POPULAR THEORIES REGARDING THE

INUNDATIONS.

This explanation, however, is too simple to satisfy the unscientific mind, which delights in gratuitous assumptions. and occult causes, the magnitude of which cannot be measured by the senses, and on which, therefore, the imagination can draw to any amount.

Bacon observes in his plain language that "men have a natural, though corrupt, love of the lie itself."* Without going the length of Bacon, and affirming that men love falsehood for falsehood's sake, we may assert that few have any love of truth for the sake of truth, and that, when their material interests are not concerned, the majority do not care to investigate and bring it to light. On the contrary, they prefer to leave it in obscurity, because wider scope remains for the play of their fancy. When Tacitus put the "omne ignotum pro magnifico" in the mouth of the Caledonian chieftain,† he shewed his knowledge of human nature, and of the feelings by which most persons are influenced in judging of men and things. This indifference to abstract truth is especially observed in matters of science, where there is no question of material loss or gain; and opinions have been current since the time of Cicero which a few simple experiments or observations, recorded for a single year, would have shewn to have no foundation in truth.‡ It is in accordance with such a

* Bacon's Essays, I.

+ Taciti Agricola XXX.

‡ Witness the popular belief in the influence of the moon on the weather and in equinoctial gales. All scientific men of note, as Arago, Herschel, Admiral Fitzroy, &c., are agreed that there is no connexion between the changes of the weather and the changes of the moon. The belief in such a connexion is nothing but a relic of the astrological superstition which attributed to that planet an influence over men as well as things; the

feeling that the damming up of the waters of the Tiber by a Scirocco wind,* and the increase of their volume by the melting of the snows on the Apennines, are the popular modes of accounting for the inundations of the river, while the rain, though it may promote the melting of the snows, is thought to contribute nothing to the mass of water. The latter notion I will examine first; for the former, though opposed to every principle of mechanical science, may seem to be borne out by what occurs in a few other rivers. It will be necessary, therefore, to examine it at greater length, and to shew how different are the conditions in the case of the Tiber and of those rivers where floods are actually produced by the action of the wind.

As the modern Romans take no account of the rain, so the ancients appear to have ignored the snow. Livy, in his notices of the floods, which occurred in different epochs of the history of Rome, always attributes them to rain. His words are invariably “magnæ aquæ erant" or "ingentes aquæ erant," "there were copious rains" or "there were great rains,” and the Tiber overflowed its banks, &c. Tacitus, also, in describing

memory of which superstition is still preserved in the word "lunatic." Yet the notion will probably hold its ground to the end of time, because nobody has any personal interest in disproving it.

The belief in equinoctial gales is as old as the time of Cicero, who alludes to it; and in the present day these imaginary gales have been assigned by the government as a reason for modifying the original plan for the autumnal manoeuvres of the troops. Yet, in the average of years, the fortnight before and the fortnight after the twenty-first September are fine and calm. The greatest storms on record have occurred either in November or nearer to the winter solstice than to the autumnal equinox. We might, therefore, with more propriety talk of solstitial than of equinoctial gales.

* This appears to have been a generally accepted theory in the latter end of the sixteenth century. In a Latin poem of the time, written immediately after an inundation, the earth is represented as complaining to the Tiber, that it is despoiled of all its productions and all its beauty by the violence of the latter; to which the Tiber replies that it is not its fault, but the fault of the south wind, which bars the passage of its water to the sea.

At si forte graves aspirat ab æquore flatus,

Egressum nostris impedit Auster aquis.

the inundation in the reign of Tiberius, uses the words continuis imbribus auctus Tiberis plana urbis stagnaverat." "The Tiber swollen by incessant rains had inundated the level parts of the city." Pliny, the younger, also describes an inundation of the Anio, the same to which we have before referred, which was caused by rains of extraordinary violence, so that great damage was done by the temporary torrents which they produced in places where there were no streams. Neither in these authors nor in Dio Cassius is there any mention of snow as connected with the rising of the river. In this they were nearer to the truth than the moderns; for though the snow aids somewhat in producing the inundation, and in exceptional cases may contribute largely to it, yet everyone who studies the weather and observes the phenomena of the river must see that it plays but a subordinate part. We have seen that the greatest flood but one recorded in modern history, was produced entirely by rains of unusual violence. On the other hand, there is no instance of any great inundation produced by the melting of the snows unaccompanied by heavy rain. In the January of 1871 there was an unusual accumulation of snow on the Apennines; yet, though melted by a warm Scirocco wind, and accompanied by gentle rains at the commencement, it only swelled the river to the level of the Ripetta.

