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was removed, steam came forth, and was exceedingly an. noying.
Below the ridge, on the further side of this vast great bed of sulphur, they saw a great deal of vapour escaping with much noise. Having crossed to the opposite side of the
. mountain, they found the surface sufficiently firm to permit their walking cautiously upon it. They had not, however, as yet, visited the principal spring, as it is called, and this was a task of much apparent danger, as the side of the moun. tain in which it lies for nearly half a mile, is covered with loose clay, into which their feet sunk at every step. In many places there was only a very thin crust, below which the clay was wet and very hot. At a small distance, a dense column of steam mixed with a little water, was forcing its way impetuously through a crevice in the rock at the head of a narrow valley, or break in the mountain. The violence with which it rushes out is so great, that the noise it produces is sometimes heard at several miles distance. Behind this column of vapour is a dark-coloured rock, which forms an advantageous back.ground to this wonderful scene.
But it is quite beyond the power of words to convey any adequate idea of the wonders and terrors of this extraordinary place. The sensations of a person, even of firm nerves, standing on a support which but feebly sustains him, over an abyss where literally fire and brimstone are in incessant action; having before his eyes tremendous proofs of what is going on beneath him ; enveloped in thick vapours; his ears stunned with thundering noises; all these terrifying and awful phenomena united, must be experienced to be understood. Our young traveller, Edward, may therefore be excused at rejoicing most heartily when he once more gained the firm ground; nor was the Doctor less pleased when he again entered their tent, free from harm.
SECTION VII. VOYAGE BACK TO SCOTLAND-THE TIDES AND SALTNESE
OF THE SEA. OUR travellers having thus gratified their curiosity, in seeing the most remarkable phenomena of Iceland, re-embarked for Scotland. « Pray Sir," said Edward, as they sat one evening upon deck, “ how is the saltness of the sea accounted for ?"
DR. WALKER.-" Why this is a subject which has been variously discussed, and I will give you some of the different opinions upon the subject. There are persons who suppose that this saltness arises from great beds of salt lying at the bottom of the sea. But others more rationally suppose it is owing to the following cause. Salt is one of the original principles of nature, and is mixed, in greater or less quantities, with most other bodies. Now all rivers run into the sea, and carry some salt with them; but no rivers run out of it, nor is any water taken from it, except by exhalation or evaporation. But chemists have demonstrably proved, that no salt can ascend in either of these ways; and, consequently, all the salt carried into the sea, by the immense numbers of rivers that run into it, remains behind, and occasions its saltness.
“ That no salt ascends from the sea, either by exhalation or evaporation, is evident from this, that rain-water, which falls from the clouds, and which was originally exhaled from the sea, is, of all kinds of water, the sweetest, purest, and lightest, and is made the standard by which philosophers judge of all other waters."
"I think the last conclusion most satisfactory," replied Edward, “and now Sir, if you will not think me very troublesome, will you have the goodness to explain the nature and cause of the tides of the sea.”
DR. WALKER. .66 Most willingly. As rivers flow and swell, so also does the sea. Like them it has its currents, which agitate its waters, and preserve them froni putrefaction. That regular motion of the sea, according to which it ebbs and flows twice in about twenty-four hours, is called its tides,
“ In its flux, the sea generally rises for about six hours, when it remains, as it were, suspended, and in equilibrio, for some minutes. At that time it is called high water.
" In its reflux, the sea falls for about six hours, when it remains, as it were, in a like manner, suspended, and in equiJibrio, for some minutes. At that tirne it is called low water.
“ We are told that Aristotle, despairing to discover the true cause of these wonderful appearances, had the folly, in spite of his philosophy, to throw himself headlong into the “ The tides are occasioned by the attraction of the moon. This doctrine remained in obscurity, till Newton explained it by his great principle of gravity or attraction.
“ The tides are greatest at the new and fuil moons, and are thence called spring tides, and least at the first and last quadratures, and are thence called neap tides, and the highest tides are near the time of the equinoxes.
“ When the moon is in conjunction or opposition with the sun, as the tides, which each endeavours to raise are in the same place ; whereas, when the moon is in the first or last quarter, the sun being in the meridian when the moon is in the horizon, depresses the water where the moon raises it ; whence the tides are then the least of all. On the full and new moons, which happen about the equinoxes, when the luminaries are both in the equator or near it, the tides are. the greatest; for example, the two eminences of water are at the greatest distance from the poles, and hence the difference between ebb and flood tide is more sensible; for if thuse eminences were at the poles, it is obvious we should not perceive any tide at all : again, if the equatorial diameter, of the earth be produced, it pusses through the moon, which diameter is longer than any other, and, consequently, there is a greater disproportion between the distances of the zenith, centre, and nadir, from the centre of gravity of the earth and moon, in this situation, than in any other; finally, the water rising higher in the open seas, rushes to the shores with greater force, where being stopped, it rises higher still.; for it not only rises at the shores in proportion to the height to which it rises in the open seas, but also according to the velocity with which it flows from thence against the shore. The spring tides, which happen a little before the vernal and after the autumnal equinox, are the greatest of all, because the sun is nearer the earth in the winter than in the summer.
