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allowance in the length of the knot is made such that it shall obviate any causes of error arising when heaving the log; hence, for practical purposes at sea, the length of the nautical mile is often taken to be 6,000 feet. Then, on this rough basis, for the 30-second glass we readily get the length of the knot; 30 seconds, or half a minute, is the 120th part of an hour, and 6,000 divided by 120 gives 50 feet, which is the length generally taken ; and it must be clear to you that if the 120th part of a mile (as measured on the log-line) runs out in the 12oth part of an hour, the ship would be dragging along at the rate of 1 mile per hour; on the same basis, if eight 120ths passed out, she would be sailing at the rate of 8 miles an hour. But you must not assume that this is more than an approximation.
RULE.For accuracy a method of computation must be adopted equally applicable and simple for all glasses. Now there are 3,600 seconds in an hour, and 6,080 feet in a nautical mile, hence
= 47 ft. 33 in. These are the correct lengths for the 30 sec., and 28 sec., glasses ; and the statement is the same for any other glass, whatever it runs to.
Rough RULE.—Throwing away the odd 80 feet in the nautical mile, we have 6,000 feet; and since the ratio of 3,600 to 6,000 is as 6 to io, or as 3 to 5, hence the rule ; affix a cipher to the seconds run by the glass, and divide by 6, for the length of a knot in feet. Or, multiply the seconds run by the glass by 5 and divide the product by 3. Example.—Suppose, a 28-sec. glass, find the length of the knot. 6)280
46-66 feet. It is well, however, always to be accurate in your work : therefore, the rate of the glasses should often be examined by the aid of the chronometer, and the length of the knots measured. It is no uncommon thing to mark the length of the knot by two copper nails driven into the deck, at the proper distance apart; so this measure is always at hand.
The subdivisions of the knot, already referred to, and erroneously called fathoms (8 to a knot), are really nautical furlongs, after the same manner as 8 statute furlongs make a statute mile. The division into tenths is preferable.
Always bear in mind that the value of the operation of heaving the log depends conjointly on the accuracy of the instruments and the care bestowed in using them.
NOTE.—The log-line no less than the glass varies in its indications.
Glass too long gives distance too long.
Knot too long gives distance too short.
If BOTH are faulty :—Multiply faulty length of knot by erroneous distance, and divide the product by faulty time that the glass runs; threefifths of the result gives the true distance.
The following 'rules for correcting the ship's run, on account of the errors in the log-line or half-minute glass, are given on a supposition that the knot ought to measure 50 feet, and the glass to run 30 seconds.
It would, of course, be much simpler to find the length of the knot for the actual number of seconds run by the glass.
I. When the Log-line is truly divided, and the Glass faulty RULE.—Multiply the distance given by the log by 30; divide the product by the seconds run by the glass, and the quotient will be the true distance.
Example 1.-If a ship sails 8 Example 2.-Suppose the disknots by the log, while the glass is tance sailed by the log be 75 miles, running out, which when measured and the glass runs out in 27 seconds, is found to run 34 seconds, what is what is the true distance run ? her true rate of sailing ?
Distance by log 75 miles.
27)2250 (83.3 true distance Distance by log 8 knots.
90 34)240 (7 knots.
II. When the Glass is true, and the Log-line faulty RULE.—Multiply the distance sailed by twice the measured length of a knot; then point off two figures to the right, and the remainder will be the true distance.
Example 1.—A ship sails 9 knots Example 2.-If a ship sails 195 in half a minute, by a log measuring miles by a log which measures 52 feet; required the true rate of 48 feet, what is her true distance sailing.
195 miles. Distance by log..... 9 knots. Twice the length of a knot 96 Twice the length of a knot 104
1170 True rate .. 9:36
1755 or 9 knots 4 tenths nearly.
III. When the Glass and Log-line are both faulty RULE.—Multiply the distance sailed by the log, by six times the measured length of a knot, and divide the product by the seconds run by the glass; the quotient, pointing off one figure to the right, will be the true distance.
Example 1.—If a ship runs 5 Example 2.-Suppose the disknots of a log-line of 45 feet to a knot, tance sailed by the log be 150 miles, while a glass of 25 seconds is running the measured length of a knot being out, what is her true rate of sailing ? 51 feet, and the glass running 28
seconds; required the true distance
run. Distance run by log ..
5 Distance by log.... 6 times the length of a
6 times length of a knot
knot Seconds run by glass 25)135.0
28)4590-0 True rate of sailing
True distance run or 5 knots 4 tenths.
45 X 6
IV. To find the Length of a Knot corresponding to a Glass running any
given Number of Seconds RULE.-Add a cipher to the number of seconds run by the glass, and divide this by 6; the quotient will be the proportional length of a knot in feet.
Example 1.-What ought to be Example 2.-Required the the length of a knot when the glass length of a knot corresponding to a runs 33 seconds ?
glass that runs 28 seconds. 6)330
46.67 or 46 feet 8 in.
