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The remark on the upper right side of Clark's sketch, "This rivine is 2 inches higher up," meant that the north ravine was actually 200 yards upriver from the location where he showed it – an arbitrary space-saving decision. That "umbrello" that Clark lost in the flash flood (line 14) is discussed in "A Matter of Melanins?". Another critical loss suffered in that incident is revealed in "Pomp's Bier", especially on the first page, titled "Breathless Moment". The omission of figures for the longitude, at lower right, was deliberate. The captains made numerous celestial observations to acquire the data from which a mathematician was to have calculated the longitude after their return, but that work was never completed.
Clark recounted his first impression of the Great Fall including its height and widths on June 17, but whether he would have had time to take the measurements on that day is not known.
we proceeded up the river passing a Sucession of rapids & Cascades to the Falls, which we had herd for Several miles makeing a dedly Sound, I beheld those Cateracts with astonishment the whole of the water of this great river Confined in a Channel of 280 yards and pitching over a rock of 97 feet 3/4 of an [inch], from the foot of the falls arrises a Continued mist which is extended for 150 yds. down & to near the top of the Clifts on L Sd. the river below is Confined a narrow Chanl. of 93 yards haveing a Small bottom of timber on the Stard Side which is definded by a rock, rangeing Cross wise the river a little below the shoot, a short distance below this Cataract a large rock divides the stream, I in assendending the Clifts to take the hith of the fall was near Slipping into the water, at which place I must have been Sucked under in an instant, and with deficuelty and great risque I assended again, and decended the Clift lower down (but few places Can be descended to the river) and took the hight with as much accuricy as possible with a Spirit Leavels &c.
In 1853 the Stevens Railroad Survey's engineers measured the distance from the Mississippi River to the Great Fall at 2445 miles, and the fall's height at 76 feet.2 The correct latitude is 47° 30' North; the longitude is 111° 18' West.
The geometric procedure Clark used to measure the widths of rivers is illustrated in the discussion of the confluence of the Snake and Columbia Rivers. Other ways of making measurements of heights, besides the one Clark said he used, are illustrated in "Heights and Distances"
The north-side route from the plains to the island below the falls, which Clark points out in line 12, probably was the one Lewis had descended on June 13. He reached the overlook at about noon, having covered some 15 miles that morning. Immediately, Lewis wrote,
1. The journalists referred to it by several names – "great Fall," "grand Fall," and "great Cateract." On U.S. Geological Survey maps today it is labeled "Big Fall." The journalists were ambivalent toward "fall" versus "falls," but "cateract" and "cascade" were always singular.
2. Isaac. I. Stevens (1818-1852), Narrative and final reports of explorations and surveys to ascertain the most practicable and economical route for a railroad from the Mississippi river to the Pacific Ocean . . . made under the direction of the secretary of war in 1853-1855.