« AnteriorContinuar »
Fiducial Temperature in lat. 45° at sea level 285° A.
Correction for 50 feet + 10°
Fiducial Temperature in lat. 45° at a height of 50 feet
Latitude correction, Table I.
Fiducial Temperature in lat. 60° at a height of 50 feet 303° A.
Uncorrected reading 1028 millibars Correction for 25° diff. between Actual and Fiducial Tempera
tures from Table II. B. + 4•1 millibars
Scale Error — nil
From the above it is obvious that a table of Fiducial Temperatures for the height of the “ Barometer Cistern" above sea level in different latitudes much facilitates the correcting of barometer readings, as, for practical purposes, there is only the correction given in Table II. for the difference of temperature between the Actual Temperature and the Fiducial Temperature, in the particular latitude the observer is in, to be applied to the reading as shown by the barometer in order to reduce the reading to Standard Conditions.
BAROMETER (ANEROID) The aneroid barometer is another instrument for measuring changes in pressure. It consists of a circular metallic chamber partially exhausted of air and hermetically sealed. By an arrangement of levers and springs a hand is worked which indicates the pressure.
This instrument is particularly useful in ships, as it can be placed in a position immediately under the eye of the officer on deck, which, generally speaking, is not a practicable or advantageous position for a mercury barometer. The aneroid should be frequently compared with the mercury barometer, and corrected, when necessary, by means of the adjusting screw at the back. Whenever such an alteration of the index error is made, the fact should be clearly stated in the log book, or on any records of observations, as a guide to persons consulting the data for use in the future.
Readings of aneroids do not require correction for temperature, but only for height above sea level and index error. The figure given for the correction of the aneroid barometer of ships in communication with the Meteorological Office is frequently a combined result, and makes allowance for both height and index error
THE THERMOMETER This instrument shows increase or decrease of temperature but is not sensibly affected by changes of the pressure of the air. It consists of a glass tube of very small bore, closed at one end, and united at the other to a bulb, which is commonly filled with mercury. Thermometers intended for use in very cold climates are filled with spirit instead of mercury, which would freeze and solidify at the low temperatures of the Arctic regions, whereas spirit would not freeze. Mercury freezes at a temperature of about - 382° F. = 39° C.; spirit (pure alcohol) becomes a thick liquid at — 130° F., and solidifies into a white mass at — 202° F. Almost all substances expand when they are heated, and contract when they are cooled, but they do not all expand equally. Mercury expands more than glass, and so when the thermometer is heated the mercury in the bulb expands, and that portion of it which can no longer be contained in the bulb rises in the tube, in the form of a thin thread. The tube being very minute, a small expansion of the mercury in the bulb, which it would be difficult to measure directly, becomes readily perceived as a thread of considerable length in the tube. When the instrument is cooled the mercury shrinks, and the thin thread becomes shorter as the mercury subsides towards the bulb. By observing the length of the thread of mercury in the tube, as measured by the graduation on the scale at its side, or marked on the tube, the thermometer shows the temperature of the bulb at the time, which thus indicates the temperature of the surrounding air, or of any liquid in which the bulb is immersed.
The indications of a thermometer are recorded in degrees, the scale for which is obtained as follows. There are two fixed points on the scale according to which thermometers are graduated, viz., that at which ice melts, and that at which water boils. In the thermometers in ordinary use in England, the distance between these two points is divided into 180 parts, or degrees. When surrounded by melting ice an accurate thermometer on this scale indicates 32°, and if placed in boiling water, when the barometer reading is 30 inches, the reading is 212°. This graduation was adopted by Fahrenheit, a native of Dantzig, in the year 1721. Other graduations were devised about twenty years later ; one by Celsius, a professor at Upsala, in 1742, and another by Réaumur, a French physicist, at about the same period. Celsius suggested that the boiling-point be called zero, and the freezing-point 100°. The modern Centigrade scale, which is an adaptation of the Celsius, is in general use at the present time in most Continental countries, the freezingpoint is taken as zero, and the boiling-point as 100°. Réaumur framed a scale similar to the Centigrade but divided the interval between the freezing and boiling-points into eighty divisions. This scale, which at one time was commonly employed on the Continent, is now almost obsolete.
The Absolute scale is yet another measure of temperature that has been introduced, based on the researches of the late Lord Kelvin, Dr. J. P. Joule, and others, who found the absolute zero of temperature to be 273° Centigrade below the freezing-point of water, or 4590 on the Fahrenheit scale. This zero of temperature is based on the doctrine of the dissipation of energy, heat having for a long time previously been recognised as a form of energy. It represents, so far as our present knowledge goes, the temperature at which the whole of the heat of any substance whatever would have been converted into some other form of energy. The principal advantage of the Absolute scale for meteorological work is that all negative values are avoided.
F° = C° X? +32°, and Co
= (F° -- 32”) xing
where F° = degrees Fahrenheit and Co
Also see Norie's Tables, Table for conversion of temperature readings of Fahrenheit and Centigrade scales to the Absolute scale.
This instrument measures the humidity of the air. There are several kinds of hygrometers, but the easiest to make and to manage consists of a pair of thermometers placed near each other. If one of these be fitted with a single thickness of fine muslin or cambric fastened tightly round the bulb, and this coating be kept damp by means of a few strands of cotton wick, which are passed round the glass stem close to the bulb so as to touch the muslin, and have their lower ends dipping into a cup of water placed close to the thermometer, it will usually show a temperature lower than that shown by the other thermometer which is near it, the amount of the difference, commonly called the depression of the wet bulb, being dependent on the degree of dryness of the air.
