Diagram No. 31 shows a different state of affairs. The Sun being at B and the Moon at A, their gravitations act at right angles to each other, and tend to balance each other; in consequence of this, the tide neither rises as high at E H and K D, nor falls so low at L F and G M as it does in the former instance, and we have Neap Tides.

Tides and Tidal Streams.-The Moon, owing to the Earth's rotation on her axis combined with the Moon's own motion, appears to revolve round the Earth in about 24 hours 48 minutes, thereby causing two tidal waves to sweep round beneath her, so that we have two high tides and two low tides in that time. These tidal waves are simply a rising and falling of the water, for little or no horizontal motion is imparted to it until, owing to the shape of the land, or the decrease in the depth of the water, this perpendicular movement is converted into a horizontal one.

Imagine yourself to be standing on the pier-head on a day when there is no wind, and the sea is glassy in its

smoothness, but when a swell is heaving in from seaward. As you stand at the end of the pier where the water is deep, you will notice that as each roller comes in the water rises, and as it passes away the water falls, there being hardly any horizontal movement. Now go to the beach, and there you will see that as a roller comes in its crest gathers velocity, and a stream of water pours rapidly in to your feet. This gives a rough illustration of a tidal. wave and a tidal stream. Evidently, therefore, tidal phenomena consist of two factors, first the undulation or rise and fall of the water-what is commonly called High Tide and Low Tide; second the stream, or horizontal movement of the water, which attends the tidal undulations. These two divisions of tidal phenomena must be considered quite independently. To know the amount of rise and fall of tide when wanting to take your ship over a bar, or into a tidal harbour, is a matter of great importance. To know how the tidal stream sets at any particular time, when you are in narrow waters and the weather is thick, is equally necessary.

I have spoken of the tidal undulations as waves, for in many respects they exactly resemble other waves; the only difference being the enormous distance between their crests. The biggest wind waves measure about 800 ft. to 1,000 ft. from crest to crest; but tidal waves would, if no land interfered with their regular progression, measure about 12,000 miles apart, or half the circumference of the globe, from crest to crest. These tidal waves follow the Moon round as far as is practicable, but the great continent of North and South America, and the enormous mass of land extending from the Cape of Good Hope to the Arctic Ocean, prevents them from obeying their natural impulse; and consequently, while proceeding more or less regularly in the South Atlantic and South Pacific

Oceans, where land does not obstruct them, they act quite irregularly everywhere else.

The tidal wave which sweeps round the Cape of Good Hope from East to West spreads in every direction, and a portion travels up the South Atlantic Ocean, proceeds onward up the North Atlantic Ocean, and finally arrives on our coast from the South-westward. That is to say the tidal wave, which, if there were no land to intervene, would always travel from East to West, reaches us from quite a different direction.

Let us consider for a moment the effect produced by this tidal wave approaching our coast from the Atlantic. At about 4 P.M. upon the day when the Moon is on the Meridian of Greenwich at about Noon, or as it is called technically at Full and Change of the Moon, the crest of the great tidal wave stretches from the West coast of France to the North-westward, curving to the Northward, and trending parallel to, and at a short distance from, the West coast of Ireland. It is quite clear that a line drawn along the crest of this wave will be the line of high water. Therefore, at all places and on any part of the sea traversed by this line, it will be high water at the same time. This line is commonly called the Co-tidal Line. At 5 P.M. on the same day this tidal wave has reached the South and West coasts of Ireland, and it curves up the Bristol Channel till within a few miles of St. Bride's Head, when it recurves South to the Scilly Islands and Land's End, and continues along the South. coast of England as far as Start Point, when, after bellying out a little to the Eastward up the English Channel, it trends Southerly to the coast of France.

The tidal undulation would on the Equator, if there were no obstruction by land or from any other cause, travel at the rate of about a thousand miles an hour, but

in our waters it advances only at the rate of from twenty to fifty miles an hour. It is divided by the coast of Ireland into two portions; the Northern portion is again divided by the coast of Scotland into two parts, one of which goes to the Northward round the Northern extremity of Scotland, while the other finds its way through the North Channel into the Irish Sea. The Southern portion passes the South coast of Ireland, and divides itself into three parts; one part goes up the Irish Channel and meets the tidal wave from the North in the centre of the Irish Sea; another part swells the water in the Bristol Channel; and the third part gives high water in succession to the ports of the English Channel. This latter portion at length meets in the Straits of Dover with the other tidal wave which has come round the North coast of Scotland, and made its way to the Southward along the East coasts of Scotland and England.

A glance at the position of affairs at 11 P.M. on the same day will be interesting. Two tidal waves have travelled up the Irish Sea, one coming from the Southward and the other from the Northward. A tidal wave has gone up the Bristol Channel, another up the English Channel, and another has swept round the Northern extremity of Scotland and has run down the East coasts of Scotland and England. The tidal waves have met in the centre of the Irish Sea, giving high water from Dublin to Carlingford Bay on the coast of Ireland, and from Liverpool Bay on the coast of England to the South of Scotland. It is also high water in the Straits of Dover, and at the mouth of the Thames, where the English Channel and North Sea tidal waves have met. And another tidal wave causes it to be high water about the North-eastern coast of Scotland. At the same time it is low water on the South-west and South coasts of

Ireland, on the South coast of England from the Scilly Islands, and Land's End to the Start, and on the East coast of England about Flamborough Head.

So much for the tidal waves at Full and Change; now let us turn our attention to the tidal streams. The Northern tidal wave enters the Irish Channel about 7 P.M., that from the South coming in one hour earlier. The tidal streams, however, which accompany these waves, have no such difference in their time of flow and ebb in any part of the Irish Sea, but are found to commence and cease simultaneously. The current sets to the Northward in its Southern portion during the flow of the tide, and the current runs to Southward in its Northern half during the same period. The exact reverse of this occurs during the ebb of the tide. A little to the Westward of the Isle of Man there is an area of between fifteen and twenty miles where no tidal stream manifests itself.

In the Irish Sea tides are referred to Liverpool, because it is a Standard Port; but as the times of slack water in the Irish Sea correspond with the times of high and low water at Fleetwood and Morecambe Bay, which occur twelve minutes earlier than at Liverpool, it is simpler in considering the tides to refer to Fleetwood.

For nearly six hours after low water at Fleetwood and Morecambe Bay the tidal streams are pouring into the Irish Sea, both from the Northward and Southward, and then for nearly six hours afterwards they are rushing outwards. Their velocity is about two-and-a-half knots an hour on an ordinary spring tide, except when the narrowing of the Channel, as at the Mull of Cantire, or the shoaling of the water in other parts, causes the speed to vary.

In the English Channel, and in the North Sea as far