The Ground Log.--An adaptation of the common log is used in shoal water when the ship is drifting in a tideway, or amidst currents, with no land visible, or no distant object is seen whereby to fix the position. A lead, of 4 or 5 lbs., is made fast to a log-line and then cast overboard ; thus the lead rests on the bottom, and the rate and drift are indicated, irrespective of current, and can be noted as usual ; especially will this log show the drift of the current as it is hauled in.
The Dutchman's Log is a very old contrivance, and perhaps not the least accurate. On a ship's rail mark off a given distance. When about to take the rate, an observer must be stationed at each extremity of the distance. A bottle or log of wood is thrown overboard, from the forward station, and forward of the direction in which the ship is progressing; as
the log passes the forward station the time is noted, and similarly noted by the aft observer as it passes the aft station. Thus there is a distance, and an interval of time; divide the distance by the number of seconds of interval, and multiply the quotient by :6; or if you wish to be more accurate multiply by •59.
Example.—Suppose, distance 190 feet, and interval of passage 15 seconds, then
Various logs have been devised at different times—for instance, screw logs, pressure logs, and electric logs. Few successful. The log best known is that originally proposed by Massey.
Patent Log.-In days gone by there were no other mode of finding the ship’s rate of progress through the water except by the use of the log and seconds' glass, but the PATENT LOG has now, to a certain extent, supersedeu the old log-ship, especially in the case of steamers. This instrument, consisting of a rotator and register is kept towing astern with sufficient length of tow-line to carry it out of the immediate wake of the ship, and then the revolutions of the rotator indicate on the register the distance run, which can be ascertained from time to time by hauling it in. The mechanism of the patent log is the same in principle as that of the screw propeller; the rotator, of three or four flanges (B), revolves more or less rapidly according to
the rate at which it is drawn through the water, and, by revolving, sets in motion a system of wheel-work which turns the hands of three indices on the. register (C). The distance run, according to the number of revolutions of the rotator, is registered first in 4 miles, then up to 10 miles, and lastly up to 100 miles.
It is needless to dwell on the patent log, as there are many different kinds by different makers, and "descriptions for use ” accompany all of them. Up to a recent date it has been necessary to haul in the log to ascertain by the register the distance run; but the latest improvement in the mechanism is the arrangement by which the rotator alone tows, and the register is connected to it on board, so that without any hauling in the distance run can be known at any moment. The makers are numerous, and the logs of each have specialities of their own—some shipmasters preferring one maker's instrument, and others another maker's.
METEOROLOGY Meteorology is now understood to be the science which deals with the conditions and changes in the atmosphere. That is weather science. To the sailor the weather is an all-important subject.
Strong winds make high seas, and flat calms are often associated with dense fogs. The weather is contained in the atmosphere which, resting upon the earth, extends upwards to about 200 miles, becoming more and more rarefied with distance from the earth.
The average weight of a column of atmosphere is about 15 pounds to the square inch at the sea level. The human body supports a pressure of about 14 tons. The atmosphere is sometimes compared to the earth as the skin to an orange, but it is more true to scale if we say that it is comparable to the tissue paper in which an orange is wrapped.
The atmosphere is composed in volume of 77 parts of nitrogen, 21 parts of oxygen. There are small quantities of carbonic acid gas, argon, and other gases. There is also much water vapour in suspension; the density is measured by the barometer (see " Barometer '). The average height at the sea level is 29.92 inches, or 760 centimetres. At the height of 18,000 feet the pressure is į
Balloons having meteorographs (a combination of barometer and thermometer) attached have recorded heights of 79,200 feet (15 miles). Modern meteorologists divided the atmosphere into troposphere and stratosphere, an inner and outer layer. The inner, the troposphere, extends from sea level to about five miles high at the poles and seven at the equator, varying also with the height of the barometer. In the inner layer of atmosphere, the troposphere, the temperature of the air varies, uniformly decreasing one degree for every 300 feet of altitude.
The peculiarity of the outer layer, the stratosphere, is that the temperature does not vary uniformly, but is more or less constant over the British Isles at a height of from five to seven miles, the temperature being from - 20° F. to — 80° F. The temperature is highest at the poles, lowest in the neighbourhood of the equator. In Central Africa (Victoria Nyanza) the temperature of the stratosphere is about — 119° F.
The study of the upper air is still in its earliest stages. It is not yet possible to speak definitely of what takes place there. There is, however, a peculiar co-relation in the troposphere between geographical positions widely apart. One of the first to attract attention was the atmospheric oscillation between Iceland and the Azores. It has been found that in a given month, if the pressure at Iceland was above the average, at the Azores it would be below the average, and vice versa. Again, there is a high pressure system over Siberia and the low pressure over the North Pacific; also there seems to be a barometric see-saw between India and