LOG-LINE AND HALF-MINUTE GLASS.

VARIOUS methods have been proposed for measuring the rate at which a ship sails; but that most in use is by the Log and Half-Minute Glass.

The Log is a flat piece of thin board, of a sectoral or quadrantal form (see Plate VI. fig. 3), loaded, on the circular side, with lead sufficient to make it swim upright in the water. To this is fastened a line, about 150 fathoms long, called the log-line, which is divided into certain spaces called knots, and is wound on a reel (see Plate VI. fig. 4) which turns very easily. The Half-Minute Glass is of the same form as an Hour Glass (see Plate VI. fig. 2), and contains such a quantity of sand as will run through the hole in its neck in half a minute of time.

The making of the experiment to find the velocity of the ship, is called heaving the log, which is thus performed :-One man holds the reel, and another the half-minute glass; an officer of the watch throws the log over the ship's stern, on the lee side, and when he observes the stray line is run off (which is about ten fathoms, this distance being usually allowed to carry the log out of the eddy of the ship's wake), and the first mark (which is generally a red rag) is gone off, he cries, Turn; the glass-holder answers, Done; and, watching the glass, the moment it is run out, says, Stop. The reel being immediately stopped, the last mark run off shows the number of knots, and the distance of that mark from the reel is estimated in fathoms. Then the knots and fathoms together show the distance the ship has run the preceding hour, if the wind has been constant. But if the gale has not been the same during the whole hour, or interval of time between heaving the log, or if there has been more sail set or handed, a proper allowance must be made. Sometimes, when the ship is before the wind, and a great sea setting after her, it will bring home the log. In such cases, it is customary to allow one mile in ten, and less in proportion if the sea be not so great. Allowance ought also to be made, if there be a head sea.

This practice of measuring a ship's rate of sailing, is founded upon the following principle that the length of each knot is the same part of a sea mile, as half a minute is of an hour. Therefore the length of a knot ought to be of a sea mile; but, by various admeasurements, it has been found that the length of a sea mile is about 6120 feet; hence the length of a sea knot should be 51 feet. Each of these knots is divided into 10 fathoms, of about 5 feet each. If the glass be only 28 seconds in running out, the length of the knot ought to be 47 feet and 6 tenths. These are the lengths generally recommended in books of navigation; but it may be observed, that, in inany trials, it has been found that a ship will generally overrun her reckoning with a log-line thus marked; and, since it is best to err on the safe side, it has been generally recommended to shorten the above measures by 3 or 4 feet, making the length of a knot about 74 fathoms, of 6 feet each, to correspond with a glass that runs 28 seconds.

In heaving the log, you must be careful to veer out the line as fast as the log will take it; for if the log be left to turn the reel itself, the log will come home and deceive you in your reckoning. You must also be careful to measure the log-line pretty often, lest it stretch and deceive you in the distance. Like regard must be had that the halfminute glass be just 30 seconds; otherwise no accurate account of the ship's way can be kept. The glass is much influenced by the weather, running slower in damp weather than in dry. The half-minute glass may be examined by a watch, with a second hand, or by the following method:-Fasten a plummet on a line, and hang it on a nail, observing that the distance between the nail and middle of the plummet be 39 inches; then swing the plummet, and notice how often it swings while the

To correct the distance when the log-line and half-minute glass are faulty.

If there be any error in the log-line or glass, the measured distance must be corrected in the following manner, supposing that a 30" glass requires 50 feet to a knot:

(1.) If the glass only is faulty, you must say, As the seconds run by the glass are to 30 seconds, so is the distance given by the log to the true distance. Thus, if a ship sails 8 knots per hour, by a glass of 36 seconds, the true number of knots per hour will be 7.1; for 36: 30 :: 8.5 7.1.

(2.) If the log-line only is faulty, you must say, As fifty feet is to the distance of a knot on the line, so is the distance run by the log to the true distance. Thus, if a ship sails 7 knots per hour, by a log-line measuring 53 feet, her true distance will be 7.4 miles per hour; because 50: 53 :: 7 : 7.4.

