No. 3.] ATOMS AND MOLECULES. 575 
direction about it, and this condition would be maintained as 
long as the vibration was maintained. Another body near the 
vibrating sphere would be subject to a pressure on its further 
side greater than on the side adjacent to it, and as a conse- 
quence would be pushed towards the sphere. It would appear 
as if the latter attracted it. The space about the vibrating 
body within which such lessening of pressure occurs, may be 
called its mechanical field, because the effects noted are mechan- 
ical effects. This expression is in accordance with usage 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 spheri- 
cal; but if the vibrating body was a rod or fork or disk or ring, 
the shape of the field would be different, and evidently would 
depend upon the shape of the body as well as upon its charac- 
teristic vibrations. A tuning-fork does not have such a uniform 
field as the imagined sphere, but presents two nodes, radiating 
from the outer edge of each prong. These nodes, or spaces of 
no vibration, may easily be detected by holding the fork near 
the ear and rotating it with the fingers while it is vibrating. 
As the lessening of density depends upon the amplitude of 
vibration, it is plain that the greatest disturbance and least 
pressure will be in the plane of vibration of the fork and near 
the ends of the prongs, and here is where it makes itself 
obvious by its apparent attraction of other bodies. If a paper 
wind-mill three or four inches in diameter be held by a thread 
near to a vibrating Chladni plate, it will be made to rapidly 
rotate by the difference in pressure above and below the arms 
of the mill. A smaller one that may be moved to different 
parts of the plate will be found to be but little affected at the 
nodal points and lines. This mechanical field of the Chladni 
plate extends an inch or more from its surface both above and 
below, and its form, of course, varies with the number of vibra- 
tions the plate is making and the energy of the components of 
the sound. 
It is here assumed that vibrating atoms must have a fe/d in 
the same sense as larger bodies, and that mechanical effects 
must be produced in them not dissimilar to what takes place in 
larger masses. The atoms, however, vibrate in the ether; but 
we know from various other phenomena presented by atoms, 
