22 Prof. Snell on new articles of Philosophical Apparatus. 
sees it simply rising and falling in its place. He is thus enabled 
to conceive of the co-existence of these two facts with a distinct- 
ness not easily acquired in any other way. 
The motion is produced in the following manner. Hach piece 
of iron is fastened at its lower extremity toa stiff iron wire, which 
is bent upward three inches just within the front, and thence 
proceeds horizontally to the back side of the box, where it is se- 
cured by a pivot, allowing only of vertical motion. Each wire 
with its strip of sheet iron forms a lever of the third order, which 
should rise and fall with the greatest freedom, while lateral mo- 
tion is prevented by perpendicular guides. The axis, turned by 
the crank (6), is placed as near the front as possible, and extends 
the whole length of the box. It is furnished with thirty excen- 
tric cams, each of which gives vertical motion to one of the wire 
levers just mentioned. Fig. 3 shows a section of the box, per- 
pendicular to its length, with a lever and its supporting cam; (a) 
represents a vibrating column; (0) the lever; (¢) its pivot; (d) 
the cam; (f) the axis; (g) the guide, which prevents lateral 
motion. 'To produce the wave motion already described, the 
cams must obviously be arranged according to a uniform law, 
the summit of the second being turned a given number of degrees 
farther round on the axis than that of the first, the third than the 
second, and so on through the whole. The axis with its entire 
series of cams will thus have the form of a helix. The exhibi- 
tion of the instrument is most satisfactorily made, by presenting 
it first in a room partially darkened, so that the intervals between 
the columns are invisible. On turning the crank the illusion is 
complete—a liquid, or at least something flexible, is seen to roll 
in dark horizontal waves, and no other motion is dreamed of. 
But on admitting the light it is immediately apparent that every 
moving particle oscillates in a perpendicular direction, and has no 
other motion whatever. 
Fig. 1 represents the instrument designed to illustrate acoustic 
waves. 'These are waves of condensation and rarefaction; and 
the molecular vibrations are made in the line of wave motion, 
and not perpendicular to it, as in sea waves, which may be termed 
waves of elevation and depression. The box is of the same 
length as the other ; its breadth and height are a little less. The 
front is constructed in the manner already described. ‘Thirty 
slips of japanned sheet iron (ee), one and a half inch long and 