To ensure correct records of the temperature and humidity of the air, the dry- and wet-bulb thermometers should be placed in a screen, the sides of which are protected from the sun and rain by narrow sloping boards overlapping each other, but with spaces between, so as to let in the air freely.
This instrument is employed for determining the specific gravity of liquids. The hydrometer used at sea is constructed of glass. If made of brass the corrosive action of salt water soon renders the instrument erroneous in its indications. The form of the instrument in common use is shown in Fig. 1. It consists of a glass tube ending in a globular bulb partly filled with mercury or small shot, to act as ballast and to make the instrument float steadily in a vertical position. From the neck of the bulb the glass is expanded into an oval or cylindrical shape, to give the instrument sufficient volume for flotation ; above this it is tapered off to a narrow upright stem closed at the top, attached to which is an ivory scale. The divisions on the scale read downwards, so as to measure the length of the stem which stands above the surface of any fluid in which the hydrometer is floated. The denser the fluid, or the greater its specific gravity, the higher will the instrument rise; the rarer the fluid, or the smaller its specific gravity, the lower it will sink.
The indications depend upon the well-known principle that any floating body displaces a quantity of the fluid which sustains it, equal in weight to the weight of the floating body itself. According, therefore, as the specific gravities of fluids differ from each other, so will the quantities of the fluids displaced by any floating body, or the depth of its immersion, vary, when it is floated successively in each.
The specific gravity of distilled water, or its relative weight, compared at the temperature of 62° F., to an equal volume of other substances, being taken as unity, the depth at which the instrument remains at rest when floating in distilled water is the zero of the scale on which its indications are recorded. If the specific gr
If the specific gravity or the density of the water be increased, as it is by the presence of salt in solution, the hydrometer will rise, and the scale is so prepared as to indicate successive increases of density up to 4 per cent., or 40 in the thousand parts. The graduations thus extend from o to 40, the latter corresponding to the mark on the scale which will be level with the surface when the instrument is placed in water, the specific gravity of which is 1.040. In recording observations, the last two figures only-being the figures on the scale are written down. There has recently been introduced an hydrometer of more open scale, which has a range of from 15 to 35 (Fig. 2), instead of from o to 40, as in Fig. 1. This change will facilitate reading, and serve nearly every purpose for observations on board ship.
The instrument is used to show the relative density of different parts of the ocean. It may float at 40 or even higher in some parts of the Suez Canal, where the water is exceedingly salt. On the western side of the North Atlantic, in the Tropics, Bay of Bengal, and Black Sea, and in the vicinity of the mouth of a large river, the hydrometer will sink much deeper, owing to the comparative freshness of the water. The water employed for taking the specific gravity of the sea should be drawn in a bucket from over the ship's side, forward of all ejection pipes, and its temperature immediately observed and recorded, so that by its aid the specific gravity may be reduced to what it would have been at the temperature of 62° F. as explained below. The hydrometer should be slightly spun in the centre of the bucket ; it soon loses any up-and-down motion ; and the scale can be read before the turning motion has entirely ceased.
Fig. 2. Whenever the temperature of the water tested differs from 62°, a correction to the reading is necessary, for the expansion or contraction of the glass, as well as for the temperature of the water itself, in order to reduce all observations
Fig. 1. to one generally adopted standard.
When using the hydrometer, it should be scrupulously clean, all dust, smears, or greasiness being got rid of by wiping the instrument with a clean soft cloth, before and after use.
THE TIDES AND ON THE CORRECTION FOR SOUNDINGS Tidal phenomena present themselves under two aspects : as alternate elevations and depressions of the sea, and as recurrent inflows and outflows of streams. Careful writers, however, use the word tide in strict reference to the changes of elevation in the water, while they distinguish the recurrent streams as tidal currents. Hence, also, rise and fall appertain to the tide, while flood and ebb refer to the tidal current. Stand should be used specifically for the period of time, at high or low water, when no vertical change can be detected; and slack for the period of time when no horizontal motion can be detected. Set and drift are applicable only to tidal currents, as indicating direction and velocity. The range of the tide is the height from low water to high water.
The cause of the tides is the combined action of the sun and moon. The relative effects of these two bodies on the oceanic waters are directly as their mass, and inversely as the square of their distance; but the moon, though small in comparison with the sun, is so much nearer to the earth that she exerts the greater influence in the production of the great tide-wave : thus the mean force of the moon, as compared with that of the sun, is as 2} to I.
The attractive force of the moon is most strongly felt by those parts of the ocean over which she is vertical, and they are, consequently, drawn towards her; in the same manner the influence being less powerfully exerted on the waters furthest from her than on the earth itself, they must remain behind. By these means, at the two opposite sides of the earth, in the direction of the straight line between the centres of the earth and moon, the waters are simultaneously raised above their mean level; and the moon, in her progressive westerly motion, as she comes to each meridian in succession, causes two uprisings of the water—two high tides—the one when she passes the meridian above, the other when she crosses it below; and this is done, not by drawing after her the water first raised, but by raising continually that under her at the time; this is the tide-wave. In a similar manner (from causes already referred to) the sun produces two tides of much smaller dimensions, and the joint effect of the action of the two bodies is, that instead of four separate tides resulting from their separate influence, the sun merely alters the form of the wave raised by the moon; or, in other words, the greater of the two waves (which is due to the moon) is modified in its height by the smaller (sun's) wave. When the summit of the two happens to coincide, the summit of the combined wave will be at the highest; when the hollow of the smaller wave coincides with the summit of the larger, the summit of the combined wave will be at the lowest.
If the earth presented a uniform globe, with a belt of sea of great and uniform depth encircling it round the equator, the tide-wave would be perfectly regular and uniform. Its velocity, where the water was deep and free to follow the two luminaries, would be 1,000 miles an hour, and