It will be easy, however, to shew by a simple calculation that, under no conceivable circumstances could the effect of the snow equal that of the rain, and that the great rise of the river on Tuesday night and Wednesday morning, the twentyseventh and twenty-eighth December, 1870, was due to the heavy rain of Monday night, the twenty-sixth. Fresh fallen and uncompressed snow occupies about fourteen times the space of water.* On the night of Monday, the twenty-sixth,

* Dove. Meteorologische Untersuchungen, Berlin, 1839, page 51.

It should be observed that the space occupied by snow depends upon the temperature at which it falls, being greatest when the temperature is low and the snow light and feathery, and least when the temperature approaches the freezing point and the snow is ready to melt.

But fourteen inches is the mean. Dove's observations agree nearly with my own.

there fell more than an inch and a half of rain. To produce the effect of this depth of rain, a foot and three-quarters of snow extending over the whole basin of the Tiber would be required. But the superficies of the Apennines covered with snow, even when the snow descends as low as two thousand feet above the sea, is not one-fourth of the area of the basin of the Tiber. The depth of snow, therefore, on the Apennines ought to have been seven feet on the level, and the snow ought to have melted in twelve hours, in order to produce the rapid rise of the river which took place on Wednesday, the twenty-eighth. Now, such a depth of snow is unknown on the Alps, below the region of perpetual congelation, except in drifts, and on the Apennines it is inconceivable, especially when a large part of it must have been dissolved by the Scirocco wind of the previous day and night; and as snow absorbs a large quantity of latent heat in the process of liquefaction, it would have required, even when aided by rain, three or four days to melt. The temperature of the plains never exceeded fifty-five or fifty-six degrees Fahrenheit, and the average temperature at an elevation of between two thousand and six thousand feet, corresponding to that temperature in the plains, would not be more than forty-three degrees Fahrenheit, a temperature at which it is impossible that the melting of the snow could proceed at a rapid rate. Anyone who has observed how long it takes for half-a-foot of snow to melt in the plains, even when a strong south-west wind is blowing, will see how impossible it is that such a depth of snow as would be required to produce the requisite quantity of water could have been dissolved in the mountains in the course of twelve hours. On the other hand, a given quantity of rain may be spread over several days, or may descend in half-an-hour with the violence of a waterspout. Sudden inundations, therefore, where the river comes down with a head of water, must always be due mainly to rain.

I now proceed to explain how tides or gales of wind may occasion floods in rivers, whose volume is not increased by rain or melted snow. When a river widens rapidly as it

*

approaches the sea, so as to form a funnel-shaped estuary, whose mouth is open to a stormy wind, the tides often rise to a great height, and floods occasioned by them or by storms of wind are of frequent occurrence in the low lying districts along their banks. When the tidal wave enters the mouth of the estuary, its progress is checked in front by the sudden contraction of the banks, and the hinder part retaining its momentum rises over it and adds to its height. The effect is repeated, as the wave advances up the estuary, until the height of the tide in the estuary far exceeds its height in the open sea. Thus the spring tides, which, according to Professor Airy, rise only eighteen feet at the mouth of the Bristol Channel, reach the height of more than thirty at Swansea and fifty at Chepstow; and in the bay of Fundy, between Nova Scotia and New Brunswick, they attain an elevation of one hundred and twenty feet.† The lower parts of London are often inundated by high tides at the time when it is low water in the German Ocean, the tidal wave by its momentum having, as it were, run up an inclined plane. In like manner, a gale of wind blowing into the mouth of a funnel-shaped bay may heap up the water at the narrow end far above its level in the open sea. In this way are produced the floods which have often devastated St. Petersburg. The stormy west wind blowing up the gulf of Finland drives vast billows into the open mouth, which rise to a great height at the narrow extremity. The waters of the Neva are dammed up, and rising above the quays overflow the city. St. Petersburg, as everyone knows, was originally a morass, but slightly elevated above the

* Encyclopedia Metropolitana.

† (Land and Sea).-Yet at Green bay, on the north side of the isthmus which separates it from the narrow end of the bay of Fundy, it rises only seven feet. In the same work there is the story of a ship which during the night was deposited by the rising tide of the bay of Fundy on a rock of considerable elevation. We may conceive the astonishment and dismay of the crew at daybreak, when they found themselves suspended high in air, like an aerial vessel.

The story does not go on to tell us whether they got afloat again at high water.

H