May 17, 1894] 



NATURE 



53 



tance of ihe Zodiacal circle ; now it is certainly true that a much 

 smaller weight will suffice to proJuce equilibrium. Suppose, 

 for instance, that the Zodiacal circle has a radius equal to the 

 distance of the moon, or sixty limes the earth's radius ; then 

 22,400^3600, say, 61b., suspended near the earth's surface at the 

 end of the suing, some 240,000 miles long, will suffice to 

 balance ico tons in the other scale pan, close up to the Zodiacal 

 circle. But if the 100 tons is also lowered down by another 

 mathematical string to the surface of the earth, then 100 tons is 

 requited to equilibrate it; and so it will for any intermediate 

 position, when the lengths of the strings are equal ; so that it 

 does not tend to any clear or useful end to say that the weight of 

 100 tons at the distance of the moon is only about 6 lb. On the 

 contrary, the weight of 100 tons is everywhere 100 tons. 



The wight of a body xsihelkal-iuhich is measured in lb., tons, 

 kilogrammes, or other standards of weight ; and these standards 

 are certain lumps of metal licensed by .^cts of Parliament, and 

 carefully preserved against change or degradation. 



The weight of the Earth, for instance, as determined by the 

 Cavendish experiment, is about 6 x 10"' tons ; and it is not 

 necessary to dig up the Earth and 10 weigh the fragments at the 

 surface of the Earth for the determination of this weight. 



It is too late now to change the meaning of a word that has 

 been in immemorial use in all languages ; such a quasi-Gjihic 

 revival would have to restore all literature, as, for insti^nce, the 

 lines of Ovid (An. Am. 3, 319) : — 



" Qua: nuncno nen hab;nt operosl signa Myronis 

 Fondus iners quond-ini duraque massa fuil." 



A. G. Greenhill. 



The Niagara Riveras a Geologic Chronometer. 



When we quote an author whose views coincide with our 

 own, we are apt to speak of him a-i an authority on 

 the subject ; but when we dissent from the views we quote, 

 we are not so apt to recognise the higli authority of 

 that author. This reflection on a phase of mental bias 

 is suggested by a personal experience with reference to 

 the age of Niagara Falls. The geologis's and others who 

 have discussed the length of post-glacial time may be rudely 

 classed as minimists, maximists, an 1 agnostics. Within the 

 past five years I have been frequently and approvingly quoted 

 by the minimists as estimating tne portion of time consumed by 

 the Niagara River in cutting its gorge at 7000 years, and the 

 reputation thus acquired has not been noted without personal 

 appreciation. But self-complaisance in that regard has been 

 somewhat impaired by the thought that the honour is ill- 

 founded, and that the insecurity of its foundation would sooner 

 or later be discovered. Not less disturbing was the fear that 

 when the maximists or agnostics look their turn at writing, I 

 should be classed with the goats instead of the sheep. There 

 can be no doubt that the manly ani in every way proper cour.-^e 

 would have been followed had I years ago disclaimed the glory 

 accidentally thrust upon me ; but it is easy to bask in the sun- 

 shine of even unmerited applause, and conscience was too weak 

 to determine action until another mo'ive was added by a blow 

 from the agnostic side. In his recent book Dr. James Geikie, 

 after quoting me as an authority for the yoooyear estimate, adds 

 that "all such estimates are in the nature of things unreliable." 

 I now hasten to declare that I never said or thought that the 

 period in question was about 7000 years. What I did incautiously 

 say was, in effect, that the time allowance for the cutting 

 of the gorge would be about 7000 y.;ars if the rate of the cutting 

 were uniform, but that there vva> good reason to believe the rate 

 had not been even approximately uniform. 



Dropping personaliiie-, which lack interest for your re.aders 

 unless they involve principles, I beg to say a few words on the 

 actual value of Niagara Falls as a chronometer. In 1844 lames 

 Hall made a map of the brink of the Falls, and established 

 bench marks to which changes could be referred. Within a few 

 years several other surveys had been m,adt: and connected with 

 the first bench marks. It has thus become known, first, that in 

 the middle of the Horseshoe Fall, where the princi|ial body of 

 water descends, the brink retrogrades at the rale of four or five 

 feet per annum ; second, that the American Fall, carrying a 

 much thinner sheet of water, retrogrades so slowly that its rate 

 is concealed by errors of survey. The gorge, which has been 

 cut since the ice sheet retreated from th; region, is six miles 

 long, and the division of this distance by the annual rate de- 

 termined for the Horseshoe Fall yields a period agreeable to Ihe 



KO. I 28 I, VOL. 50] 



