point diametrically opposite to the handle. I do the same at a point 90° from the handle; the pitch of the note in both these cases is the same. In both cases the handle occupied the middle of a vibrating segment, loading that segment by its weight. But I now draw the bow at an angular distance of 45° from the handle; the note is sensibly higher than before. The handle in this experiment occupies a node; it no longer loads a vibrating segment, and hence the elastic force, having to cope with less weight, produces a more rapid vibration. The experiment here made with a jug Chladni executed with a tea-cup. Now bells often exhibit round their sound-bows an absence of

FIG. 75.

uniform thickness, tantamount to the want of symmetry in the case of our jug; and we shall learn subsequently, that the intermittent sound of many bells, noticed more particularly when their tones are dying out, is produced by the combination of two distinct rates of vibration, which have this absence of uniformity for their origin.

There are no points of absolute rest in a vibrating bell,

SONOROUS RIPPLES.

153

for the nodes of the higher tones are not those of the fundamental one. But that the various parts of the soundbow, when the fundamental tone is predominant, vibrate with very different degrees of intensity, is easily demonstrated. Suspending a little ball of sealing wax a, fig. 75, by a string, and allowing it to rest gently against the interior surface of this inverted bell, it is tossed to and fro when the bell is thrown into vibration. But the rattling of the sealing wax ball is far more violent when it rests against the vibrating segments than when it rests against the nodes. Permitting the ivory bob of a short pendulum to rest in succession against a vibrating segment and against a node of the Great Bell of Westminster, I found that in the former position it was driven away five inches, in the latter only two inches and three-quarters when the hammer fell upon the bell.

Could the Great Bell' be turned upside down and filled with water, on striking it the vibrations would express themselves in beautiful ripples upon the liquid surface. Similar ripples may be obtained with smaller bells, or even with finger and claret glasses, but they would be too minute for my present purpose. I have here a large hemispherical glass which emits a full deep note. I fill it with water and pass the fiddle-bow across its edge; crispations immediately cover its surface. When I draw the bow vigorously, you see the water rising in a copious spray of liquid spherules from the four vibrating segments. I will endeavour to show you these sonorous ripples. The broad beam from the electric lamp being permitted to fall upon the tranquil water is now reflected at the proper angle, and in the path of the reflected beam I place this large lens, which throws a magnified image of the water surface upon the screen. I now pass the bow gently across the edge of the glass, or I rub my finger gently along the edge; you hear this low sound, and at the same time you observe the

ripples breaking in visible music over the four sectors of the liquid surface.*

When bisulphide of carbon is employed instead of water, its spherules, in consequence of the greater weight of the liquid, bound from it with greater momentum, and the exquisite mosaic upon its surface is longer retained. But a more beautiful effect is produced when one of the lighter volatile liquids is made use of. You know the experiment of Leidenfrost which illustrates the spheroidal condition of water. You know that if water be poured into a red-hot silver basin, instead of flashing at once into steam, it rolls about upon its own vapour. The same effect is produced if we drop a volatile liquid, like ether, on the surface of warm water. The drop retains its spheroidal shape. Filling a bell-glass with ether or alcohol, a sharp sweep of the bow over the edge of the glass detaches the liquid spherules, which, when they fall back, do not mix

with the liquid, but are driven over the surface on wheels of vapour to the nodal lines. The warming of the liquid, as might be expected, improves the effect. M. Melde, to whom we are indebted for this beautiful experiment, has given the drawings, figs. 76 and 77, representing what

*For the illustration of this subject, I am indebted to the kindness of Mr. Bird, of Birmingham, who executed several airs on his magnificent set of bell glasses, which had been sent to London for this express purpose.

FARADAY'S AND MELDE'S FIGURES.

155

occurs when the surface is divided into four and into six vibrating parts. With a thin wine glass and strong brandy the effect may also be obtained.

The glass and the liquid within it vibrate here together, and everything that interferes with the perfect continuity of the entire mass disturbs the sonorous effect. A crack in the glass passing from the edge downwards would extinguish its sounding power. The same effect is produced by a rupture in the continuity of the liquid. To demonstrate this, I have placed in this glass a solution of carbonate of soda. I strike the glass, and you hear this clear musical sound. But I now add a

little tartaric acid to the liquid; it foams, and this dry unmusical collision takes the place of the musical sound. As the foam disappears the sonorous power returns, and now that the liquid is once more clear, you hear the musical ring as before.

The ripples of the tide

FIG. 78.

leave their impressions upon the sand over which they pass. The ripples produced by sonorous vibrations have been proved by Mr. Faraday competent to do the same. Attaching a plate of glass to a long flexible board, and pouring a thin layer of water over the surface of the glass, on causing the board to vibrate, its tremors chase the water into a beautiful mosaic of ripples. A thin stratum of sand strewn upon the plate, is acted upon by the water, and carved into patterns, of which fig. 78 is a reduced specimen.

SUMMARY OF LECTURE IV.

A rod fixed at both ends and caused to vibrate transversely divides itself in the same manner as a string vibrating transversely.

But the succession of its overtones is not the same as that of a string, for while the series of tones emitted by the string is expressed by the natural numbers 1, 2, 3, 4, 5, &c.; the series of tones emitted by the rod is expressed by the squares of the odd numbers 3, 5, 7, 9, &c.

A rod fixed at one end can also vibrate as a whole, or it can divide itself into vibrating segments separated from each other by nodes.

In this case the rate of vibration of the fundamental tone is to that of the first overtone as 4: 25, or as the square of 2 to the square of 5. From the first division onwards the rates of vibration are proportional to the squares of the odd numbers 3, 5, 7, 9, &c.

With rods of different lengths the rate of vibration is inversely proportional to the square of the length of the rod.

Attaching a glass bead silvered within to the free end of the rod, and illuminating the bead, the spot of light reflected from it describes curves of various forms when the rod vibrates. The Kaleidophone of Wheatstone is thus constructed.

The iron fiddle and the musical box are instruments, whose tones are produced by rods, or tongues, fixed at one end and free at the other.