An Essay on the Encroachments of the German Ocean Along the Norfolk Coast / With a Design to Arrest Its Further Depredations

by William Hewitt

This is the reason that two great spring tides never take place immediately after each other; for if the moon be at her least distance at the time of new moon, she must be at her greatest distance at the time of full moon, having performed half a revolution in the intervening time; and, therefore, the spring tide at the full will be much less than at the preceding change. For the same reason, if a great spring tide happens at the time of full moon, the tide at the following change will be less. [14c]

The spring tides are highest and the neap tides lowest about the beginning of the year; for the earth being nearest the sun about the first of January, must be more strongly attracted by that body than at any other time of the year: hence the spring tides which happen about that time, will be greater than at any other time, and should the moon be new or full in that part of her orbit, which is nearest to the earth at the same time, the tides will be considerably higher than at any other time of the year.

The tide which happens at any time while the moon is above the horizon, is called the superior tide, and when below the horizon, the inferior. When the moon is in the equinoctial, the superior and inferior tides are of the same height, but when the moon declines towards the elevated pole, the superior tide is higher than the inferior. If the latitude of the place and the declination of the moon are of contrary names, the inferior tides will be the highest. But the highest tide at any particular place is when the moon’s declination is equal to the latitude of the place, and of the same name, and the height of the tide diminishes as the differences between the latitude and declination increases, therefore the nearer any place is to that parallel whose latitude is equal to the moon’s declination and of the same name, the higher will be the tide at that place. In comparing the height of tides at different places, it is supposed that the sun and moon are at the same distances from the earth, and in the same position with respect to the meridian of these places. [15a]

The above observations relative to the regularity of the tides could only result by supposing the earth to be covered with the waters of the ocean to a great depth, but as this is not the case, it is only at places situated on the shores of large oceans where such tides exist. [15b]

From local circumstances the tides are subject to great irregularities, such as meeting with islands, headlands, passing through straits, &c. In order that they may have their full motion, the ocean in which they are produced ought to extend 90° from east to west, because that is the distance between the greatest elevation and the greatest depression produced in the waters by the moon.

Hence it is that the tides in the Pacific Ocean exceed those of the Atlantic, and that they are less in that part of the Atlantic which is within the torrid zone between Africa and America, than on the temperate zones on either side of it where the ocean is much broader. [16a]

Tides are not perceptible in lakes and most inland seas, and deep and extensive as is the Mediterranean, are scarcely sensible to ordinary observation, their effects being quite subordinate to the winds and currents. In some places, however, as in the Straits of Messina, there is an ebb and flow to the amount of two feet and upwards; at Naples and at the Euripus, of twelve and thirteen inches, and Rennell informs us, at Venice, of five feet. [16b]

The ebb and flow of the ocean is very slight in islands remote from any continent, as for example, at St. Helena, where it seldom exceeds three feet. Tides are remarkably high on the coasts of Malay, in the Straits of Sunda, on the open coast of Patagonia, along the coasts of China and Japan, at Panama, in the Gulph of Bengal, and at the mouth of the Indus, where the water rises thirty feet in height. Tides are greatest in any given line of coast, in narrow bays and estuaries; and are least in the intervening tracts where the land is prominent. [16c]

On the authority of the late Captain Hewett, R.N., at the entrance of the estuary of the Thames, the rise of the spring tides is eighteen feet; but when we follow our eastern coast from thence northward; towards Lowestoft and Yarmouth, we find a gradual diminution, until at the place last mentioned the highest rise is only seven or eight feet. From this point there begins again to be an increase, so that at Cromer, where the coast again retires towards the west, the rise is sixteen feet; and towards the extremity of the gulph called “the Wash,” as at Lynn and in Boston Deeps, it is from twenty-two to twenty-four, and in some extraordinary cases, twenty-six feet. From thence again there is a decrease towards the north; the elevation at the Spurn Point being from nineteen to twenty feet, and at Flamborough Head, on the Yorkshire coast, from fourteen to sixteen feet.

It is also recorded, on the authority of Captain Beaufort, R.N., that at Milford Haven, in Pembrokeshire, at the mouth of the Bristol Channel, the tides rise thirty-six feet, and at King-road, near Bristol, forty-two feet. At Chepstow, on the Wye, a small river which opens into the estuary of the Severn, they reach fifty feet, sometimes sixty-nine, and even seventy-two feet. [17]