the height of tide inconsiderable. But even the Atlantic is not broad enough for the formation of a powerful tide-wave. The continents, the variation in the direction of the coastline, the different depths of the ocean, the narrowness of channels, all interfere to modify it. At first it is affected with only a slight current motion towards the west-a motion which only acquires strength when the wave is heaped up, as it were, by obstacles to its progress, as happens to it over the shallow parts of the sea, on the coasts, in gulfs, and in the mouths of rivers. Thus the first wave advancing meets in its course with resistance on the two sides of a narrow channel, it is forced to rise by the pressure of the following waves, whose motion is not at all retarded, or certainly less so than that of the first wave; thus an actual current of water is produced in straits and narrow channels; and it is always important to distinguish between the tide-wave, as bringing high water, and the tidal stream-between the rise and fall of the tide, and the flow and ebb. In the open ocean, and at a distance from the land, the tide-wave is imperceptible, and the rise and fall of the water is small among the islands of the Pacific 4 to 6 feet is the usual spring rise. But the range is considerably affected by local causes, as by the shoaling of the water and the narrowing of the channel, or by the channel opening to the free entrance of the tide-wave. In such cases the range of tide is 35 to 45 feet or more, as in the Bay of Fundy, in the river Severn, and at St. Malo, and where the tidal stream is one of great velocity. It may under such circumstances even present the peculiar phenomenon called the bore, as in the Hooghly and the Amazon Rivers, where a wave comes rolling in with the first of the flood, and, with a foaming crest, rushes onwards, threatening destruction to shipping, and sweeping away all impediments lying in its course. It is certain that in the open ocean the great tide-wave could not be recognised as a wave, since it is merely a temporary alteration of the sealevel. The progress of the tide-wave as it circulates round the globe is shown on a physical chart by a series of irregular curves called co-tidal lines; and thus these curves pass through all such places as have, at full and change, a contemporaneous tidal hour. In enclosed seas, as the Baltic and the Black Sea, there are no tides: in the Mediterranean and Red Sea, open at their entrance, the range is small. Shortly after the time of conjunction, when the moon is new-and at opposition, when the moon is full-the moon and sun are in such positions that their attractions produce the greatest effect on the waters, and the result is the highest tides, called the spring tides. When the action of the moon and sun are contrary, as when the moon is in quadrature, the tides have the least range and are called neap tides— the moon's action being then the least possible, and the sun's the greatest possible. High water occurs on the average of 28 days comprising the lunar month, at about the same interval after the time of the moon's crossing the meridian. This nearly constant interval, expressed in hours and minutes, is known as the lunitidal interval. The observed interval at the time of full and change at any port is the establishment of the port, which is an element necessary for the determination of the tidal hour to be derived from standard tables of reference. The corrected establishment, used in the United States Coast Survey, is the mean of all the intervals of the tides and transits of half the month. The time elapsed between the original formation of the tide and its appearance at any place is called the age of the tide, and sometimes the retard. The difference between the lunitidal interval and the correct establishment is the semi-monthly inequality. The height of the tide is subject to considerable perturbation from the weather; and the effect of winds from different directions in raising or lowering the mean level of the water is well known. The water also stands higher with low, and lower with high, barometer; to what exact extent is uncertain; estimates vary from 7 to 20 inches rise of water for an inch fall of the mercury. Again, the times of high and low water must not be considered to always coincide with the times of slack and change of current, the two phenomena being frequently quite distinct. In estuaries and rivers the water often still runs up-stream for long after the tide has turned, and when the water-level is falling; the converse is true of ebb and low water: the current in the offing compared with that near the shore often presents these peculiarities. In many estuaries and rivers the water rises much more rapidly than it falls. In some places there is a double high water, called tide and halftide; the second high water occurring within an hour or two of the first, making four high tides in the day, generally caused by some peculiarity in the coastline. Southampton has a double tide caused by the tide flowing in first at the Needles then again round St. Catherine's. The possibility of computing an accurate tide-table depends on the knowledge of certain tidal constants appropriate to the port, and most of these constants must be obtained from a series of observations at the port. The basis of the computation, whether for time or height of high water, chiefly depends on the time of the moon's transit, the semi-monthly inequality, and the corrections for the moon's parallax, the moon's declination, the sun's declination, and the sun's parallax. And in not a few places the tides are affected by a diurnal, and even by a semi-diurnal, inequality, the effect of the latter being that one high and one low water may be succeeded by a second high and low water of considerably diminished range; and again at certain of the moon's quadratures, in years of large lunar declination, the inferior tide may disappear altogether, and there will be only one high and one low water during the day. It is the custom of the Admiralty to publish, in advance, annual “ TIDE TABLES," which are indispensable to the navigator. These Tables give, for every day in the year, the predicted times and heights of the tides at all the principal ports of Great Britain and Ireland, and the times of low water at several prominent ports; also the times and heights of the tides at the principal ports of the world; and then by a system of "Tidal Constants" or tidal differences the times and heights at any port can be readily calculated. At the end of the Tide Tables is also given a description of the general set of the tides in the