between Metallic Masses having different Temperatures. 447 



will be obtained ; as in the case of a silver spoon, used as a bar, 

 where the bowl of the spoon rests upon the block. No vibra- 

 tions will take place if the handle, which is the other point of 

 support, terminate with a round end ; should it, however, termi- 

 nate with an ornamental device, which affords two points upon 

 which it can rock, the necessary impulse will be gained ; we pre- 

 sume, therefore, that the time of contact of two points of the metals 

 must be longer than that of the intermediate portions. This condi- 

 tion is readily fulfilled by a vast variety of forms of apparatus, 

 and the rudest masses of metal, such as a poker, when duly heat- 

 ed and placed upon lead, will produce active vibrations. The 

 variations of tone produced upon the apparatus by mechanical 

 interference is easily explained ; if a slender rod, with metallic- 

 balls at its extremities, be placed across a vibrating bar at right 

 angles to its axis, the time and the arcs of oscillation will be ex- 

 tended, the matter being thrown more to the sides ; hence the 

 note will become much lower, and vibrations previously quite in- 

 sensible will become visible. Again, if while a bar is in a state of 

 active vibration, it be gently pressed from above, the extent of 

 its vibrations will be diminished, and the time will be reduced ; 

 hence the note will rise. 



38. As it appeared essential to the experiment that the vibra- 

 tion should take place between two points which were longer in 

 contact with the block than the other portions, it seemed import- 

 ant to determine whether the connection of these points was 

 essential. With a view to determine this, I constructed a bar of 

 lead of the same figure as .those which I usually employed, such 

 as AB, Fig. 5. I let into it a stud of copper a, of which the sur- 

 face corresponded with that of the rest of the bar, and similarly 

 two small ones b c, forming the two solid angles upon which the 

 bar was to vibrate, but totally distinct from one another. Whe- 

 ther upon the complete bridge a, or upon the divided bridge b c, 

 the bar, when heated, and placed upon a block of cold lead, vi- 



