SCIENCE. 



573 



with irresistible force, and having sand, gravel and boul- 

 ders beneath it, or frozen into it, was the most potent 

 agent of erosion known. The eroding power of the 

 ancient glaciers, which once reached southward to Tren- 

 ton and Cincinnati, was attested not only by the planed 

 down rocks, but by the immense sheet of transported 

 debris left by the glacier in its retreat. 



The glaciated, planed, and polished rocks in the West- 

 ern States are generally covered by a thick layer of clay, 

 abounding in glaciated boulders. There are also other 

 water-worn materials which have been transported, per- 

 haps thousands of miles, representing the gravel bars, 

 sand beds, etc.,- produced by sub-glacial rivers. Al- 

 though the materials are entirely of glacial origin, all 

 the stones are here usually rounded. We find in these 

 deposits, called kames or eskers, the evidences of the 

 action of running water produced by the melting of ice, 

 their accumulation in heaps, ridges, etc., having been 

 effected by local causes, waterfalls, streams upon or un- 

 der the ice, etc. 



The finer material produced by the same grinding ac- 

 tion has been deposited along our coast in the vast masses 

 of the Champlain clays. It is well known that the drain- 

 age of all glaciers results in milky streams ; e. g„ those 

 which descend from the Alps impart an opalescence to 

 the Lake of Geneva, and the streams from the Cascade 

 Mountains are clouded with silt derived from the small 

 glaciers at their heads. So, during the Glacial period all 

 the fine material was sometimes washed out of the glacial 

 drift, leaving banks and ridges, kames, hogsbacks, etc., of 

 gravel and boulders, and carried by streams to the coast 

 and there deposited along shore in the Champlain clays. 

 The fine flour and bran ground by the glaciers have been 

 sometimes referred to different epochs, but they are pro- 

 duced simultaneously. The Glacial or Champlain clays 

 are of great economical importance to the city as they are 

 the brick clays of Croton Point, Haverstraw Bay, and 

 other points along the Hudson. Their thickness reaches 

 100 feet along the lower portion of the Hudson river, 400 

 feet on Lake Champlain, 500 feet at Montreal, 800 feet at 

 Labrador, 1000 feet at Davis' Strait, and 1800 feet at 

 Polaris Bay. This indicates that the continent was de- 

 pressed to this extent at each of these points, that the 

 waters of the ocean extended through these valleys, and 

 that here was dead water into which the glacier drainage 

 flowed and was deposited. 



In the vicinity of New York City it is evident that the 

 glaciers everywhere overrode and disregarded the under- 

 lying topography. All the surface of the island is strewn 

 with materials derived from the N. N. W., and the rock 

 has been planished and striated with grooves running in 

 that direction. The hills back of Yonkers are covered by 

 trap boulders which have been conveyed across the river 

 from the Palisade range on its western side, and it is plain 

 that the Glacier completely disregarded the depression of the 

 Hudson valley, filled it up to a greateror less extent with de- 

 bris, and so rode smoothly over it. Afterwards this and the 

 other valleys were more or less cleared out by the present 

 streams, but a portion of their contents is generally left 

 in their beds, the tunnel between this city and Hoboken 

 being now driven in fact through a part of this clay de- 

 posit. On the east side of the city a narrow canon, 300 

 to 400 feet deep, has been proved to underlie the East 

 River ; and it would have been a wiser and cheaper plan to 

 construct a tunnel through the clay bottom, for communi- 

 cation with Brooklyn, in place of the present costly and 

 to some extent insecure bridge. 



Dr. Newberry finally expressed his interest in the care- 

 ful study of the erosion and sculpture of the Catskills and 

 desire for its continuance. 



Mrs. J. M. Fiske has left a bequest of $40,000, to es- 

 tablish a hospital for the use of the students of Cornell 

 University. 



RINGING FENCES* 

 By Prof. S. W. Robinson, Ohio State University. 



This sketch is mainly of a simple fact of observation. 

 My attention was one day suddenly arrested while walk- 

 ing on a hard road alongside a picket fence by the pecu- 

 liarity of the sound which reached my ear .immediately 

 following each step. This sound was first noticed to be 

 very different from that perceived at other parts of the 

 sidewalk. On instituting an inquiry'for the cause of this 

 difference the only one discoverable was^a change in the 

 construction of the yard fences along the sidewalk. 



The peculiarity observed in change of sound was very 

 marked when passing from a portion of the sidewalk 

 opposite a board fence to parts opposite a picket fence. 

 In the former position a quick drop of the foot upon the 

 walk was accompanied by a simple sound or noise of 

 short duration. But when opposite the picket fence the 

 noise following each footstep was prolonged into a 

 curious musical tone of initial high but rapidly lowering 

 pitch, and with a duration of perhaps a quarter of a 

 second. 



This singular musical tone following, and due to the 

 noise of a simple foot step, could only be accounted for 

 on the supposition that each picket of the fence reflected 

 the sound reaching it from the foot, the rapid succession 

 of which, from the several pickets of the fence, resulted 

 in the sound observed. 



The duration of the sound reflected from the pickets at 

 each step is evidently due to the different distances of the 

 pickets from the ear of the observer, and the greater 

 length of time required for the sound to travel to and from 

 the more distant pickets. For instance, suppose the 

 observer is walking along a stone or mastic walk at a 

 a distance of eight feet from the fence, the latter 

 extending either way some distance along the street. 

 The sharp noise of the footsteps returns from the nearest 

 pickets first. Here the differences in distance from the 

 adjacent pickets is slight, and hence the succession of re- 

 flected noises is rapid. But from more remote pickets 

 the difference of distance is greater, and the succession in 

 reflection less rapid. 



In studying the nature of the resulting tone it is at once 

 seen that the initial pitch is due to an almost infinite 

 number of reflections or vibrations per second, while at 

 the end of a quarter, or half, second the lines of advance 

 and return of the sound are nearly parallel to the fence 

 and hence the pulsations have an interval of time equal" 

 twice the constant distance between pickets divided by 

 the velocity of sound. For instance, if the pickets be 

 four inches apart, or one-third of a foot, the terminal 

 pitch would be one of about 1500 vibrations per second 

 The law of retrogression of pitch may be of interest. To 

 express it as a function of passing time, let 



d = distance from observer to fence. 



a = constant distance between pickets. 



v = velocity of sound. 



n — number of vibrations per second. 



t — the time following the initiation of the reflected tone 



Then by aid of a diagram we easily obtain the follow- 

 ing relation between the above quantities viz ■ 



-4- = i- £ 



4 a- « 2 ( v t + 



The curve for this equation is not easily classified 

 But by computing quantities and constructing a curve it 

 is found to be very much like a hyperbola referred to its 

 asymptotes, which indicates that the pitch falls rapidly at 

 first, and less so subsequently. 



Not only is the above described phenomenal reflection 

 observed in connection with fences, but from any series 

 of flat surfaces in steps, as, indeed, in the case of stairs 

 under proper conditions. Such echoes have been ob 

 served from the steps in front of the State House at 

 Columbus, O. 



*Read at A.A.A.S., Cincinnati. 



