156 GROVE KARL GILBERT— DAVIS [ " CMWa " [ |8OTE 



Science erred in attributing to him a definite age for the falls. He did, however, point out 

 that the chief recession was at the apex of the Canadian or Horseshoe falls, where the retreat 

 is so rapid that the American falls seem not to recede at all in comparison; so rapid, indeed, that 

 if the young couple whose "wedding journey" was described by Howells had, instead of behav- 

 ing foolishly on an islet in the upper rapids, gone to the upland on the farther side of the river 

 and observed the form of the horseshoe reentrant in a sensible manner, they might, if Niagara 

 had been included in their silver wedding journey, have easily detected a visible change in 

 the vertex of the reentrant on repeating their observation. 



In this connection Gilbert, then or later, pointed out that at no very distant epoch in the 

 future, the Horseshoe falls will, on receding above the head of Goat Island, draw off all the 

 water from the American falls; and thereupon Goat Island and the dry channel behind it will 

 repeat on a larger scale the features already found at Wintergreen flats, on the Canadian side 

 of the gorge below the whirlpool ; for there also the original river was divided into a larger and 

 a smaller channel by an island; and there also, when the cataract receded to the island, it was 

 divided into two falls of unequal breadth; and there the recession of the broader falls long ago 

 accomplished just such a diversion of current from the narrower falls on the Canadian side 

 as the broad Horseshoe falls of to-day will in the future accomplish with respect to the narrow 

 and slowly receding American falls. It was by the introduction of realistic items of this sort 

 that Gilbert made his account of Niagara so vivid. 



A great advance was made when the volume of Niagara River, which Gilbert had treated 

 as a possible variable in his analysis of 1886, was shown by another investigator in 1888 to have 

 actually suffered a great diminution of volume for a considerable period by reason of the di- 

 version of the discharge from the upper Great Lakes through a temporary short-circuit outlet 

 that was opened across the Province of Ontario by the retreat of the ice sheet, before the post- 

 glacial upheaval of the land in the northeast had raised that district to a higher level than the 

 outlet of Lake Huron past Detroit; and at about the same time Gilbert himself discovered a 

 second path of diversion of the upper lakes through another short-circuit outlet of later adop- 

 tion farther north than the first. The problem thus became beautifully complicated. Un- 

 fortunately there is no adequate record of Gilbert's method of work upon it or of his progress; 

 but those who occasionally worked with him believe that he was constantly as active in hypo- 

 thetical speculation as he was in outdoor observation, and that his various speculations were 

 continually tested by comparing their "consequents," as he called them, with appropriate 

 facts. His observations were therefore not made at random; they were guided by what the 

 sagacious Playf air called the " clue of theory," and were thus led to " those instantiae cruris . . . 

 that exclude every hypothesis but one, and reduce the explanation given to the highest degree 

 of certainty." 



THE TORONTO LECTURE: 1889 



But although his method of progress is not recorded, the conclusions that he reached after 

 several years of work were most beautifully summarized in a lecture that he gave at the Toronto 

 meeting of the American Association for the Advancement of Science in the summer of 1889. 

 He wrote at that time to a friend: "Fair weather, the hospitality of the Canadians, and a rather 

 good attendance conspired to make the meeting successful"; but he said nothing of his own 

 lecture, which was a part of that conspiracy and a very delightful part. Its presentation was 

 popular in the sense that various important conclusions, based on others' work as well as on 

 his own, were presented briefly and without full proof, but he had full proof in reserve. The 

 story told was fascinating in that it gave a brilliantly rational and coherent meaning to various 

 matters of fact which, more or less known as isolated items, had never been fully understood 

 in their relations to each other; and the story was all the more appealing because it centered 

 upon the great cataract of Niagara, shared and enjoyed by the peoples on its two sides as 

 the chief natural wonder of the eastern United States, during that period of American history 

 when the West was the Far West and when its greater natural wonders were known only to the 

 adventurous few. So lively indeed was the interest excited by the lecture that it was given 



