Читать книгу The Niagara River - Archer Butler Hulbert - Страница 23

In size the cave is about one hundred feet wide, a hundred feet deep, and about one hundred and sixty feet high. At one point in the cave, on a bright day, by standing in the very edge of the spray, one becomes the centre of a complete circle of rainbows, an experience probably unequalled elsewhere.

About half-way between the stairway and the cave is the point from which, in 1829, Sam Patch made his famous leap, elsewhere described.

On the side of the Horseshoe Fall is to be found a fine position from which to view the mighty force of the greater mass of waters. For some distance along the front of the fall immense masses of rock have accumulated. The trip over these rocks is fraught with danger and is taken by very few. For those who care to take the risk, the sight is well worth the effort. Just above at the crest are Terrapin Rocks, where formerly stood Terrapin Tower. Professor Tyndall went far out beyond the line of Terrapin Rocks to a point which has been reached by very few of the millions of visitors to this shrine. Passing along the cliff toward Canada, Porter's Bluff is soon reached, which furnishes one of the grandest views of the Horseshoe Fall. Fifty years ago, from this point one could see the whole line of the graceful curve of the Horseshoe; since that time the rapid erosion in the middle of the river (where the volume is greatest) has destroyed almost all trace of what the name suggests. The sides meet now at a very acute angle, the old contour having been entirely destroyed.

One of the most interesting experiments conducted under these great masses of falling water was essayed by the well-known English traveller Captain Basil Hall in 1827. It seems that Babbage and Herschel had said that there was reason to expect a change of elastic pressure in the air near a waterfall. Bethinking himself of the opportunity of testing this theory at Niagara during his American tour, Captain Hall secured a mountain barometer of most delicate workmanship for this specific purpose. In a letter to Professor Silliman the experimenter described his experience as follows, the question being of interest to every one who has attempted to breathe when passing behind any portion of this wall of falling water:

I think you told me that you did not enter this singular cave on your late journey, which I regret very much, because I have no hope of being able to describe it to you. In the whole course of my life, I never encountered anything so formidable in appearance; and yet, I am half ashamed to say so, I saw it performed by many other people without emotion, and it is daily accomplished by ladies, who think they have done nothing remarkable.

You are perhaps aware that it is a standing topic of controversy every summer by the company at the great hotels near the Falls, whether the air within the sheet of water is condensed or rarefied. I have therefore a popular motive as well as a scientific one, in conducting this investigation, and the result, I hope, will prove satisfactory to the numerous persons who annually visit Niagara.

As a first step I placed the barometer at a distance of about 150 feet from the extreme western end of the Falls, on a flat rock as nearly as possible on a level with the top of the "talus" or bank of shingle lying at the base of the overhanging cliff, from which the cataract descends. This station was about 30 perpendicular feet above the pool basin into which the water falls.

The mercury here stood at 29.68 inches. I then moved the instrument to another rock within 10 or 12 feet of the edge of the fall, where it was placed, by means of a levelling instrument, exactly at the same height as in the first instance.

It still stood at 29.68 and the only difference I could observe was a slight continuous vibration of about two or three hundredths of an inch at intervals of a few seconds.

So far, all was plain sailing; for, though I was soundly ducked by this time, there was no particular difficulty in making these observations. But within the sheet of water, there is a violent wind, caused by the air carried down by the falling water, and this makes the case very different. Every stream of falling water, as you know, produces more or less a blast of this nature; but I had no conception that so great an effect could have been produced by this cause.

I am really at a loss how to measure it, but I have no hesitation in saying that it exceeds the most furious squall or gust of wind I have ever met with in any part of the world. The direction of the blast is generally slanting upwards, from the surface of the pool, and is chiefly directed against the face of the cliff, which being of a friable, shaly character, is gradually eaten away so that the top of the precipice now overhangs the base 35 or 40 feet and in a short time I should think the upper strata will prove too weak for the enormous load of water, which they bear, when the whole cliff will tumble down.

These vehement blasts are accompanied by floods of water, much more compact than the heaviest thunder shower, and as the light is not very great the situation of the experimenter with a delicate barometer in his hand is one of some difficulty.

By the assistance of the guide, however, who proved a steady and useful assistant, I managed to set the instrument up within a couple of feet of the "termination rock" as it is called, which is at the distance of 153 feet from the side of the waterfall measured horizontally along the top of the bank of shingle. This measurement, it is right to mention, was made a few days afterward by Mr. Edward Deas-Thompson of London, the guide, and myself with a graduated tape.

While the guide held the instrument firmly down, which required nearly all his force, I contrived to adjust it, so that the spirit level on the top indicated that the tube was in the perpendicular position. It would have been utterly useless to have attempted any observation without this contrivance. I then secured all tight, unscrewed the bag, and allowed the mercury to subside; but it was many minutes before I could obtain even a tolerable reading, for the water flowed over my brows like a thick veil, threatening to wash the whole affair, philosophers and all, into the basin below. I managed, however, after some minutes' delay to make a shelf or spout with my hand, which served to carry the water clear of that part of the instrument which I wished to look at and also to leave my eyes comparatively free. I now satisfied myself by repeated trials that the surface of the mercurial column did not rise higher than 29.72. It was sometimes at 29.70 and may have vibrated two or three hundredths of an inch. This station was about 10 or 12 feet lower than the external ones and therefore I should have expected a slight rise in the mercury; but I do not pretend to have read off the scale to any great nicety, though I feel quite confident of having succeeded in ascertaining that there was no sensible difference between the elasticity of the air at the station on the outside of the Falls and that, 153 feet within them.

I now put the instrument up and having walked back towards the mouth of this wonderful cave about 30 feet, tried the experiment again. The mercury stood now at 29.68, or at 29.70 as near as I could observe it. On coming again into the open air I took the barometer to one of the first stations, but was much disappointed though I cannot say surprised to observe it full of air and water and consequently for the time quite destroyed.

My only surprise, indeed, was that under such circumstances the air and water were not sooner forced in. But I have no doubt that the two experiments on the outside as well as the two within the sheet of water were made by the instrument when it was in a correct state: though I do not deny that it would have been more satisfactory to have verified this by repeating the observations at the first station.