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SCIENCM 



[N. S. Vol. XXVIII. No. 726 



away from the gorge in the recession of the 

 falls, which is occasioned by the undercutting 

 in the soft strata, and not by the abrasion 

 from an infinitesimal quantity of detritus. 

 The capacity of streams for transportation 

 varies as the sixth power of the velocity, while 

 the erosion varies only as the square of the 

 velocity. I do not understand the ground for 

 comparing the clear-water Niagara with the 

 experiments based upon transportation of 

 muddy water of other streams. But we can 

 get an inkling as to the relative abrasion at 

 Niagara. Only since the falls have receded 

 the last mile and a half have the Upper 

 Rapids come into existence (1,500 years ago 

 or less). In the meanwhile the river, with its 

 dissolving waters, ice, detritus and currents, 

 has not made any true channel next to the 

 head of Goat Island, while at the foot, after 

 the rapids have descended over fifty feet, all 

 of these forces together have only cut in the 

 rock to a depth of four feet — and this while 

 the falls have been receding a mile and a 

 half. If the recession were proportional to 

 the detritus, which he assumes as ground for 

 quadrupling my computations, we should cer- 

 tainly have to raise his conjectural age into 

 millions of years. The proposition of meas- 

 uring the work of Niagara Falls by transporta- 

 tion of detritus is certainly irrelevant. 



Concerning the soundings, it is further 

 stated that my apparatus was of the Kelvin 

 type. This statement requires correction. 

 That used by me was invented and patented 

 by Lieutenant Blish and Commander Tanner, 

 officers of the U. S. Navy. It is entirely 

 a different and better device for recording the 

 depths than that of Kelvin, whose instru- 

 ment could not have been used in fresh 

 water. Commander Tanner kindly lent me 

 some of the apparatus. 



Dr. Gilbert says: 



But while the discovery of the real law of effi- 

 ciency would be a notable contribution to the 

 problem, it would not remove every difficulty. In 

 its proper application there would be need to take 

 account of various qualifying conditions, not all 

 of which are easily evaluated. 



He then specifically mentions five. Here 

 indeed I not only accept his conditions, but 



I did anticipate them, giving the most de- 

 tailed measurements of them, with many 

 sections drawn to scale, and other illu- 

 strations. These I shall take up consecu- 

 tively. 



(1) " Width of gorge as affecting quan- 

 tity of erosion," and I may add depth. These 

 subjects are fully discussed, with numerous 

 sections determined instrumentally, in chap- 

 ters V. to X. and parts of XII. and XIV. 

 (2) " Depth of the gorge from the crest of 

 falls to bottom of pool." This has been 

 measured for the first time, by me, and de- 

 scribed in a headed page (56), figured sec- 

 tion (p. 48), further explained (p. 86), and 

 applied in calculations (p. 367). (3) " Con- 

 centration of flow as affecting efficiency." 

 This is discussed, in relation to points raised, 

 in chapters IX., XII. and XIV. (and p. 368). 

 (4) " Thickness of the capping limestones as 

 affecting efficiency." This is shown in tabu- 

 lar form at many points (p. 90), in many 

 figured sections (Chap. VIII. and pp. 50, 343), 

 and in other references and with a time allow- 

 ance (p. 369). (5) "The relation of the 

 Medina sandrock to efficiency." A table is 

 shown to illustrate this (p. 90), also many 

 figured cross-sections (Chap. VIII.) and longi- 

 tudinal ones (pp. 50 and 343). These show 

 that the Medina sandrock had no relationship 

 whatever to the uppermost cataract upon which 

 was based the recession during the first 35,000 

 years, and in the last 3,500 years, to only an 

 infinitesimal amount. These features may be 

 redescribed by others, but any student wiU 

 find all of them described in my book as no- 

 where else, many for the first time, which will 

 furnish him with data for working out spe- 

 cific problems that I may have deemed imma- 

 terial. Finally he says that " Spencer's com- 

 putations do not include data bearing upon 

 these variables." I am surprised that any one 

 should have read my book and failed to take 

 account of the distinct and articulate chapters, 

 headed paragraphs and illustrations, covering 

 all of these points. He also mentions my dis- 

 cussion of the present stability " of the land in 

 the Great Lakes region, with the conclusion 

 that no earth movements have occurred in 

 modern times," without further comment, as 



