III. TIDE-OBSERVATIONS.

936. It is evident, from what has been said (Nos. 919, 922), that the establishment cannot be truly deduced from the notice of a solitary high water; and that observations, continued, at least through a semi-lunation, are necessary for even a tolerable approximation. But the true establishment cannot be successfully determined from a series of observations involving the semi-menstrual inequality, the various effects of changing declinations and parallaxes, with temporary and local circumstances, except by persons not only thoroughly versed in arithmetical operations on an extensive scale, but well exercised in the particular intricacies of these laborious calculations. We have, therefore, confined ourselves here to merely indicating the details which should accompany tide-observations.

(1.) The exact spot of observation must be specified.

(2.) The instant of both high water and low water should be stated, with the height, or difference of the two levels, in feet and inches. As the water hangs for some time towards the turn of the tide, and as the tide-current may be independent, it is necessary to note the instant at which the water passes a fixed mark, both in rising and falling; the means of these times are the instants of high and low water respectively. The marks should be fixed in some place to which the water passes slowly, because the waves, however small, continually washing over the marks, render it difficult to detect a small rise or fall of the water.

The observations of both low and high waters of the 24" are necessary for determining the Diurnal Inequality; but as the time of this inequality is of less importance than the height, it will often be enough, in respect to this particular point, to note the height

alone.

About mean water (or half tide) the surface rises or falls with greater velocity than at any other time, and accordingly the instant at which the water passes a fixed mark or a given horizontal line may be observed with greater precision than at any other time. Hence it has been recommended to notice the instant of passing one or two such marks, instead of the times of high and low water."On the Law of the Rise and Fall of the Sea's Surface during each Tide."-Phil. Trans., Part II. for 1840.

It has been proposed to place the marks at half-tide, but this does not answer, especially where the diurnal inequality is consider

able. The intervals should be short on either side, of high and low water, because the tides do not rise and fall with equal velocity.

(3.) The times of slack water should be noted.

(4.) The direction, and, in general terms, the force of the wind, should be stated, as, also, the height of the barometer.

As the effects of winds and atmospherical changes are not confined to the particular hours during which such causes are in action, it will be proper, when only a short series of observations can be obtained, to add further a brief notice of the state of the weather for some time previous.

Observations continued for a fortnight afford a first approximation to the Tide-hour; and when carried on for some months, this, with some other principal elements, may be obtained with considerable accuracy.

937. The custom has prevailed of noting the establishment as the hour of the day; but it obviously should, as recommended by Dr. Whewell (Phil. Trans. 1833, p. 229), be considered merely as an interval. Since the correct establishment is measured from twelve o'clock, it may, indeed, appear to be indifferent whether we call it an absolute time or an interval; but the absolute time of the tide is in all cases referred to the instant of the moon's transit, and it is absurd to talk of adding two absolute times together; as, for example, adding three o'clock of the day to five o'clock of the day. Also, by considering the establishment as an interval only, we avoid confounding mean and apparent times.

938. The soundings on the charts are the depths at "low water;" but this term may imply indifferently the mean low water of the whole year, or of the equinoctial spring-tides, of which the average is not always identical, or of those low waters only which were observed during the operations of survey. Since these may differ considerably from each other, the computed depth may be in error by the same difference. It might appear less equivocal if the lowest of all the low waters were understood; but this, though a natural phenomenon, and, so far, preferable to an imaginary standard, as an average, is still defective, since it is affected by winds. It would appear, therefore, as Capt. Beechey proposes,† that the standard low water should be identified as so many feet and inches below the mean level, which appears to be the only element nearly constant.

The mean level may, it appears, be found approximately by observations of four consecutive tides, which include the diurnal inequality.

Adm. Bayfield (to whom I am indebted for some important remarks and corrections here and elsewhere in the former editions) informs me that in the St. Lawrence the alternate ebbs do not fall to the half-tide mark at all when the diurnal inequality is considerable. Also Adm. Beechey acquainted me, as the result of numerous observations, that at Plymouth the half-interval of time between the passages over the half-tide marks requires of the whole int. to be added to it for the correct time of high water, in consequence of the unequal rise and fall.

