the common term implies swerving from, and, in some other northern languages, mis-shewing. The sense is fully conveyed in the compound out-of-the-way-ness; that is, de-via-tion.

The term deviation is thus, if a new name is to be adopted, sufficiently precise and descriptive; and local deviation springs directly from it as the effect due to local circumstances. But the word declination is, at least by all scientific analogy, singularly unlucky; for magnetic declination, in the phraseology appropriated to the circles of the sphere, is an arc perpendicular to the magnetic equator.

Again, as to the dip, it is the fashion now to call this Inclination. This term, indeed, indicates angular position with respect to the horizon; but it does not, like the old word dip, answer the further object of directing the mind to that extremity of the needle which is below the horizon; because, in general, the term inclination relates to the direction of a line, and depression to the position of a point. We have, it is true, on the other hand, the term " depression of the horizon;" but these can never be confounded; and the question, moreover, is not the establishment of new terms for new things, but the changing of old terms for old things. Besides this, the term inclination seems the best adapted to the position of the ship herself; a consideration absolutely necessary in all questions of the compass, though perpetually overlooked: for which, therefore, it must be reserved, as its synonyme heel means, also, the foot of the sternpost.

The new terms are employed incidentally in the instructions given by the Council of the Royal Society to Captain Sir J. C. Ross on his expedition to the antarctic regions. But the assimilation of the scientific language of different countries, to the extent of two words, is no reason why we should abruptly depart from our oldestablished sea-terms to follow those of other nations less essentially maritime. It is accordingly to be hoped that scientific and intelligent seamen will strongly oppose all sudden changes in our marine vernacular, introduced on scientific or any other grounds, but especially one so ill considered as this, which tends directly to throw into confusion the slender vocabulary of those seamen who navigate thousands of our ships with the minimum of scientific knowledge, by entailing on us all the perpetual necessity of distinguishing between the declination of the sun, or any celestial body, and the declination of the compass.+

* This last term seems rather adapted to the action of machinery than of a physical cause; to a clock, for example, rather than the compass.

+ The word variation is clearly a misnomer. A thing varies when it does not remain the same. A clock varies when it does not shew the same time from day to day, whether the time shewn be true or not; but no one ever thinks of calling this error of the clock its variation. Again, when the course is wrongly shaped we do not call the error by the name of variation. What we mean by the variation of the compass is simply an error, but the word error is too vague; and we require to denote, out of all the errors to which the compass is liable, that particular one which has its source in the nature of magnetism, and this seems satisfactorily represented by the term deviation. We propose accordingly, after a proper period, to adhere systematically to the deviation of the compass, the dip of the needle, the depression of the horizon, and the inclination of the ship.

908. In this chapter we shall attempt merely a general enumeration of the principal phenomena of the tides, with such other matters as are of direct practical importance.*

I. PHENOMENA OF THE TIDES.

909. The connexion observed in all ages, and, with particular exceptions, in all places, between the succession of high waters and the moon's meridian passage, has established the belief that the moon is the cause of the tides. The principle of gravitation, on which the motions of the earth and the celestial bodies are calculated, and their figures explained, has confirmed, and at the same time corrected, this belief, by shewing that sensible effects must be produced not only by the moon, but also by the sun, though, from her greater nearness, the moon has by far the greater influence; and the general result would, naturally, until the observations were analysed, be attributed exclusively to her.

910. The attraction of the moon acting most strongly on those parts of the ocean which are nearest to her, that is, over which she is vertical, tends to draw these parts towards her, while their place is supplied by the water at the sides of the globe. And since the central parts are likewise more affected in the same action than the surface at the opposite or farthest side, the figure of the earth becomes elongated in the direction of a line drawn towards the moon; that is, the water is accumulated at the point exactly under

* The reader may refer, for additional information, to various papers, by Sir John Lubbock and the Rev. Dr. Whewell, in the Philosophical Transactions, &c., 1833, particularly to "An Essay towards a Map of Cotidal Lines," followed by other dissertations by Dr. Whewell; to Sir John Lubbock's "Elementary Treatise on the Tides," and to the "Annuaire des Marées" for 1839.

