This table may be used when we know the age of the tide. Thus let the age of the tide be a day and a quarter, and the mean lunitidal interval 2 h. 11 m.; let the moon's transit take place at 4 h.: then at the birth of the tide, a day and a quarter earlier, the transit took place at 3 h.; therefore the correction of the lunitidal interval is, by the table, 41 m., and the interval so corrected is 1 h. 30 m., which, added to 4 h., the time of moon's transit, gives 5 h. 30 m. as the time of high water.

To find the Establishment at any place when the Hour of High Water on a given day is observed.

On the given day, the time of moon's transit is known, and hence the lunitidal interval; and, by the above tables, the correction by which this differs from the establishment is known.

Thus, if high water occur at 5 o'clock when the time of moon's transit is 3 h., the lunitidal interval is 2 h.; and the correction (if the first table be applicable) is - 47 m.; hence the establishment is 2 h. 47 m.

NOTE (B).

NOTE TO ARTICLE 25.

The Rule of the Diurnal Inequality.

The Diurnal Inequality depends upon the moon's declination, as has been said already. It increases from 0 up to its maximum, and decreases to 0 again, as the declination does so; following these changes at an interval of one, two, or three days, according to the age of the tide. The rule is expressed in this way :

For north declination of moon,

Add to the tide following moon's south transit;

Subtract from the tide following moon's north transit.

For south declination of moon,

Subtract from the tide following moon's south transit;

Add to the tide following moon's north transit.

The south transit is the superior transit in the northern hemisphere, and the north transit the inferior. The contrary is the case in the southern hemisphere.

, Long.
Fixed scale in open water?
Tube with float?

Self-registering gauge?

Mode of deducing H. W. and L. W. Mere looking?

Ordinates

every 5m. near max.?

The general progress of the tide-wave (Art. 35) along even the most frequented shores is still imperfectly known; and about the connexion of the tides over the general areas of large oceans, we are as yet entirely in the dark: there is therefore an ample field of important and useful discovery in this subject, even by means of brief and scattered series of observations: still more is this the case if simultaneous or connected observations can be made, such as are described in Articles 39 and 40.

The main general features of the progress of the tides, as hitherto ascertained, are the following:

The tide-wave which brings the tides to the coasts of Europe comes from the Atlantic, and brings high water to the western coast of Spain and Portugal about 2 hours after the moon's transit; to the western coast of France about 3 hours; to the western coast of Ireland and to the Land's End about 4 hours. The tide-wave then runs along the south coast of England, and the north coast of France, to the Straits of Dover, which it reaches about 11 h. after the moon's transit. It runs also along the west coast of Ireland and Scotland, and reaches the Orkneys about 9 hours after the moon's transit. From hence it enters the German Ocean, and runs along the east * These columns to be filled at leisure (see Arts. 13, 41).

coast of Britain so as to reach Peterhead about twelve hours after the moon's transit, and Harwich in about 12 hours more; where it meets the tide-wave which had come through the Straits of Dover derived from the same Atlantic wave about 12 hours earlier. The tides of the German Ocean are produced by the mixture of these two tide-waves, and hence follow complicated laws: as for the same reason do those of the Irish Channel.

The tide-wave which brings the tides to the eastern coast of North America appears to reach the southern parts about 7 hours, and the northern parts of the United States about 11 hours after the moon's transit; but its course has not yet been distinctly traced.

How the tides on the eastern and on the western shores of the Atlantic are connected, has not yet been clearly shown. It is difficult to explain the tides of the Atlantic Islands (Madeira, Teneriffe, &c.) by any simple form of a tide-wave.

It is remarkable that the European tide-wave, though following the moon's transit at a definite interval (nearly), moves (at first) in a direction opposite to the moon; namely, from west to east.

If we go to the Pacific, we find the same phenomenon. The tides on the western shore of South America, near Cape Horn, also move from west to east. They are simultaneous with the moon's transit at Chiloe; 1 hour after at Cape Pillar; and at Cape Horn it is 3 hours later than this.

Along a large portion of the west coast of the Pacific, it seems difficult to say whether the tide-wave travels northward or southward. From the Isthmus of Panamá, however, it appears plainly to travel to the northward, occupying about 12 hours to run from Realejo to Nootka Sound.

In the western parts of the Pacific, the tide-wave runs to the westward, as we learn by its progress along the coasts of New Zealand and Australia, where the movement is better known than on any coasts out of Europe. It visits New Zealand about 6 hours, and Australia about 10 hours after the moon's transit at Greenwich.

In the central parts of the Pacific the tides are small and anomalous (for they do not clearly depend on the moon), and hence it is still more difficult to connect the littoral tides than in the Atlantic Ocean.

The outer regions of the Pacific, broken by large islands, and the Indian Ocean, have tides, of which the laws of progress are more complex, and have not yet been disentangled.

The Diurnal Inequality (Art. 23, &c.) adds to the complexity of the tides. This inequality appears very conspicuously in the tides on the west coasts of Europe and the east coasts of North America ; but its maximum in those two regions does not appear to be simul

K

taneous. It is very large in the Indian Ocean and on the coast ot Australia, having different phenomena at different places, as noted in Art. 26 and 27.

The movement of the tide along the surface of the ocean may be in some measure represented in the following manner :-Draw lines through all the places where it is high water at the same time; that is, one line (generally it will be a curved line) through all the places where it is high water at One o'clock; another line through all the places where it is high water at Two o'clock; and so on. These lines, being the lines at which the tide is contemporaneous, are called cotidal lines. They represent the form of the tide-wave which carries the tide from one point of the shore to another.

Such cotidal lines have been drawn (in the Phil. Trans. for 1833 and 1836, by Dr. Whewell) for those shores on which the tides are best known, and especially for the coasts of Europe.

But it appears that we cannot, by means of such cotidal lines, express the movement of the tides in oceanic spaces. The cotidal lines can only be drawn in the neighbourhood of coasts.-(Phil. Trans., 1848, Part I.)

A

2

The best way to disentangle the phenomena of the tides when we are observing them at any place, is to refer the time of high water and low water to the time of moon's transit; and to do this at once, while the series of observations is going on. For want of following this rule, it has very often happened that long series of tide observations have been made, which could not be turned to any use afterwards; and in almost every case, the usefulness of such observations is by this method much increased, and the labour much diminished.

SECTION V.

GEOGRAPHY.

BY W. J. HAMILTON, Esq., PRES. R.G.S., 1848-49.

In drawing up the following remarks for the use of those requiring information as to the principal points to which, in respect of geographical investigations, their attention should be mainly directed, the first thing which strikes us is the greater consideration now bestowed on this science than was the case not very long ago. As the study of the science of geography has of late years rapidly advanced, it has also risen in public estimation. Nor when we consider the nature of the subject, and our own national position, with our colonies extending to every quarter of the globe, our ships navigating every sea, and our travellers exploring every country, is there any reason to be surprised at such a result. It is no longer considered sufficient to possess correct maps of every state, with their political subdivisions and boundaries carefully laid down; it is no longer enough to have attained a perfect delineation of all the continents and oceans of the globe, with their bays and islets, their rivers and their mountains, and to have correctly fixed the position of all those physical features which constitute the surface of our earth. This is but the commencement of our science. The most perfect maps are but the