534 Prof. W. Beetz on the Tones produced 



heat developed in a metallic disk rotating rapidly in the exhausted 

 receiver of an air-pump*. That the periodic retardation of Encke's 

 comet is due to the resistance of the universal medium is highly 

 probable ; but this experiment, if it proves anything in the same 

 direction, proves a great deal too much : for if such be the vis- 

 cosity of the medium hypothetically present in the receiver, that 

 the addition of a notable quantity of air (5 or 10 per cent, as 

 was stated) makes no sensible difference in the heat generated 

 by friction, it is dime alt to conceive how any of us have hitherto 

 escaped resolution into our gaseous elements. On the contrary, 

 when our earth and its envelope enters a probable atmosphere of 

 orbitating fragments (of which we have recently had such a mag- 

 nificent experience), some of these crumbs of the universe which 

 have for an indefinite period harmlessly traversed the sethereal 

 medium of infinite space, at the enormous velocity of perhaps 

 30 or 40 miles in a second, become immediately ignited, and 

 probably consumed by friction in the confines of our atmosphere, 

 which must there be attenuated to a degree never yet attained, 

 except perhaps in the vacuum-tubes of Mr. Gassiot. 



Permit me in conclusion to repudiate (needlessly perhaps) any 

 claim to originality in the general idea that the molecules of pal- 

 pable mater are permeable to the undulations of light, which has 

 long since been ably advocated by Euler and by Mr. Grove f. 



I remain, 



Yours faithfully, 



Charles Brooke. 



LXXI. On the Tones produced by Rotating Tuning-forks. 

 By Professor W. Beetz J. 



THE brothers Weber, in their investigations into the laws of 

 waves, have described the following experiment §:— " If a 

 tuning-fork is fixed in a lathe so that it can be made to revolve 

 about the longitudinal axis of the stem, the sound of the vibra- 

 ting fork is observed to cease when the speed of revolution has 

 reached a certain point, but it becomes again perceptible if the 

 driving-wheel of the lathe is suddenly stopped. This pheno- 

 menon cannot be explained by supposing it to be due to the 

 drowning of the sound of the fork by the noise of the lathe ; for 

 even when one end of a cylindrical tube is brought near the 

 prongs, and the ear is put at the other end of the tube, it becomes 



* Proceedings of the Royal Institution, vol. iv. p. 563. The writer has 

 since learnt that the aether-friction theory has been judiciously withdrawn 

 by its author for further consideration. 



t Correlation of Physical Forces, p. 163, &c. 



% Communicated by the Author, from Poggendorff's Annalen, vol. exxviii. 

 p. 490 (August 1866). 



§ Wellenlehre, p. 519. 