“ When the moon is in the northern hemisphere, it produces a greater tide while it is in the meridian above the horizon, than when it is in the meridian below it; when in the southern hemisphere, the reverse is the case.
" For the like reason, when the moon is in the southern signs, the greatest tides on the other side of the equator will be when it is below our horizon, and the least tides when it is above it.
“ These things would happen uniformly, were the whole surface of the earth covered with water ; but since there is a a multitude of islands and two vast continents, which intere rupt the natural course of the water, a variety of appearances are to be met with in different places, which cannot be explained, without regarding the situation of shores, shoals, and other objects, which contribute in producing those appearances.
“ There are frequently streams or currents in the ocean, which set ships a great way beyond their intended course. There is a current between Florida and the Bahama islands, which always run from north to south. A current runs constantly from the Atlantic, through the straits of Gibraltar, into the Mediterranean. A current sets out of the Baltic sea, through the sound or strait between Sweden and Denmark, into the German ocean ; so that there are no tides in the Baltic.
“ About small islands and head-lands in the middle of the ocean, the tides rise very little ; but in some bays, and about the mouths of rivers, they rise from twelve to fifty feet.
“ Perhaps it may be said, that as a current constantly runs from the Atlantic into the Mediterranean, the waters of that sea ought to increase. By no means. The water extracted from it in vapours, is more than sufficient to counterbalance the influx. It has been found by calculation, that in a summer's day, there may be raised in vapours, from the Mediterranean, 5280 millions of tuns of water. Yet this sea does not receive, from all its nine great rivers, above 1827 millions of tuns per day, which is but one third of what is exhausted in vapours ; so that, were it not for the influx from the Atlantic, the Mediterranean would soon be rendered dry.
• The tides flow from east to west, for they must necessarily follow the moon's motion, which is from east to west.
“ The course of the tides, however, is sometimes interruptted by continents, and other large tracts of land. The tide, for instance, in the Indian ocean, being stopped by the eastern coast of Africa, must necessarily flow south, towards the Cape of Good Hope, which having passed, it then runs northward along the Western coast of Africa, to that of Spain, Portugal, and France, till it enters the English channel ; there meeting the tide from the German ocean, running a contrary way, it is necessarily stopped, and produces a very great swell of water.
“ These two tides, thus flowing in opposite directions, and meeting a little irregularly, have sometimes occasioned evra
tides, the one immediately after the other, in the river Thames, which, though proceeding from a natural cause, and consequently very easy to be explained, has been looked upon as a prodigy.
- So much for the tides, Edward. Now I dare say, although you were inquisitive as to the saltness of the sea, you never reflected upon another of its properties. I mean its fluidity. How would you describe a fluid?” "
EDWARD." Why I do not exactly know; but I think I should define it as something which always eluded my grasp.”
Dr. WALKER.-" That is no bad definition. A fluid is scientifically defined to be a body whose parts yield to any impression, and in yielding, are easily moved amongst each. other.
“ Fluids are of two kinds : non-elastic and incompressible fluids, such as water, oil, mercury, &c.; and elastic and compressible fluids, as air of different sorts.
• The cause of fuidity is not perfectly known. Some are of opinion, that the particles of fluids are spherical, and in consequence of their touching each other, in few points only, cohere very slightly, and easily slip or slide over each other. But that the particles of fluids are of the same nature or figure as those of solids, seems probable from the very fre. quent conversion of the one into the other. Some have not thought it rational to suppose, that the particles of gold, lead, glass, &c. when in fusion, are rendered spherical by the action of the fire, but the sparks of steel, when struck with a Aint, if caught on a piece of white paper, and examined with a microscope, will be found spherules, which could only result from their having been in a state of fusion. The original cause of fluidity may not, after all, consist in the figure of the particles, but simply in their want of cohesion.
“ If the particles of a body cohere strongly together, it is evident that they will not easily move amongst each other. An imperfect cohesion must, therefore, be one of the properties of a fluid mass.
“ Modern philosophers suppose, that a certain portion of heat combined in some way or other with bodies, occasions fluidity, and that the relative proportions of heat contained in fluids and solids, is the cause of the difference between them.
" It is from the imperfect cohesion of Auids, that, when in