280 



The substances which lowered the pitch in the above experi- 

 ments have one common property, namely, porosity, and, as far it 

 could be ascertained, the depression of pitch was in proportion to this 

 condition. Is it possible that the small vacuities included in the 

 substance, acting as resounding cavities, and reflecting the vibrations 

 from their walls, may so direct them that they may somewhat interfere 

 with one another, and thereby be somewhat diminished in number ? 

 The following experiment tends in some degree to support this con- 

 jecture. It is well known that if the vibrating fork be held obliquely, 

 resting on the table, " a loud resonance is audible ; but if the tuning- 

 fork be moved parallel to itself along the surface of the table, the 

 resonance of the table immediately ceases from the interference of 

 the planes of vibration with each other ;" but if the fork is moved 

 so slowly, and so that the resonance is not completely destroyed, 

 the pitch falls slightly. 



Again, if the fork be applied to the head, and listened to first with 

 the ear open, and afterwards with the ear closed, the pitch is found 

 to be slightly lowered. 



In all those experiments in which the pitch was elevated by con- 

 duction, it was found that there was diminished intensity in pro- 

 portion to the elevation of pitch ; thus it would appear that all bodies 

 raise the pitch in proportion to the difficulty with which they receive 

 and conduct vibrations. 



Dr. Scott Alison has proved in some recent experiments, that the 

 conductivity of media, as regards rapidity, does not correspond with 

 that of intensity. Of all tried substances, iron was the worst con- 

 ductor as regards intensity, and this was found to raise the pitch 

 most. 



The above explanation is rendered somewhat probable from the fact 

 that in all cases the elevation was greater with a weak note than a 

 strong one. Dr. Wylde tells me that it has long been noticed by 

 musicians that a weak note is somewhat higher pitched than a strong 

 one, it being under these circumstances caught through the medium 

 of the air. Those bodies which, on the contrary, lower the pitch, 

 do so to a greater extent with a weak note. 



These explanations are offered with the utmost diffidence, on ac- 

 count of my very limited knowledge of acoustics. 



On looking into the literature of the subject, the only reference to 



