756 



SCIENCE 



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



whom he quotes, mentions that in numerous 

 trials the efficiency of different wheels varied 

 from 66 to 80 per cent. The loss actually 

 questioned is the difference between these 

 figures, which shows that the relative work 

 performed by the worst machine is only 17.5 

 per cent, below that of the best. In the tur- 

 bine wheel there is a loss from leakage as well 

 as from friction, which does not occur in the 

 natural falls, where, however, there is wind 

 effect. As the American Falls represent 

 200,000 gross horse-power, which is very much 

 greater than any turbine, the relative loss of 

 power should be very small compared with 

 even the best engine. In testing the relative 

 loss of power in turbines, Kent^ mentions that 

 in diminishing the supply of water to haK the 

 flow, in one kind of turbine, the useful work 

 fell six per cent. only. This is the best kind 

 of illustration for comparing the efficiency of 

 the changing volume of the cataract with a 

 man-made engine. The loss of work to the 

 extent of six per cent, in the American Falls 

 over that of the Canadian, due to smaller 

 volume, would cause only an increase in the 

 computed age of Niagara Falls of less than 

 2,000 years (or a difference in efficiency of ten 

 per cent., would add less than 4,000 years) in 

 excess of the 39,000 years. This only possible 

 increase, if not counteracted by other minor 

 elements, is provided for in the anticipated 

 variation of ten per cent. (p. 369). The small- 

 ness of the variation suggested is better appre- 

 ciated when it is compared with the discoveries 

 of increased energy (1,000 per cent.), which 

 has raised the supposed age of the falls from 

 less than 7,000 years (Gilbert, formerly) to 

 39,000 years (Spencer). 



However, if any allowance should be made, 

 as above set forth, there is another physical 

 component of about equal importance, the 

 effect of which would counterbalance any rela- 

 tively inferior efficiency of the smaller falls. 

 Both are small factors compared with the great 

 changes of energy discovered. 



The second point raised is a plea for meas- 

 uring the age of the whole gorge on a basis of 

 the recession of the American Falls. He 

 says : " The American Fall does not differ 



- Kent's " Engineering Pocket Book," p. 596. 



greatly from the computed energy of the main 

 cataract during the longest division of its his- 

 tory," and further states that if I had used 

 these falls for the basis of my calculations (or 

 a formerly provisional rate based upon incom- 

 plete data obtained from measurements made 

 in 1868, since abandoned for complete ones), 

 the age would have been nearer 20,000 than 

 39,000 years, but I may say that with the full 

 treatment of the subject, from even the partial 

 data published, any one would have found the 

 computation to have reached 32,000 years. 

 But no computations based on the American 

 Falls were attempted by me. He also says 

 with regard to the recession of the American 

 Falls : " It may with confidence be said that 

 0.06 foot a year [in recession] is nearer the 

 truth than 0.60, but no definite estimate is 

 warranted." This latitude in comparison 

 reaches 0.32 foot (thirty-two hundredths), 

 which appears to be what he meant, because 

 in his official bulletin," page 22, he gives this 

 figure as one of his conjectures, but in the 

 controversy one would think that he meant 

 little more than the lesser amount (0.06). 

 Using his coefficient of 0.32 foot, the age of 

 Niagara would be over 37,000 years (not 20,- 

 000 years). 



In my Niagara book, I stated that the 

 American Falls had receeded 0.60 foot between 

 1842 and 1890, which was true for that period. 

 More recent surveys show that afterwards, 

 down to 1906, no further important changes 

 had occurred. Thus the measured rate is re- 

 duced to 0.47 foot a year. This must be 

 further diminished to 0.35 foot in comparing 

 the work with that of the Canadian Falls. 

 The mean total longitudinal recession has been 

 only 29 feet since the first measurements were 

 made, while that of the greater cataract has 

 been 265 feet, and the discharge measurements 

 show that it carries 95 per cent, of the volume 

 of the whole river. 



The recession is intermittent; in the one 

 case the amount is so small that we do not 

 know whether the cycle has been completed 

 during the period of observation or not. Here 

 a small error from any cause would prevent 



= Bulletin U. S. Geological Survey, No. 306, 

 1907. 



