486 ON THE ABSORPTION OF LIGHT 



fasten with sealing-wax, on one of them one, and on the 

 other two, disks of card (all equal in size), on the inner 

 surfoces, having the plane of the card perpendicular to 

 that of a section of the fork through the axes of both its 

 branches. The cards on that fork which has two, should 

 have their surfaces about a tenth of an inch asunder, and 

 their centres just opposite ; and the other fork should be 

 brought into unison with it by loading its undisked 

 branch with additional wax, equal in weight to the disk 

 and wax on the other. Now strike the forks, and a re- 

 markable difference will be perceived in the intensity of 

 their sounds. The fork with one disk will utter a clear 

 and loud sound, while that of the other will be dull and 

 stifled, and hardly audible, unless held close to the ear. 

 The reason of this difference is that the opposite branches 

 of the fork are always in opposite states of motion, and 

 that in consequence the air is agitated by either the two 

 branches vibrating freely, or by both loaded with equal 

 disks, with nearly equal and opposite impulses \ whereas 

 in the case of a fork furnished with only one disk, a 

 greater command of the ambient medium is given to the 

 branch carrying it, and a much larger portion of uncount- 

 eracted motion is propagated into the air. Here, then, we 

 have a case in which a vibrating system in full activity 

 is rendered, by a peculiarity of structutre, incapable of 

 sending forth its undulations with effect into the sur- 

 rounding medium ; while the very same mass of matter, 

 vibrating with the same intensity^ but more favourably dis- 

 posed as to the arrangement of its parts, labours under 

 no such disability. 



