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.