Слике страница
PDF
ePub

character of which we have spoken, we should not attempt any experiments in boring, because the different rocks of which it is composed have no alternate strata of limestone and clay, or of permeable and impermeable beds.

19th. When we examine the jets d'eau which sometimes spring up from a depth of three hundred feet, through pipes which have been introduced to the bottom of the bored hole, and when we reflect on the difficulties which must be overcome in boring to so great a depth, we must be convinced that great care and precision are necessary in the operation, both in boring the earth and in sinking the boxes or wooden frames which are used to shut out the quicksand so frequently encountered in boring for water."

We have given these few extracts from M. Garnier's work, to show how pointedly and constantly he expresses his opinion on the probable cause of the rise of waters in these Artesien or bored wells. We are sorry that we cannot present more of the work to our readers, but we trust they will soon be gratified with a good translation of the whole. If patient investigation, ingenious and philosophical application of facts, and accuracy and minuteness of detail should inspire confidence, M. Garnier's work will be read and valued by all who are interested in this curious and highly important branch of hydraulics.

So far as the author has advanced in speculation we entirely agree with him; we acknowledge that water may and does rise by atmospheric pressure, but phenomena frequently occur which render it necessary to seek for the intervention of another power. We are only surprised when the subject so naturally presented itself that the ingenious and sagacious author did not raise a question in his own mind concerning it. It was in an investigation somewhat similar, though not to the same extent, that we arrived at the conclusion that some of the waters from the central reservoirs owe their elevation to the surface to other agents than atmospheric, or irregular gaseous pressures.

M. Garnier distinctly observes, in several places, that unless there be a reservoir higher than the surface whence we intend to bore, we cannot hope to obtain an overflowing fountain. In a note he remarks that gases which are disengaged from certain minerals, may force up certain waters, such as are called mineral springs, but he does not allow himself any further excursion into the regions of speculative philosophy. For instance, he has never followed those pent up streams that flow between permeable and impermeable strata, to the tops of those mountains where he says they originate, and on which mountains are to be found those reservoirs that are to supply all lower levels.

If he had carried his thoughts to those high points, it must have occurred to him that the vast reservoirs found there,

amounting to lakes of great magnitude, ought to receive their supply of waters from sources still higher; and, finally, he would then have ascended to the highest known mountain, such as Himalaya, for instance, and there he would be quite at a loss, for even on these highest points he would find not only reservoirs, but tremendous cataracts. It has been conjectured by sensible travellers that the quantity of water discharged from some of these cataracts in one day is of such vast amount, that it could not be supplied by all the vapours that might fall on the mountain in one year.

The cataract issuing from Himalaya, or, as it is sometimes called, Roodroo Himala, is the source of the river Jemma, near Jungotree, a rapid and large river; and, in fact, many of the largest rivers in the world proceed immediately from mountains, or from lakes that are formed by cataracts.

The rivers Ganges, Nile, Indes, Hoangho, Kiamku, Senegal, Rhine, Rhone, Vistula, Elbe, Loire, Garonne, Douro, Guadiana, Guadalquiver, Po, Adige, Arno, Severn, Don, Ain, Wherse, Swale, Forth, Tay, Monongahela, and many others, both in America and Europe, have their source in mountains. They receive no increase from tributary streams, but issue in such astonishing abundance from the rocks, as to overflow and fertilize the countries through which they pass.

The American essay on boring contains the first hint of the probability that water may rise above the surface independently of atmospheric pressure. Of course so novel a suggestion would be received with distrust, and be neglected; for if it were considered plausible it would dispute the long acknowledged theory of the origin of springs, and oblige our professors and lecturers to alter their notes.

But although neglected here, it was seized upon immediately in England, for we perceive the following explanation in the twenty-third number of the Library of Useful Knowledge, April 1st, 1828, on the subject of physical geography.

"The origin of the numerous springs which break forth from the earth's surface cannot be referred to one exclusive cause. The internal reservoirs by which they are supplied are in many cases derived from the water which the earth absorbs from rain and melted snow; from those reservoirs, wherever there is uneven or mountainous ground, the water flows out by minute fissures from the sides of hills. But when we see springs rising up in plains, it is evident they must have ascended, that is, they must have travelled in a direction contrary to that produced by the force of gravity. This no doubt is sometimes to be attributed to water flowing under ground from distant elevations, and to the natural tendency of a liquid to find its level.

"But the rising up of springs in plains cannot always be VOL. XXII. NO. 44.

43

accounted for in this manner, and it has therefore been supposed that the earth contains capillary tubes, the effect of which, in attracting liquids upwards, is explained in chapter eighth of the treatise upon Hydraulics. It is also evident that such springs as suffer no diminution, even from the longest continuance of dry weather, must be derived from a source quite independent of rains and other external means of supply. They must proceed from some vast body within the earth, and it has with apparent reason been concluded, that many springs arise from the ocean and filter through the pores of the earth, the salt particles being lost in the passage."

Thus we perceive that scarcely two years after the new theory was made known in the United States, it found its way to England and was well received there. There are some mistakes, however, and some misconstructions in the above exposition, which we shall presently state and endeavour to rectify. In the American essay, it is suggested that in some cases water may rise by "centrifugal repulsion," but the English philosophers choose to call the power "capillary attraction," a term which our author disclaims. Whatever may have been the theory of the origin of springs before the hypothesis of Halley, certain it is that since his day there has been no other than the one he suggested, and that is atmospheric pressure. The rise of water in capillary tubes was accounted for by modern philosophers on a principle very different from that which the American theorist supposes may elevate fluids to the surface.

