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.