352 NATURAL SCIENCE. May. 



note which is audible to man to have been one of them — i.e., a note 

 produced by sixteen vibrations per second, or thereabouts. In water 

 this note has a wave-length of half-a-furlong, i.e., nearly four 

 thousand inches. One five-billionth of this is one one-thousand-two- 

 hundred-and-fifty-millionth of an inch (T^socfooooo inch). This can 

 hardly be the length of the " violent " vibration which is said to have 

 rendered the hairs invisible. Suppose the language used in des- 

 cribing the result to be not greatly exaggerated, we may safely say 

 that a movement at least one million times greater than this was 

 referred to. This is, however, the greatest movement producible in 

 water by the passage of any audible note when the intensity is such 

 as to render the note just audible at a distance of half-a-mile. The 

 employment of notes of this low pitch, and of higher intensity than 

 this, would be fraught with great danger, for the apparatus of the 

 middle ear of man could hardly stand much more. To reach 

 the intensity required for the production of the above result, a 

 steam "devil" such as is used on large steamers would be re- 

 quired, and would have to be close to the ear of the observer 

 who was watching the hairs under the microscope. The man 

 who would risk putting his ear within a foot or two of such 

 a source of sound when this was put into operation to the utmost 

 limits of its powers would indeed be a bold man. I doubt, however, 

 if the most powerful of these " whistles " ever made has ever been 

 heard at a distance of thirty miles. Supposing y/go of an inch to be 

 the extent of the violent movement rendering the hairs invisible, i.e., 

 a little over twice the diameter of a red blood-corpuscle, then the 

 intensity of the sound required, supposing the note to be C,v( = i6 

 vibrations per second), would be such as to render it audible at a 

 distance of 500 miles in air, and much further in water. Supposing 

 it to be at such distance from the head of the observer as to have an 

 intensity equal (in air) to that operating (in water) on the hairs, then 

 the air in contact with his head would move with an amplitude of 

 about eight inches, and with a force which no human skull could 

 withstand for a moment. A microscope would be instantly destroyed 

 by it. I doul:)t whether even any ordinary building could resist its 

 destructive power, and do not for a moment believe it possible to 

 construct an instrument capable of "sounding a scale" with such 

 intensity as this. 



We may, therefore, in spite of the authority of so eminent a 

 physicist as Helmholtz, rest perfectly assured that the effects de- 

 scribed have never been either produced or repeated or verified by 

 either Hensen or anybody else. It is, however, not the record of the 

 experimental results which is at fault, but the explanation of them. 

 Vibrations in the water produced by the sound-waves in air, or by 

 vibrations communicated from the source of sound tlirough the table, 

 microscope, etc., have been mistaken for sound-waves in water. 



It is, perhaps, inaccurate to assume, as I have done, that the 



