ivithin the Cataract of Niagara. 365 



sequence to have some method of ensuring this position 

 throughout the observations. Mr. Thomas Adie, instrument 

 maker, in Edinburgh, in conjunction with Mr. Jardine, the em- 

 inent civil engineer, devised a small fixed circular spirit level 

 on the top of the instrument, the bubble of which is made to 

 stand at the centre, when the tube is perfectly upright. In 

 order to bring it to this position, four screws are necessary at 

 the collar, near the centre of motion, by which not only the 

 requisite adjustments are made, but the instrument can after- 

 wards be firmly secured in its place. In other respects, it did 

 not differ from the best mountain barometers, where both sur- 

 faces of the mercurial column are capable of being observed; 

 and where, consequently, the observation being direct, no al- 

 lowances or corrections are required. 



Some days after reaching Niagara, I went behind the sheet 

 of water, on the Canada side of the Falls, and although cir- 

 cumstances did not promise very favorably, I resolved to try 

 what could be done with the barometer, in a place where no 

 similar instrument had probably ever been set up. 



I think you told me that you did not enter this singular 

 cave on your late journey ; which I regret much, because I 

 have no hope of being able to describe it to you. In the 

 whole course of my life, I never encountered any thing so 

 formidable in appearance : and yet, I am half ashamed to say 

 so, as 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 con- 

 troversy, 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 had 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 per- 

 sons who annually visit Niagara. 



,As a first step, I placed the barometer at the distance of 

 about one hundred and fifty feet, from the extreme western 

 end of the Fall, on a flat rock, as nearly as possible on a level 

 with the top of the ' talus 1 or bank of shingle, lying at the base 

 of the over-hanging cliff, from which the cataract descends. 

 This station was about thirty perpendicular feet above^ the 

 pool or basin into which the water falls. The mercury here 

 stood at 29.63 inches. I then moved the instrument to another 

 rock, within ten or twelve feet of the edge of the fall, where 