+ This principle is that there subsists amongst all particles of matter a mutual attraction, whose intensity is inversely as the square of the distance, or in the proportion described in No. 233, third paragraph.

the moon, and at another point distant from the former 180° in latitude and longitude. The moon, in her progress to the westward, causes thus, at each meridian in succession, a high water, not by drawing after her the water first raised, but by raising continually that under her at the time.

The opposite high water, or, as it is called, the inferior tide, would, if the moon's action was uninterrupted, follow the other, or superior tide, after the interval of half a lunar day, or 12h 24m on the average.

Again, the sun, acting in the same manner, though with less force than the moon (in consequence of his distance more than counterbalancing his greater magnitude), produces two tides, which would follow each other, if uninterrupted, after an interval of half a solar day, or 12 hours.

911. But, instead of four separate tides produced by the inde pendent actions of both bodies on the mass of waters in their original form, the effect produced is the same as if, after one of the bodies, as the moon for example, has given a form to the waters, the sun alters that form, the two separate actions thus producing a joint result. Hence the place at which it is high water is that at which the sum of the heights of the tides produced by the two bodies is greater than any where else.

912. When the sun and moon are on the meridian together, their actions concur, and the tide is higher than at any other time. The same holds when they are in opposition. These highest tides are called spring-tides, and occur after new and full moon. Again, when the sun and moon are 90° apart, their actions tend to neutralise each other; and the neap-tides, which occur after the first and third quarters of the moon, are the smallest of all. (See No. 919.)

913. Since the sun and moon act with greater force as they are nearer, the effect of each body in raising the tide is greater as its parallax is greater (No. 436). The highest spring-tides would occur, therefore, in January, about the time of the month when the moon's hor. par. is greatest. But the effect of both bodies is greater, generally speaking, as their alts. are greater, since when vertical the effect is greatest. This period, therefore, depends on circumstances.

But

914. If the actions of the sun and moon were, as we have hitherto supposed, uninterrupted by obstacles or forces of any other kinds, the tides would be regular, and their calculation certain. from the unequal depth of the ocean, and the barriers presented by continents which stand across the natural progress of the tides, their motion is interrupted, and the tide-wave (as the accumulation of waters is called), abandoned by the forces which originated it, becomes subjected to the mechanical action proper to waves in general.

915. It is necessary to distinguish between the motion of a wave and that of a current. A wave is not an absolute transfer of the body of moving water in the direction of the motion of the waves, but is a motion perpendicular to the surface, or up and down. The

notion of waves is represented in the fluttering of a flag and the shaking of a sail. It is easy to see that this kind of motion is compatible with immense velocity, without any appreciable current in the water itself; thus the tide-wave appears to pass from the Cape of Good Hope to Cape Blanco in twelve hours.

916. The motion of waves is quicker as the water is deeper. Also, the largest waves are the swiftest; a fact illustrated by the superior velocity of a heavy sea over that of the rippling of a pool. When the water shoals, the wave is retarded and becomes steeper on the advancing side, as is seen in the approach of waves to a shelving shore, and in the bores of rivers. The velocity of waves is also considered to be greater as their length (or distance from hollow to hollow) is greater; thus the tide-wave, though inferior in height to the waves of an agitated sea, yet travels with prodigiously greater velocity. Waves of different size and velocity merge into one another, as is known to those who have endeavoured to follow with the eye the waves of the sea. Lastly, when the waves meet with obstacles, such as sand-banks or reefs, the directions of their motions, as well as their figures, are changed. Several of the anomalies which the tides present are attributed to these and like circumstances.*

917. The current which accompanies the tide, and changes its direction with the ebb and flow, is the effect of the alteration of the level of the water during the passage of the tide-wave. Also, when a body of water in a channel has been set in motion, the motion does not immediately cease with the cause that produced it. Hence the tide-current does not necessarily, and in all cases, change with the tide; and thus, under certain circumstances, the current of the ebb continues to run for some hours after the flood-tide has made.

