366 Notice of the pressure of the Atmosphere, <$>c. 



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 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 

 overhang? the base thirty-five or forty 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 

 bv 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. By the assistance of the guide, how- 

 ever, who proved a steady and useful assistant. I managed to 

 set the instrument up, within a couple of feet of the ; - termina- 

 tion rock, ri as it is called. w r hich is at the distance of one hun- 

 dred and fifty-three feet from the side of the water fall, meas- 

 ured horizontally along the top of the bank of shingle. 



This measurement, it is right to mention, was made a few 

 days afterwards, by Mr. Ed. Deas Thompson, of London, the 

 guide, and myself, with a graduated tape. 



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 con- 

 trivance. I then secured all tight, unscrewed the bag, and al- 

 lowed the mercury to subside : but it was many minutes be- 