minimists. Had the conditions remained uniform, no fault 

 could be found with this estimate, but there is reason to believe 

 that the conditions have varied enormously in nearly every 

 particular. The thickness of the resistant bed at the crest of 

 Ihe Falls is far from uniform, and it was aliogether wanting for 

 part of the distance. During the period of gorge-cutting, the 

 height of Lake Ontario, which gives bast-level to the river, has 

 varied through a range of several hundred feet. The volume of 

 the river has doubtless varied somewhat through climate, but 

 it has probably varied enormously by reason of changes in drain- 

 age systems, resulting chiefly from differential uplift. The 

 Niagara now carries surplus water from the basins of Lakes 

 Erie, Huron, Michigan, and Superior. There was probably a 

 post-glacial epoch during which three of these lakes discharged 

 their water in other directions, and only the basin of Lake Erie 

 fed the Niagara River. During that epoch the volume of the river 

 was so small that canyon-cutting was effected only by the feeble 

 process now illustrated by the American Fall, instead of the 

 vigorous process illustrated by the Horseshoe Fall. These con- 

 siderations, which the inquiring reader may find more fully set 

 forth in the annual report of the Smithsonian Institution for 

 1890, tend strongly 10 sustain the agnostic view of the Niagara 

 River as a geologic chronometer. G. K. Gilbert. 



Washington, April 30. 



The Teeth and Civilisation. 



On the Sth inst.. Dr. Wilberforce Smith read a short com- 

 munication before the Anthropological Institute on the teeth of 

 ten Sioux Indians attached to the Wild West Show. His 

 investigation showed that in regard to molars and premolars 

 (the only teeth examined), these Indians were wholly free from 

 caries. In the discussion which followed the reading of the 

 paper, it was mentioned that the same fact was revealed in the 

 skulls of the Fourth Egyptian Dynasty brought to England by 

 Dr. Flinders Petrie, and in some skulls examined by Dr. 

 Wilberforce Smith himself, which were derived from the ruins 

 of Pompeii. The teeth of the Indians, both old and young, 

 and those in the skulls just referred to, all showed more or less 

 wear of the cusps, which is a most unusual circumstance in the 

 teeth of modern civilised people, and it was thought that some 

 difference in the food, or its mode of preparation, would be 

 required to account for the absence of signs of wear in our time. 



Now, it has never been proved that the increasing preva'ence 

 of caries is due to weakness of the teeth owing to comparative 

 disuse, but there is nevertheless great probability in the infer- 

 ence, especially as signs of wear and freedom from caries appear 

 to occur together, and vice vcrsii. There is, however, a further 

 point in regard to the existing liability to the attacks of caries 

 which I think can he best explained by a transference of nourish- 

 ment to other parts governed by the same nerves. On inquiry 

 of several dentists, I find that the teeth most subject to decay 

 are the molars, and of these the upper molars are more often 

 attacked than those in the lower jaw. The molars of the upper 

 jaw are fed by a branch of the filth nerve, and in m idem life 

 this nerve has, perhaps, more strain put upon it than any other 

 in the body. We use our eyes, partly supplied by the ophthal- 

 mic branch of this nerve, not at intervals, but often closely 

 throughout a long day. And it seems, therefore, that with so 

 many increasing calls on this bundle of nerve fibres, the 

 filaments sent to the teeth are, by an automatic economy of 

 expenditure, robbed of the energy necessary to perform their 

 functions properly. The teeth through lack of use may not excite 

 the nerves to natural action, and thus from both sides there is 

 a failure of function, and the teeth are consequently more and 

 more unable to resist the attacks of caries. I am disposed to 

 attach some importance to this explanation, as I find that those 

 who have great calls on their nervous energy are more liable to 

 caries than people of quieter habit and slower temperament. 

 Dr. Wilberforce Smith mentioned the alarming increase of 

 ilental decay amongst hospiial nurses, whose occupation is 

 certainly one demanding a constant drain on their nervous 

 energy. It was also noted that people in towns lose their teeth 

 more rapidly than those living in the country, which also bears 

 out the idea here suggested. On th,; other hand, the savage is 

 seldom required to strain his facial nerves continuously for any 

 length of time, and in reference to general nervous expenditure 

 he enjoys long periods of rest which are wholly denied to the 

 civilised man in towns. No doubt in consequence of the 

 excessive calls on our nervous energy the distribution of it is 



