The moon's average distance from the centre of the earth (A to C, in the diagram) is about 240,000 miles, or about 60 times the earth's radius (B C), which is a little under 4000 miles, consequently if the distance from the moon to the centre of the earth (A C) be expressed by 60, the distance to the nearest surface (A B) will be 59, and the farthest surface (A D) 61. The moon will attract the point B with a force of 1/592 or 1/3481, the point C with the force of 1/602 or 1/3600, and the point D with the force of 1/612 or 1/3721. Now as the force at C is greater than at D, the earth itself will be pulled away from the water at D, and leaving it a little behind, will produce a high tide there, this tide is known as the "inferior tide" (2). Again as the force at B is greater than at C, the water at B will be pulled away from the earth and produce a still higher tide, called the "superior tide" (1), or that which takes place when the moon is due south. This heaping-up of the waters by the moon, to the height of 4½ feet (on an average) causes corresponding depressions to the same depth in those parts of the earth which are situated between the raised parts, making a difference between high and low-water of nine feet due to the moon; precisely the same effects are produced by the sun, but though his attractive power is so much greater than the moon's, his distance is also so much greater, that the heap of water he can raise is somewhat less than 2 feet, and makes therefore less than 4 feet difference between high and low-water, due to the sun. When the sun and moon are both on the same side, or on opposite sides of the earth (as at new or full moon), then the attraction of both act together and produce "spring tides," with a difference of 13 feet; at first and last quarter the sun and moon oppose each other, acting in cross directions, and the difference is then only 5 feet. But these effects could only take place in the wide open space of an ocean, and when other forces do not interfere, such as obstruction to the tide-waves by land, &c.
DIAGRAM OF THE TIDES.
Inland seas (as large as the Mediterranean even) have but very little tide, while pinched-up rivers may have a great rush of tide, as at Bristol, where it rises to 45 feet sometimes, and at Anapolis in Nova Scotia, to so much as 120 feet! Wind will also accelerate or retard both the speed and height of the tides. London receives a much greater tide than its due, for the tidal-wave being interrupted by Ireland and England, rolls up the western coast (taking a northerly direction) with such force as to swing round the Shetlands and all down the eastern coast of Great Britain. The direct Atlantic wave also, when split by our island, rolls along the south coast from Land's-End to Dover, rushes through the "Straits," and meets the backward wave which has been travelling round by Scotland (as before mentioned), the two then pour into the Thames and give an otherwise quiet river a high and useful tide; so that London receives two tides, the southern one nearly 10 hours after Cornwall, and the roundabout one not till nearly 23 hours after it first reaches England.
RIVER THAMES.
The waters are the great highway throughout the whole world, and what an easy transit it has formed to those nations sufficiently civilised to require a further knowledge of the world, and thence procure for themselves luxuries not obtainable at home—at the same time, spreading civilisation wherever they go!
If the oceans, instead of forming one great concourse of waters, had existed in the form of inland seas or great lakes, the great chains of mountains, deserts, and uninhabited tracts of earth, would, in some cases, have formed impassable barriers, whilst in other parts civilisation would have been impeded from the easy access of the more savage tribes, who would have despoiled the more fortunate by inroads amongst them, mingling with or exterminating them, and checking the onward progress of civilisation. And this has always existed where great tracts of land are, which are the last parts of the world to be civilised, and in many parts remain as rude as they were thousands of years ago. All this is owing to the good and wise arrangement of water and land, and is an instance of the care and foreknowledge of God, who has provided for the onward progress of that creature to whom alone such a progress has been allotted; but as they now exist the oceans form impenetrable barriers to the savage, so as to restrict his brute force and uncultivated habits spreading, while they form easily-traversed highways to the more civilised nations, enabling them to spread their civilisation without contamination to themselves, but to the advancement of others.
The oceans are now navigated by thousands of vessels engaged in mercantile objects, and it is perfectly astonishing with what rapidity and safety these great tracts of water are crossed.
In one of Chambers's "Papers for the People," called "Ocean Routes," is the following, speaking of the perfecting of the steam-vessel:—