neighbourhood of several parts of the coast, including tides on the west coast of Scotland; a full account of the streams among the Orkneys, and through the Pentland Firth; the development of the movement of the great tide-wave up the English and Irish Channels, and into the North Sea; to which has been added a description of the set of the tides in the vicinity of Rathlin Island on the north coast of Ireland, and remarks on the tidal streams among the Channel Islands. The time of high water at Dover for every other day of the year is given upon a detachable leaf to be used with a pocket atlas of tidal streams round the British Isles. Lastly, there is appended for various places on the globe, arranged according to the apparent progress of the tide-wave, as well as alphabetically, the time of high water on the days of full and change; with the rise of the tide at springs and neaps. The United States Coast Survey publishes similar Tide Tables, with the necessary differences, for the Atlantic and Pacific coasts of the United States; and the Indian Government also issue, annually, very elaborate Tide Tables for the Indian ports and Hong Kong, giving the times and heights of both high and low water. By the aid of these special Tide Tables and the given differences, or through the difference of the establishments of two ports, the time of high water (H.W.) and the height of high water on any given day may be found, as will be seen in the sequel. Those who wish to know more about the tides should consult any of the following works :-Professor Haughton's "Manual of Tides and Tidal Currents"; "An Elementary Treatise on the Tides," by James Pearson, M.A., F.R.A.S.; or the Article on Tides by Professor G. H. Darwin, in the new edition of the Admiralty Manual of Scientific Enquiry; Captain Ruthven, and others. Time of High Water by Admiralty Tide Tables A port other than the standard one being given, to find the a.m. and p.m. times of high water on a certain day, proceed as follows: Turn to the Admiralty Tide Tables; seek for the given port, take out the time difference, and note its sign (+ or -) as well as the standard port of reference; turn to the given month, seek out the standard port, and for the given day will be seen the morning and afternoon times of high water at that port. If a blank or (—) occurs in either column it indicates no high water, and consequently there is but one high water on that day. At all ports there is always one high water every day, and since the greatest tidal difference never exceeds 8 hours, it follows that if a tidal difference be added to the morning time of high water for any day at the port of reference, the resulting time must remain in that day. Or again, if the tidal difference be subtracted from the afternoon time of high water for any day at the port of reference, the resulting time must still be in that day. And if the port of reference has only one time of high water on any day, this must be within one hour of noon, hence in this case also, whether the tidal difference be added or subtracted, the resulting time must be in the day used. Therefore one time of high water on a certain day can always be found by the following rule. When the time difference is +, that is, additive to the tidal hours at the port of reference, write down the morning time at the port of reference on the given day, and add the difference. If the sum is less than 12 hours the resulting time is a.m., but if the sum exceeds 12 hours, reject 12 hours and call the remainder p.m. When the difference is, that is, subtractive from the tidal hours at the port of reference, write down the afternoon time at the port of reference on the given day and subtract the difference, borrowing 12 hours if necessary. If 12 hours are borrowed, the resulting time is a.m., if not, then it is p.m. When there is only one time in the tables at the port of reference on the given day, write that time down and add or subtract the difference according to its sign, rejecting or borrowing 12 hours as necessary. If the operation is performed without rejecting or borrowing 12 hours the time keeps its name; but if 12 hours are rejected or borrowed the time changes its name. To find the Time of the other Tide (if any) When the time found as above is a.m. take from the tables the time following the one already used and apply the difference, rejecting or borrowing 12 hours as necessary. If the operation is performed without rejecting or borrowing 12 hours the time keeps its name; but if 12 hours are rejected or borrowed the time changes its name. When the time first found is p.m., take from the tables the time preceding the one already used and apply the difference, rejecting or borrowing 12 hours as necessary. If the operation is performed without rejecting or borrowing 12 hours, the time keeps its name; but if 12 hours are rejected or borrowed, the time changes its name. If the time now found has a different name to the first found time, it is the time required. But if it has the same name as the first found time, reject it, for it shows that there is no second time of high water on the given day. The following examples illustrate the use of the "Admiralty Tide Tables" Jan. 8th, 1913. Find the time of high water at St. Malo, a.m. and p.m. Jan. 29th, 1913. Find the time of high water at Dartmouth, a.m. and p.m. Jan. 20th, 1913. Find the time of high water at Gravesend, a.m. and p.m. London Bridge, Jan. 20th H. M. H. M. II 44 a.m. O 19 a.m. 21st 0 53 II 26 p.m. 20th Jan. 25th, 1913. Find the time of high water at Picton, a.m. and p.m. Jan. 18th, 1913. Find the time of low water at Liverpool, a.m. and p.m. H. M. Jan. 18th H. M. o 46 a.m. I 26 p.m. Jan. 7th, 1913. Find the time of high water at Annan Fort, a.m. and p.m. Use 1915 Tide Tables. Jan. 1st St. Malo. Find the a.m. and p.m. tides at the following places. Jan. 6th Chatham. Jan. 22nd Heligoland. 2nd Fowey. 7th Margate. 4th Antwerp. The soundings marked in small figures on the chart only agree with the lead line when a cast is taken at low water ordinary spring tides. At any other time the lead line will measure more or less water than is shown on the chart-mostly more. Less water can be shown only by a cast taken at dead low water extraordinary spring tides. It follows that at most times a correction is necessary, especially where the range of the tide is large. The correction is called the Reduction to Soundings. The following figure will give an idea of what is required. M.T.L. is the mean level of the sea, and the line worked from; the tide (generally speaking) rises three hours from this line and falls three hours below it. |