HAY 13, 1S85.]
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��taking tbiit as the dindeod, if we can deter- mine the annual rnte at which the falls recede, and take that for the divisor, our quotient will re]>resent the time that ha^ elapsed since the glacial perioil. The accompanying map gives a more definite idea of that divisor than we have ever before had. The lower doited line represents the margin of the horse-shoe fall as mapped lw the New-York geological aiir- vej" in 1^41, under the direction of Professor James Hall, The upper line is that made in l«7o for the U.S. geodetic survey. Bj' comparing the two, a jiietty correct calculation may be made as to the amonnt of recession of the horseshoe fall iu the interval of thirty- four years. This cannot vary much from a hundred feet upon the whole line, being, as the commissioners calculate, two hundred and seventy feel at certain points.
Until this last survey, the attempts to esti- mate the time required for the cataract to recede l^oni ijueenston to its present ]>osition have becu based upon verj- insufticient data. Mr. Bakewcll, an eminent English geolc^st, gave i)ersonal attention to the problem ns early as 1830. and, from every thing he could learn lit that time, estimated that the falls had re- ceded about a hundred and twenty feet in the forty yeara preceding. He recurred to the problem again in 1846, 1851, 1856 {American jouninl of science, January, 1857, pp. 87, 93), and was each lime confirmed in the belief that the apex of the horseshoe fall was reced- ing, on an average, three feet a year. On the other hand. Sir Charles Lyell, upon his first visit, iu 1«11, 'conceived' (upon what basis he does not tell us), that, at the utmost, the rate could not be more than one foot a year, which would give us thirty-five thousand years as the minimum lime. But, as it appears, the result of the recent survey is to conBrm the estimate of Mr. Bakewell, thus bringing the period down to about seven thousand years.
Two elements of uncertainl}', however, tend- ing to lengthen the estimate, should be noticed. In the first place, the recession may have been somewhat slower while the hard stratum. No. 3, was exposed. In the second place, the de- posits of gi-avol running southward from St. David's, and conesponding to the lake-ridges. indicate that subsettnenl to the glacial period this wbole region was slightly submerged be- neath a shallow body of water; in which ease, the recession of the gorge would liave begun only U|X)n the eniei^ence of the land. And we have no means of telUng how long an inter- val may have elapsed between the withdrawal of the ice and the withdrawal of the water.
��On the oLiier hand, it is probable that the channel of the preglacial stream extended somewhat above the whirlpool, thus reducing amount of work done since glacial time.
The a!K>ve estimates are conflnned, also, by the small amount of change that has taken place in the sj^cies of animals during that jjcriod. The molluska found in the river above the fallsat the present time, are identical species with the shells found in the deserted river- channel at the top of the escarpment opposite the whirlpool, while nearer the falls the bones of the mastodon have been found in the same deposits; all which corresponds with a vast amount of other evidence, going to show thai the present species are, in the main, identical with those existing at the close of the glacial jieriod. The thcorj- of evolution is relieved from a heavy burden by snpgrasing a recent date for the close of the glacial epoch; for the changes since that epoch have been so slight, that the time allowed by the physicists is insufflcient for the whole development of organic forms, unless the rate of change is more rapid than must be the ease if the glacial period is thrown very far back.
G. FtcEPERicK Wright.
��The first suggestion of the possible employ- ment of Niagara Falls as a source of electrical energy, and the distribution of this energy in the shape of light and power, is due to C. W. .Siemens. It was a large suggestion; and it took root speedily iu what may be termed ' cosmical minds.' The way, however, to it« fulfilment, has not been made plain to business enterprise. The most noteworthy remarks upon the subject were made by Sir William Thomson in 1881, at the York meeting of the British association. Sis remarks and calcula- tions were in substance as follows; With the idea of bringing the enet^ of Niagara Falls to Montreal. Boston. New York, and Philadel- phia, a total electromotive force produced by the dynamo- machines at the falls was taken at 80,000 volts. This was between a good earth connection at the falls, and one end of a solid copper wire of half an inch in diameter, and three hundred statute miles in length. The resistance of the circuit was so arranged that there should be an electTO motive force of 64,000 volts at the remote end. between the wire and the earth connection. The calcula- tions showed that a current of ^40 webers
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