Dr. W. Knight on the Vibration of heated Metals. 329 



affords a familiar example. We have often heard of a poker 

 producing tone when heated and the point rested on a knot 

 in a fir board in the floor of a room ; also the singing in a tea- 

 kettle is another example. In distilleries, shortly after the fire 

 is put to the cool coppers, a very loud note is given out, and 

 continues until the liquor boils*. 



John Robison, Esq., Secretary to the Royal Society of Edin- 

 burgh, informed me that he once let a heated bar fall from his 

 hand: it alighted on a painted shelf of wood, when he was sur- 

 prised at hearing sounds ; but they soon ceased. 



The following theory of the cause of earthquakes and vol- 

 canic eruptions strikes me as being not at all unlikely. Earth- 

 quakes, and the sounds accompanying them, may be caused 

 by vibration, occasioned by heat generated far below the sur- 

 face of the earth, in some enormous metallic mass, which be- 

 ing in contact with some cool substance, not a very good con- 

 ductor of heat, the latter is violently agitated, thus producing 

 the vibration felt in earthquakes. By its intensity chasms are 

 opened on the face of the earth ; and below, caverns filled with 

 condensed combustible matter and liquid lava are torn open, 

 and the contents, by their enormous expansion, and having 

 found an egress more easily upwards, rise to a great height 

 above the surface of the earth. 



The following interesting remarks on the vibration of 

 heated metals are copied from a letter received by my brother 

 from Dr. W. Knight, Professor of Natural Philosophy in 

 Marischal College, Aberdeen, dated June 8, 1 833. 



" I regret that I have not written sooner in reply to your 

 letter of the 9th ult., but I waited until I should find a conve- 

 nient opportunity for repeating many of the experiments which 

 I hinted the general nature of to you in my letter of the 19th of 

 April. These opportunities have unfortunately not occurred 

 yet, from myself being engaged with a daily class here, and 

 my family at a country residence at some distance ; nor are 

 they likely to occur soon, so that I must content myself with 



* The sounds from the statue of Memnon and the mountain at Carnac, 

 and those from machinery and a fire-grate, have already been explained, 

 nearly in the above words, by Sir John Herschel, (see Lond. and Edinb. 

 Phil. Mag. vol. i. p. 221,) but without reference, however, to the new phe- 

 nomena of vibrating metals, with the cause of which, indeed, contrary to 

 Mr. Trevelyan's opinion, we apprehend that they are not essentially con- 

 nected. The sounds heard on the granite rocks of the Orinoco are attri- 

 buted by Sir J. Herschel, in the same letter, to sonorous vibrations of the 

 air passing through small orifices, either subterranean or communicating 

 with the atmosphere. The singing in a tea-kettle, and the note given out 

 by stills, are referrible, we conceive, to another order of causes, connected 

 with the rapid condensation of vapour, and quite distinct from all the for- 

 mer as well as from those concerned in the vibration of heated metals. — Ed. 



Third Scries. Vol. 3. No. 17. Nov. 1833. 2 U 



