July 31, 1908] 



SCIENCE 



149 



Goat Island, of 192 feet. Borings also were 

 made, in the region of the pre-Niagara 

 channel from the Whirlpool to St. David; 

 and the crest of the Canadian fall was remapt. 



The oldest view of the falls, a view based 

 on the observations of Father Hennepin in 

 1678, represents a jet as pouring from the 

 western shore athwart the face of the Cana- 

 dian sheet. In a general way the sketch is 

 crude, exaggerated and untrustworthy, but 

 this particular feature is of so unusual a char- 

 acter as to encourage the belief that it cor- 

 responds to something that Hennepin actually 

 saw — some peculiarity in the cataract which 

 no longer exists. Spencer has been able to 

 connect it with an old hollow or channel on 

 the Canadian shore, a hollow now filled and 

 efEaced, and by means of this connection in- 

 fers the approximate position of the cataract 

 more than two centuries ago. He thus ob- 

 tains an additional datum for the computa- 

 tion of the average annual recession of the 

 falls in modern times and secures a rough but 

 ■valuable confirmation of the result based on 

 the definit surveys of later years. 



There is wide interest, both popular and 

 scientific, in the problem of the age of 

 Niagara, or the time that has elapst since the 

 cataract began, at the cliff near Queenston 

 and Lewiston, the excavation of the gorge; 

 and the fact of that interest is the reviewer's 

 excuse for giving special attention to the 

 author's discussion of this question. Spencer 

 treats the subject at considerable length, and 

 has much confidence in the result of his com- 

 putations — an estimate of 39,000 years. 

 " Slight variations on one side or the other 

 are probable, but under the conditions, all of 

 which are now apparently known, the error in 

 calculations will not exceed ten per cent." 

 (p. . 11). The reviewer unfortunately fimds 

 himself unable to share this optimistic view. 



The general plan followed in the computa- 

 tion is this : The present average annual rate 

 of recession of the Canadian fall is estimated 

 from maps made in 1842 and 190^5. This 

 rate is associated with a particular hight of 

 the fall and with a particular volume, or dis- 

 charge, of the river. At earlier stages in the 

 history of the cataract its hight was different, 

 and the discharge was different; and the com- 



putations make allowance for these differ- 

 ences. The differences in hight were con- 

 nected chiefly with the southward dip of the 

 strata, and with the variable altitude of the 

 base-level afforded by standing water in the 

 Ontario basin; and the range of bights was 

 from 35 to 280 feet, the present effectiv hight 

 being 180 feet. The differences in volume 

 were all in one direction.' During the early 

 part of the period of recession the water of 

 the Huron and higher basins reacht the sea 

 by a different route, the Erie drainage only 

 flowed thru the gorge, and the discharge was 

 15 per cent, of the present. At a 'later epoch 

 there may have been a temporary diversion of 

 a fraction of the Huron discharge, reducing 

 the river to 67 or 75 per cent, of its present 

 volume. To combine these various factors, 

 the gorge is divided into sections, each section 

 is computed by itself, and the whole is 

 summed. 



The principle on which allowances are 

 made for differences in the hight and dis- 

 charge of the cataract is thus stated (p. 350) : 

 " According to mathematical laws, erosion is 

 proportional to the hight of the falls and the 

 volume of the river, provided other conditions 

 remain constant." The context interprets 

 " erosion " in this formula to mean rate of 

 recession, and " volume " to mean discharge ; 

 so that the law may be more definitly stated: 

 Rate of recession is proportional to the hight 

 of the falls and the discharge of the river. 

 As the energy of the cataract (per unit time) 

 is measured by the product of hight, or head, 

 into discharge, it is implied that the rate of 

 recession is proportional to the. energy of the 

 cataract. (In a footnote Spencer says " The 

 erosion varies with the mass and square of 

 velocity," which also implies that it is pro- 

 portional to the energy.) 



I put the law into this form for the sake 

 of comparing it with the experience of me- 

 chanical engineers. The cataract is a natural 

 engin, and the erosion and recession cor- 

 respond to what Eankine calls " useful work " 

 in the discussion of artificial engines. As the 

 ratio of the useful work rendered by an engin 

 to the energy it receives is the " efficiency " of 

 the engin, so the quantitativ relation between 

 the recession of the cataract and its energy is 



