LIEUT. SAXBY’S PREDICTION.

Marlborough Express, Volume IV, Issue 197, 2 October 1869, Page 4

LIEUT. SAXBY’S PREDICTION.

To the Editor of the Marlborough Exp r ess. Sm, —I notice in your paper a letter signed “ Common Sense.” The writer says that several persons are preparing to go from the Boulder Bank to the ranges on the sth October, as according to that “old woman, Saxby,” there is to be an earthquake and tidal wave. I beg to inform your correspondent that Lieut. Saxby has not predicted any earthquake ; and of course, if by tidal wave is meant an irruption of the sea on the land as was caused by the earthquake on the coast of Peru, he has predicted nothing of the kind. What that gentleman has stated is, that on or shortly after the sth October there will be an extraordinary high Spring tide, and the reasons he gives are that on that day it is new moon—the moon is in perigree The moon also on the same day crosses the equator ; and the sun, although not on the equator, has only about 5 deg. of South declination. I beg to say that there wants but one element to cause the highest possible Spring tide—that is, that the sun also should be in perigree. This, however, only happens once a year—in January—while the moon goes through all its changes of right acension and declination, and is in perigree every twenty-nine days, or thereabouts. As everybody knows, the cause of the tides is the attraction of the moon and sun. The mean distance of the moon from the earth being only 240,000 miles, while that of the sun is over 95 000,000 miles, the attraction of the moon is much the most powerful. If the planet on which we live were one sheet of water, the tidal wave would follow the apparent course of the moon from east to west, the water being highest at the two points where the moon had shortly before been in the zenith and nadir. At the new moon the sun and moon are in the same circle of right ascension, or the moon passes the meridian at apparent noon ; their attraction is combined. It is greatest on the meridian where they are vertical, and least where they are in the nadir. Thus, wh Je it would be high water at any meridian, it would be low water at 90 degrees both east and west of it, and also high water at the lower meridian. At full moon; the moon is 180 degrees from the sun in right ascension, or she passes the meridian at midnight. The moon then raises the tide where vertical, and the sun

raises them on the opposite meridian, where the moon’s attraction is less than at the points 90 degrees east and west. This is the theory, and as the force of attraction is in inverse proportion to the square of the distance of the objects, it follows that the tides will be greatest when both objects are in perigree, the moon, being by far the most powerful attracting body. As, however, the earth is not one sheet of water, the tidal wave as it meets the land is diverted in all directions. On the east coast of Scotland and England the flood tide comes from the north. In the English Channel, from the west; in this part of of Cook Straits, from the. S.S.W. The time when the sun and moon are in opposition and conjunction on the full and change of the moon is that of the spring tides. The time when they have 90 degrees difference of right ascension, or when the moon passes the meridian at 6 a.m. or 6 p.m., is that of neap tides. The time of highest springs being generally a day or two after the full and change, and of the lowest neaps the same after the quarters. Lieut. Saxby is really doing an important service by calling attention to this. Equinoctcal spring tides are often very high ; and though the equinox is past, the sun will only have about five degrees of south declination on the sth October, and it is only a fortnight from the time of the equinox. There will thns be only an angle of five degrees, as measured from a geocentric position between the sun and moon. The rise of tide here is very small—about four feet on the springs. The moon being in perigree, will cause some increase in this, and the moon crossing the equator is a disturbing cause. The wind has also great effect on the tide. A S.E. gale in the Straits will at any time raise the tide a foot at Gouland’s Ferry, and a N.W. gale will depress them as much ; the effect will be a little different airthe Boulder Bank.

Altogether, I can see nothing to fear hereabouts ; even with a S.E. gale, nothing more than a tide 18 inches or 2 feet above the level of ordinary springs is to be expected. If the wind be from the N.W., perhaps no extra rise. But the case is very different in some other parts of the world. On the east coast of England a great deal of land is only protected by embankments from the sea and tidal waves—as the. Feus. k of Lincolnshire and Norfolk. In Lynn t)eep and the Humber ordinary spring tides rise from 22 to 23 feet, and as the extra rise caused by the coincidence of full moon and perigee must be some proportional part of the ordinary rise, the necessity for securing the embankments, &c., in time is evident. The same is the case with a large part of Holland ; and there the rise and fall is large—lß feet at Ams erdam, and 15 feet at Flushing, on Wale' eren Island. Again, aLi fh spring tide causes a very rapid stream in tidal rivers. At Liverpool, no vessel can lie in the stream of the river in a strong tide, except in the S'.oyne, on the Cheshire side of the Mersey. The ground is so hard elsewhere that anchors will not hold. It will be neceesary for all vessels anchoring in that port about the sth or 6th October to dock at once. In the Bristol Channel ihe spring tide rises 30 feet at Lundy Island At King’s Road (Bristol) it rises 40 feet, and up at Chepstow 70 Fapl; At latter — 4 ~ £ * J — * tide runs in a perpendicular wave, called by seamen a “ Bore ” This is caused by the resistance of the fresh water to the rising tide. The same phenomena is common on the river Seine, between Honfleur and Rouen, in France ; and in the rainy season, when the Hooghly is swelled with fresh water between Calcutta and Diamond Harbor. The navigation of sucli rivers as the Severn, the Seine, and the Hooghly—at all times difficult—is on the spring tides dangerous It is not uncommon for ships at Calcutta in July, August, and September to wait a few days until the spring tides are over, the pilots refusing to take charge; at such times the ebb tide will run 10 knots About the end of September, or early in October, the S. W. monsoon breaks, and generally after some very stormy weather, and very often a cyclone, the N.E. monsoon and fine weather sets in in the Bay of Bengal. Horsbury, in his East India Directory (sailing directions for the Indian and Chinese seas), says that when new moon and its perigee commences, the chances are 5 to 1 that there will be a change of weather ; full moon and perigee, 4 to 1 ; new moon aud apogee, 3 to 1 ; and full moon and apogee, 2 to 1. This was first observed by M. de Pontecoulant, a French astronomer. I may observe that as our time in New Zealand is before that of Greenwich, the new moon is at 153 a.m. of the 6th, Nelson time, and the highest tide can hardly be expected here before the 6th or 7th ; much, however, depends on the direction of the wind. 1 trust I have shown that Lieut. Saxby is no quack, and that his notice is of great service. As to the Boulder Bank, I presume that the Pilot is the best judge as to whether there is any risk of a high spring tide washing over it. I am, &c. Sept. 25, 1869. Henry Cooke# \ "■■■ S