A Report of Observations," &c.

NAVIGATING THE SHIP.

I. SHAPING THE Course. II. PLACE OF THE SHIP. III. DETERMINING THE CURRENT. IV. STORMS. V. MAKING THE LAND.

939. IN the preceding part of this volume each point of the subject has been treated separately. The present section, which will conclude the PRACTICE, and to which the former chapters may be considered subservient, contains matters of general reference in conducting the navigation of the ship.

I. SHAPING THE COURSE.

940. As soon as the ship is clear of the land, and circumstances permit, her head is put upon the course to be steered, the log hove, and the departure taken.

When the course is to be shaped for a distant port, recourse is had, in defect of personal experience, to the Sailing Directions,* in order to learn what point to steer for, so as to profit by particular winds or currents, or to avoid dangers. The bearing of such point is then worked for by parallel, middle latitude, or Mercator's sailing, according to the case; or, a ruler being laid on the chart over the place of departure, and the point in question shews the course,

No. 381.

941. When the wind is foul, reference will be made to No. 299; but, in the case of a prevailing foul wind, the proper line of proceeding will be indicated in the Sailing Directions.

A steam-vessel will generally preserve her course without regard to the wind, except in long passages.

* The Sailing Directions contain descriptions of ports and anchorages, with accounts of the winds, currents, and tides, for various coasts and seas. Besides these and other particulars, necessary for navigation alone, works of this kind contain well-selected passages from voyages and travels, by which the reader may obtain clear ideas of the physical aspect of the shores, climate, and natural phenomena of most parts of the world, and derive considerable information respecting the manners and customs of the inhabitants, the productions, and articles of merchandise.

1. Shaping the Course in a Current.

942. When the whole or any part of the voyage lies through a current, having everywhere the same direction and velocity, it is proper to shape that course which shall keep the port on the same bearing (No. 294), because the ship will thus cross the current in the shortest possible time. But if the current be different in different parts of the voyage, this rule does not hold good. This point cannot be pursued further in this volume.

When the current, setting the ship away from her port, is so strong, or the wind so light, that the ship cannot preserve the bearing of the port unaltered, she will be kept so that the course made good shall not be more than eight points from the bearing of the port; because, though she cannot thus near the port till circumstances change, yet she will not increase her distance from it, as would result from shaping any other course.

The application of all such rules must, accordingly, depend upon the circumstances of the case.

943. When the ship, having a foul wind, is in a current of which the direction and rate are known, she should be kept as much as possible on that tack on which the current tends most to drift her to windward, or is least unfavourable in drifting her to leeward.

For example. Suppose the course to be steered is S.W., the wind S.S.W., the current S.S.E., 2 knots. Then, on the larboard tack, lying west, and going, suppose, 6 knots, she will make good S. 70° W. 5.5 miles, No. 292. On the starb. tack, lying S. E. and going knots, she will make good S. 39° E., 8 knots. The distance made good in the direction of the port when her head is S E. is o'8 miles per hour, No. 285; when lying west, this quantity is 5 miles.

In this case the current tends to drift the ship to windward on both tacks; but the larboard tack is the most favourable.

2. Shaping the Course on a Great Circie.

944. When the sains on the arc of a great circle, the distance traversed in passing between any two points in her track is (as observed in Nos. 336, &c.) less than if she had sailed on a rhumb-line. A distinction of greater importance between these two tracks is, however, that every point of the great circle lies in a higher latitude than any point, having the same longitude, on the rhumb-line. Thus, if two ships sail from St. Helena to C. Horn, the one upon the great circle, and the other on the rhumb-line, altering their longitude by the same quantity, the ship on the circle will be 430 miles to the southward of the other, when the two vessels are most widely separated; that is, when the vessel on the circle is at the point of maximum separation in latitude (No. 345). Now the difference of distance is only 76 miles in 3740 (No. 337, Ex. 1); whereas the difference of 430 miles in latitude may place the vessels in different winds.

945. A course taken anywhere between the great circle and the rhumb-line will always be attended with at least some saving of

distance.