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.