Prof. Snell on new articles of Philosophical Apparatus. 23 
three eighths of an inch wide, project upward nearly their whole 
length between the two front boards. ‘These slips are taken to 
represent a series of atmospheric particles. It may be supposed 
that a line of balls, supported on slender white wires, would be 
a more appropriate representation. This was tried, but found not 
to make a sufficiently distinct impression of waves. A broad 
band rather than a delicate line needs to be seen in motion. The 
points of greatest’ condensation are at (d,d,d); and those of 
greatest rarefaction at (7,7). As the crank is turned, the waves of 
condensation and rarefaction advance regularly in one direction, 
constantly succeeded by similar waves, that are every moment 
forming themselves anew. It is more difficult in this case than 
in the former to render the molecular vibrations invisible, and to 
fasten the observer’s attention wholly upon the waves. This 
effect is best accomplished by employing oblique vision. Let the 
observer look directly at some object about two feet above or 
below, while his attention is still directed to the instrument, and 
he will, without much distraction from the motions of the indi- 
vidual parts, receive an impression of dark waves travelling over 
the length of the box in regular and constant succession. Then, 
on turning the eyes upon the machine, each molecule is readily 
seen vibrating back and forth in the line in which the waves are 
running. A similar formation of condensed waves occurs in the 
legs of the centipede when walking. 
The operation of this instrument is less interesting to the casual 
observer than the other; but in the hands of the lecturer it is 
far more valuable to the pupil, because the subject to be illus- 
trated is not so easily understood. But here, as in the other case, 
a glance of a few moments will give one a clearer conception of 
the manner in which a minute vibration of every particle of a 
substance in regular order, I will not say occasions, but zs the 
same as a succession of waves advancing through it, than can be 
obtained in many hours by means of verbal description. Indeed, 
I believe many individuals, by witnessing this experiment, soon 
comprehend the circumstances of a phenomenon, of which they 
would never have formed a distinct conception without such aid. 
The several pieces of iron (ee) receive their motions from a 
cylinder three inches in diameter, running lengthwise through the 
box near its front, and turned by the crank (g). The surface of 
the cylinder is cut by thirty grooves, three sixteenths of an inch 
