542 THE POPULAR SCIENCE MONTHLY. 



Baltimore, the alluvial plain gives place to a narrow valley with 

 steep and wooded hills on either side, and walled with conglomerate 

 or metamorphic rocks. Above Ilchester, a few miles farther up the 

 valley, begin the beds of solid granite, which are excavated by sev- 

 eral quarries near Ellicott City, fifteen miles from Baltimore. It is 

 worthy of observation that these granite beds are replaced, a few miles 

 above, by separate rocks composed of the ingredients of the granite, 

 or resulting from their decomposition. In the cuttings of the railroad 

 five miles above Ellicott City, large masses of nearly pure silica are 

 found, and in the same vicinity feldspar is abundant. The mica is 

 sufficiently stratified to furnish a supply worth working for market ; 

 and asbestos, in more or less advanced stages of formation, is quarried 

 to some extent for purposes not requiring a perfect fibrous structure. 

 Kaolin of very good quality is found, but has as yet been little worked. 

 These rocks will at some future time w r ell repay exploration. 



The object of the present paper is to record a phenomenon observed 

 at Ellicott City, which lies in a narrow and deep valley, nearly at a 

 right angle to the Patapsco, which is here a stream of some sixty yards 

 in width, flowing very rapidly over a broken and rocky bed. About 

 an eighth of a mile below the town, the turbulent stream enters a 

 straight channel with a comparatively even bottom, and flows for 

 nearly half a mile with steady current and a depth of three to six feet. 

 The river is then checked by the dam of a large mill, and presents a 

 fall of about ten feet, with a sheet of water seldom more than six 

 inches in depth pouring over the edge of the dam. 



The stream is here over sixty yards wide, and the body of falling 

 water produces a sound which can not be heard more than four hun- 

 dred yards below, or half that distance above the dam. The impact 

 of the fall is so trifling that it causes no perceptible vibration in the 

 immediate vicinity. Yet,' in the valley where Ellicott City is built, 

 more than half a mile above the fall, the houses on either hill-side, 

 which have foundations deep enough to reach the granite bed-rock, 

 are subject to occasional vibrations which shake their walls and cause 

 a continuous rattling of the windows. This effect has, of course, been 

 ascribed by the people, in an indefinite way, to the river, but it was 

 some time before I collected the physical data for the explanation 

 which is here offered. 



In making the investigation, it was observed that the current of 

 the river above and near the town was broken by rocks, and, although 

 rapid, could produce no rhythmic vibrations because of the irregular- 

 ity of its impacts. The apparently quiet flow of the stream below did 

 not seem to offer any solution of the problem, and the fall of water 

 over the dam had evidently a very trifling force of impact. But a 

 more careful observation of this little water-fall convinced me that it 

 was the motive power. First, a peculiar shimmer in the reflection of 

 light from the edge of the dam, and on nearer approach a distinct 