(3.) If the log-line and glass are both faulty, you must say, As 50,* multiplied by the length of the glass, is to 30, multiplied by the length of the line, so is the measured to the true distance. Thus, if a ship sails 6 knots per hour, with a glass of 24 seconds, and a log-line of 60 feet per knot, her true velocity will be 9 miles per hour, because 50 X 24: 30 X 60 :: 6 : 9.

*Instead of multiplying the length of the glass by 50, and the line by 30, you may multiply the former by 5, and the latter by 3. If any one chooses to mark the log-line at less than 50 feet for a glass of 30 seconds, he must put his estimated length of the knot, instead of 50, in ali the above rules.

DESCRIPTION AND USE OF A QUADRANT OF REFLECTION.

MR. JOHN HADLEY was the first who published a description of the Quadrant of Reflection, for measuring angular distances; and the instrument still bears his name, although it has been ascertained that Sir Isaac Newton invented a similar one some years before, but never made it public. One of our countrymen, Mr. Thomas Godfrey, of Philadelphia, had also contrived an instrument, on the same principles, some time before Mr. Hadley made known his discovery.

Plate IX., figure 1, represents a quadrant of reflection, the principal parts of which are, the frame ABC, the graduated arc BC, the index D, the nonius or vernier scale E, the index glass F, the horizon glasses G and H, the dark glasses or screens I, and the sight vanes K and L.

The graduated arc BC is an octant, or eighth part of a circle, but, on account of the double reflection, is divided into 90°, numbered from 0° towards the left, and each degree is commonly divided into three equal parts, of 20 minutes each. The gradua tion on the limb is continued a few degrees to the right of 0°. This portion is called the arc of excess, and is found very convenient for several purposes.

The index D is a flat bar, commonly made of brass, movable round the centre of the instrument, and broader towards the axis of motion, where is fixed the index glass F; at the other end is fixed the nonius or vernier scale, used in estimating the subdivisions of the arc; at the bottom or end of the index, there is a piece of brass which leads under the arc, having a spring to make the vernier lie close to the limb, and a screw to fasten it in any position. Some quadrants have a tangent screw affixed to the lower part of the index to adjust its motion. The vernier is a small, narrow slip of brass or ivory, fixed to that part of the index which slides over the graduated arc, and usually contains a space equal to 21 or 19 divisions of the limb, and is divided into 20 equal parts. Hence the difference between a division on the limb, and a division on the dividing scale, is one twentieth of a division of the limb, or one minute. Therefore, if any division on the vernier is in the same straight line with a division of the limb, then no other division on the vernier can coincide with a division of the limb, the extreme divisions excepted. Some time ago, it was usual to reckon the divisions on the vernier from its middle towards the right, and from the left towards the middle; but, this being found inconvenient, a more commodious method has been introduced of numbering from right to left. Hence the degree and minute pointed out by the vernier, may be found thus:-Observe what minute on the vernier coincides with a division on the limb; then this minute, being added to the degree and parts of a degree on the limb immediately preceding the first division on the vernier, will be the degree and minute required. Thus, suppose 10' on the vernier coincides with a division on the limb, and that the division on the limb preceding the first division of the vernier is 8° 20′; the division pointed out by the vernier will be 8° 30'.

The index glass F is a plane speculum or mirror of glass, quicksilvered and set in a brass frame. It is so placed that the face of it is perpendicular to the plane of the instrument, and is fixed to the index by the screw M; the other screw N serves to replace it in a perpendicular position, if, by any accident, it has been put out of order. The use of this mirror is to receive the rays from the sun, or other object observed, and reflect them towards the horizon glasses.

The horizon glasses G and H are two small speculums. G is called the fore horizon glass, from its being used in the common or fore observation, where the observer's face is turned towards the object; and H the back horizon glass, being used in the back observation, where the observer's back is turned towards the object. These mirrors receive the reflected rays from the index glass, and reflect them to the eye of the observer. The horizon glasses are not entirely quicksilvered. The fore horizon glass G is only silvered on the lower half, the other half being transparent, and the