The question yet to be decided is the true explanation of the phenomenon called capillary attraction. The generally received opinions are that it is an attractive process, but we consider it to be a repulsive effort alone; an innate repulsive power, which not only keeps matter from gravitating too near the centrifugal point, but forces fluids, and gases, and all other matter to the surface. The ability which the central power has to force off all gravitating matter is universally acknowledged; and were it not for the constant exercise of this power, the matter of which the globe is composed would by atmospheric pressure be compressed into an infinitely small space. The effort, therefore, of the central power is to counteract the undue influence of the opposing one-atmospheric pressure or gravity; and by this means all matter which is gravitating is forced upwards, and thus gases and fluids are kept in a state of motion. The dense matter of the earth being more or less porous, is capable of sustaining and transmitting all volatile and fluid substances to the surface.

On turning to the eighth chapter, page twenty-fourth, of Hydrostatics, in the first number of the Library of Useful Knowledge, we find the following explanation of the phenomenon

called capillary attraction. We trust the time is approaching when this question will be better argued.

"Hitherto we have seen no exception to the general rule, that all the parts of a liquid stand always at the same height if left to themselves, and that, consequently, no liquid can of itself rise higher in the inside of a tube than it stands outside. But there is an exception to this rule which must now be explained.

"If a drop of water, or any liquid of a like degree of fluidity be pressed upon a solid surface, it will wet that surface and stick to it, instead of keeping together and running off when the surface is held sloping. This shows that the parts of the liquid are more attracted by the parts of the solid than by one another. In the same manner, if you observe the edge of any liquid in a vessel, as wine in a glass, and note where it touches the glass, you will see that it is not level close to the glass, but becomes somewhat hollow, and is raised up on it so as to stand a little higher at the edge than at the middle and other parts of its surface. It appears, therefore, that there is an attraction at a very small distance from the edge, sufficient to suspend the part of the fluid near it, and prevent it from sinking to a level with the rest. Suppose the wineglass to be diminished so as to leave no room for any of the wine in the middle, which lies flat and level, but only to leave room for the small rim of liquor raised up all round on the sides of the glass; in other words, suppose a very small tube, placed with its lower end just so as to touch the liquor, it is evident that the liquor will stand somewhat higher in the tube than outside, and if the tube be made smaller the liquor will rise higher, there being always less weight of liquid to keep it from rising and counterbalance the attraction of the glass."

We think it quite impossible that any philosopher can now believe that "water is raised in a capillary tube and supported by the ring of glass immediately above the water's surface," or that "water has a stronger attraction for the particles of glass than it has for its own particles."

Water, like gas, cannot be applied to any particular point, unless it have a medium through which it can traverse; it must flow through a tube or over an inclined plane. But although this be necessary to the motion of water it does not follow that water has a greater affinity for the tube or the inclined plane. than it has for open space. It is the interstices of the medium which allow of the elevation of fluids. It is into these interstices that gases which form a part of water, and are always more or less present in it, elevate themselves to a certain extent, and make a lodgment both for themselves and the fluids with which they are charged.

If we put a capillary tube in water, the fluid will rise to a

certain height in the tube, above the level of the water in which the tube is immersed. It remains stationary at that height, because the gases are then incapable of overcoming the pressure of the atmosphere. They can no longer avail themselves of the aid which the slight indentations or interstices of the surface of the tube afford them. But if in this capillary tube we introduce a thread, we shall perceive that the gases will soon elevate the water to the top of the tube, even if it be six or seven inches long, and if the thread is wet, the water will be the sooner elevated. If the capillary tube were rough within, the water would rise to the top precisely on the same principle that it does when a thread is introduced, proving that in all cases an interstitial medium allows of the easy transmission of fluids.

We know that if a capillary tube is broken off below the point at which the water stood, the water will not overflow; but the true cause of this phenomenon is, that there is no further conductor. No doubt it evaporates, even on so small a surface, but we shall see it flow over fast enough if we introduce a thread. It is well known that if we suspend a strip of linen in a bowl of water, so that the bottom of the linen shall touch the bottom of the bowl, the water will rise to the top of the linen. The same thing occurs with sugar, sponge, and sand.

Gases take advantage of all rarefied interstices, for by this means they themselves, and the fluid particles with which they are charged, have their continuity broken up, as it were, and by this division of the particles they move with greater freedom. On the old doctrine of attraction we cannot account, philosophically, for the fact that water does not overflow when the capillary tube has been broken below the point of elevation. According to that docrine, the water should overflow, for the same capacity exists in the "rings" of the tube, the same disposition to attach themselves to the tube continues in the lower particles of water, both when it is broken and when of the full length. The lower particles would, in both cases, have the power of elevating themselves when the pressure of the upper was removed.

The term capillary attraction, therefore, is not a suitable one, for whenever water rises without being forced to its level by gravity, it is by as compulsory a process as when acted upon by atmospheric pressure.

All the phenomena connected with the elevation of water in bored and other wells, have hitherto been explained by the theory of atmospheric pressure, a theory established by Dr. Halley, who attributed the origin of springs to this cause alone. We are still unconscious of the many errors that may have crept in from the limited notions which this peculiar theory obliged

« ПретходнаНастави »