It is considered probable that many of the anomalies in recorded times of tide have arisen from thus confounding the time of high or low water with the time of slack water.

Admiral Beechey, who bestowed much attention upon the complicated niovements of the tides on our Western coasts, states that though each point of the coast in the Irish Channel has its proper time of high water, yet the turn of the stream takes place simultaneously to all, namely, about the time of high water at Morecombe Bay. This time is nearly that of Liverpool; accordingly, in order to know whether the stream is setting into the Irish Channel or out of it, it is necessary merely to find whether the tide is rising or falling at this place. Thus while the tide-wave, in coming in, is making it high water at the different places succeeding each other in its progress, the stream is, nevertheless, running out.

* Among the most curious of these effects are those called interferences, whereby twc distinct sets of waves may, in their combination, produce apparent rest. See Phil. Trans 1833, p. 154. On this principle are explained, also, tides which occur at irregular intervals † A Report of Observations made on the Tides in the Irish Sea, &c., by Capt. F. W Beechey, R.N., Phil. Trans. 1848; see also Naut. Mag. 1849, p. 70.

918. The height of the tide is the difference between the level of high water and that of low water.*

The height of the tide in the open ocean is supposed to be very small; and the great heights observed on some shores are evidently due to the shoaling of the water and the narrowing of the channel. The tides are insensible or very small in inland seas; as also in high latitudes, except from local causes.+

919. It is found, in general, that the tide is not due to the moon's transit immediately preceding, but to a transit which has occurred some time before. The time thus elapsed between the transit at which the tide originated and the appearance of the tide itself is called the retard, or age of the tide.

Thus the tide on the western coasts of Spain and France is a day and a half old; that at London is two days and a half old. It appears certain that the age of the tide on the W. coast of Ireland is 2 days (p. 38), and on the S. W. coast 1d 20h (p. 110).‡

It would appear further that changes in the parallax and declinations of the sun and moon produce their several effects on the time and height of the tide after particular intervals.

It is thus constantly necessary to discriminate between a tide which may happen after any particular transit and the tide which really corresponds to that transit; thus, for example, if the moon passes the meridian at 4 P.M. to-day, and the high water occurs at 7 P.M., this tide will not in general be that which corresponds to the transit 3 hours before, but may have had its origin several transits back. The transit to which the tide really corresponds is found by examining the observations of the several preceding tides, the highest of which, being due to the united actions of the sun and moon, is known to correspond to the moon's transit at 12 o'clock, noon or midnight.

920. The mean level of the sea is the middle between the levels of high water and low water.

Though the heights of high water and those of low water may vary considerably, yet the mean level seems eonfined to very narrow limits. Thus, at Singapore, where the heights of two consecutive low waters differ sometimes six feet, the mean level varies only a few inches.-Phil. Trans. 1837.

Hence it follows that heights measured above the sea should be referred to the mean level as the standard or zero, instead of that of either low or high water.

It is not, however, to be supposed that the middle point between any two consecutive tides is the mean level. This will be the case

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*The term range would be preferable to height, as it implies a distance between boun daries, as, for ex., the range of the barometer. The height of the tide" is continually, in common discourse, used for the height of the water.

† Sir John Ross found a rise and fall of 8 feet in lat. 74° N.

On the Law of the Tides of the Coasts of Ireland, by G. B. Airy, Esq., Astronomer Royal, Phil. Trans. 1845. This paper refers to a most extensive and complete series of observations made in 1842 under Gen. Colby, director of the Trigonometrical Survey, chiefly for the purpose of referring the elevations observed to the level of the sea.