i8 9 i.] 
189 
[Dolbear. 
can reduce the pressure of the medium about it may be under- 
stood by remembering that pressure is proportional to density. 
If the vibration lessens the density it lessens the pressure. Let 
the inner circle in the diagram fig. 2 represent 
an elastic sphere capable of extension to the 
dimensions of the outer circle. When it is 
thus expanded of course it excludes the air 
from the space it occupied. When it con- 
tracts again the air or other medium will 
follow. Suppose the contraction should take 
place at a rate greater than it was possible 
for the air to move, than a vacuum would be formed between 
the surface and the advancing air. 
At whatever rate the contraction took place there would still be 
a partial vacuum next the surface of the sphere, else there would 
be no movement of the air towards it, that is to say, the pressure 
is less next to the surface than at a distance from it. Suppose the 
sphere to dilate and contract between the limits of the lines say 
100 times per second then the density would be less not only be- 
tween those limits but beyond them, and this lessened density 
would lessen the pressure upon the sphere in all directions about 
it, a condition that would be maintained as long as such motion 
continued. Another body near this vibrating body would be 
subject to a pressure on its further side greater than on the side 
adjacent and as a consequence would be pushed towards the sphere. 
It would appear as if the sphere attracted it. The space about a 
vibrating body within which such lessening pressure occurs may 
be called its mechanical field , because its effects are purely me- 
chanical effects. This expression is in accordance with the termin- 
ology employed in electrical and magnetic phenomena. The 
magnetic field is that space about a magnet within which magnetic 
effects take place. In the case of the sphere the shape of the 
field would be spherical, but if the vibrating body were a rod, or 
fork, or disk, or ring, evidently the shape of the field would nec- 
essarily be different. Indeed the shape of the field would depend 
upon the shape of the vibrating body and also upon its character- 
istic vibrations. 
A tuning fork does not have such a uniform field as the imag- 
ined sphere but presents two nodes radiating from the outer edge 
of each prong. These nodes or places of uniform pressure may 
easily be detected by holding the fork near the ear and rotating 
