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: