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[Lewis] 
September 1st—
 

       The Pilott informed me that we were not far from a ripple which was much worse than any we had yet passed, and as there was so thick a fogg on the face of the water that no object was visible 40 paces he advised remaining untill the sun should acquire a greater altitude when the fogg would asscend and disappear; I conscented; we remained untill eight Oclock this morning when we again set out—    these Foggs are very common on the Ohio at this season of the year as also in the spring but do not think them as freequent or thick in the spring.    perhaps this may in some measure assist us to account for the heavy dues which are mor remarkable for their freequency and quantity than in any country I was ever in—    they are so heavy the drops falling from the trees from about midknight untill sunrise gives you the eydea of a constant gentle rain, this continues untill the sun has acquired sufficient altitude to dessipate the fogg by it's influence, and it then ceases.    the dues are likewise more heavy during summer than elsewhere but not so much so as at this season.—    the Fog appears to owe it's orrigin to the difference of temperature between the air and water the latter at this seson being much warmer than the former; the water being heated by the summer's sun dose not undergo so rapid a change from the absence of the sun as the air dose consiquently when the air becomes most cool which is about sunrise the fogg is thickest and appears to rise from the face of the water like the steem from boiling water—  [1]    we passed the little horsetale ripple or riffle with much deficulty, all hands laboured in the water about two hours before we effected a passage; the next obstruction we met was the big-horse tale riffle,  [2] here we wer obliged to unload all our goods and lift the emty Boat over, about 5 OCock we reach the riffle called Woollery's trap,  [3] here after unloading again and exerting all our force we found it impracticable to get over, I therefore employed a man with a team of oxen with the assistance of which we at length got off    we put in and remained all night having made only ten miles this day.—  [4]




 

1. Lewis's reflections on fog and dew and their determinants over the next several days bring him to posit a theory for the occurrence and to test his hypothesis by taking temperature readings of air and river during the time. Apparently cold air is moving down the valley sides and passing across the warm water of the Ohio River in a situation known as cold air drainage. Since the air is heated from below as well as moistened, the fog takes the form of rising streamers, and the phenomenon is generally called steam fog. Neiburger, Edinger, & Bonner, 123–24. (Return to text.)

 

2. Zadoc Cramer's guide to the Ohio River refers to the first ripple and the second, which he calls Horsetail ripple; he places them along today's Neville Island, in Allegheny County, Pennsylvania. Cramer (5th), 29–30. (Return to text.)

 

3. Cramer calls this Woolery's ripple. Ibid., 30; Baily, 55. (Return to text.)

 

4. Apparently just downstream from Woollery's Trap, in Allegheny County.  (Return to text.)












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