NATURAL PHILOSOPHY. 251 



the Natural History Society, opposes the theory presented by Professor 

 Snell, and considers that it is entirely disproved by a dam at Lcwiston, 

 where the water falls over an inclined plane, leaving no space for air under 

 it, and yet the vibrations are very decided. 



According to the views of Mr. Stodder, the phenomenon is caused by that 

 property of falling fluids by which they assume the globular form, which may 

 be seen at the Kauterskiil Falls on the Catskill Mountains, where the whole 

 body of the falling water is broken into drops. Applying this principle to 

 the fall over an artificial dam, the water at the very commencement of its 

 descent begins to assume that form, and the further it descends the nearer 

 it approaches to it. In passing over a dam like that at Iladley, the water 

 presents a uniform depth throughout the whole length of the clam, and if 

 we imagine the current of water to be an infinitude of small streams of uni- 

 form depth in contact with one another, each having the same tendency, the 

 result must be to produce swellings and contractions throughout the whole 

 extent of the dam. AVhen each of these waves strikes the bottom, it gives 

 a blow proportioned in force to the body of water falling from the height of 

 the dam. Every variation in the depth of the water causes a variation in 

 the size and distance of the waves, each of these causes a concussion in pro- 

 portionate intensity to the weight of water in it, and in rapidity to their dis- 

 tance apart. These effects of falling water should be expected in general 

 only on artificial falls, such as mill dams." Respecting natural falls, he 

 says : " As their faces are rarely vertical, but are broken with angular 

 rocks, causing various depths of water on them, and as every variety of 

 depth alters the conditions to form the concussive pulsations, there is no co- 

 incidence among them, so that the waves of one part strike the bottom in 

 the intervals of those of another part, and thus the concussion of one neu- 

 tralizes the other. At Hadley, the dam is one right line from bank to bank, 

 the bed of the river is solid rock, and the top of the dam is level. The 

 waves or pulsations of falling water are uniform, and strike the bottom 

 with sychronous concussion from one end of the dam to the other. It is 

 not surprising that the earth should be felt to vibrate at a great distance." 



RAPIDITY OF THOUGHT, OR NEETOUS ACTION. 



The method of transforming the valuation of time into space by the rapid 

 revolution of a cylinder, proposed by Mr. Fizcau, has been applied to the 

 measurement of the rapidity of nervous impulse. Such a cylinder rotating 

 1000 times a second, and divided into SCO degrees, may measure 1-360,- 

 000th part of a second; or rotating 1500 times a second, l-540 ; 000ih part 

 of a second ; and even this may be subdivided by a microscope, ?o as to ob- 

 tain the ten-millionth or perhaps 100-millionth part of a second. By this 

 extreme minuteness of subdivision of time, it is not difficult to measure even 

 the rapidity of a nervous impulse. If an electric shock be given to the arm, 

 it produces a sensation, and a contraction of the muscles. Hence by noting 

 the interval of time between the shock and the contraction, the time oc- 

 cupied by the transmission of the sensation and the action of the brain, how- 

 ever quick, will be determined. By tiying the experiment with different 



