To First Commissioner Works

REPORT¹ on the Great Bell of Westminster,² by John Tyndall, F.R.S., | dated 20th December 1859.

SECTION 1. —PRELIMINARY OBSERVATIONS.³

1. IF a circular plate of metal be fixed at its centre, and a violin bow drawn across its edge, it will, as is known, divide itself into an even number of vibrating segments, each separated from its neighbor by a line of rest, commonly called a nodal line. If the plate be horizontal, and if fine sand be strewn upon it, the sand will be ejected from the vibrating parts, and will arrange itself along those radii with which the nodal lines coincide.

2. When a bell is struck it divides itself in a similar manner. Thus, as regards the great bell of Westminster, we should conclude that the point on which the hammer falls lies in the middle of a vibrating segment, while two other points on the soundbow,⁴ the one 45 degrees to the right and the other 45 degrees to the left of the point struck by the hammer, would be nodes. At a further distance of 90 degrees from each of these a node would occur, thus making four in all. The circumference of the soundbow being, in round numbers, 28 feet, it follows that the distance between two adjacent nodes is seven feet.

3. Knowing, however, that, in practice, the conditions of theory are rarely strictly fulfilled, I thought it advisable to examine, by direct experiment, the condition of the bell while in a state of vibration. I therefore divided the circumference of the soundbow into 28 equal parts, and ascertained the comparative amplitudes of the vibrations at the several points of division.

4. This examination shewed that while the middle of each vibrating segment gave the maximum amplitude of oscillation, the nodal lines were, by no means, lines of absolute rest, but shared, to a considerable extent, the general agitation.

These remarks are made partly to give those unacquainted with the principles of acoustics an intelligent conception of the mode of the bell’s vibration, and partly to assist those conversant with the subject to draw their own conclusions, should they desire to do so, regarding one of the principal defects of the bell.

SECTION 2. —ON THE CAVITIES IN THE BELL.

1. On the soundbow, and diametrically opposite to the hammer, a number of cavities occur, forming a group which is included in a space of 3½ in. by 1½ in. The centre of this group (which is the principal one) is about 12½ in. distant from the edge of the bell. The cavities (six in number, excepting one or two very small ones), vary from three–tenths of an inch to less than one–tenth of an inch in diameter, and the depth of the deepest of them is two–fifths of an inch. For the purpose of future reference, I will denote this group by the symbol (G₁).

2. To the left of this group (i.e., to the left of an observer facing the group), and at a distance of about eleven inches from it, two cavities occur, the largest of which measures two–tenths of an inch across, at the surface, and is two–tenths of an inch deep. The diameter of this cavity underneath the surface augments to three–tenths of an inch.

3. At a distance of a foot further to the left, and 16 inches from the edge of the bell, is a group of pits, the largest diameter of the largest of them measuring, at the surface, one–fourth of an inch, the largest inner diameter being four–tenths of an inch: the depth of this cavity is one–eighth of an inch. All the others are small; and with the exception of one little outlier, the whole group would be covered by an area of a square inch.

4. Between this group and the point struck by the hammer; at a distance of four feet nine inches from the latter, and 14 inches from the edge of the bell, a cavity occurs which measures five–eighths of an inch across: it is filled with a foreign metal, and I am therefore unable to state its depth.

5. At a distance of 10 inches to the right of the group G₁, a cavity occurs one–fourth of an inch in diameter, and two–tenths of an inch deep.

6. At a distance of six inches above the latter, and in a line which inclines from it to the east, a cavity occurs, measuring, in its longest direction, one–eighth of an inch at the surface, but running under the surface until it attains about three–eighths of an inch in diameter.

7. Finally, at a distance of three feet four inches to the right of G_1, and at 14 inches from the edge of the bell, a cavity occurs, measuring, in its longest direction, one–fourth of an inch across, and having a depth of two–tenths of an inch.

8. All these cavities appear to have been produced by the rising of air bubbles through the metal while in a state of fusion; and their occurrence along the soundbow is, I think, due to the fact that the slope of the mould, at this place, formed a kind of roof over the bubbles and prevented their escape.

SECTION 3. —ON THE CRACKS IN THE BELL.

1. The principal cracks are two in number; the first, which I will call C_1, is diametrically opposite to the hammer, and, hence, in the middle of a vibrating segment. It is nine inches long, and runs transverse to the soundbow. It does not reach the edge of the bell, but terminates two inches above the edge.

2. The second crack, C₂, occurs at a distance of two feet four inches to the left of C₁. It also runs transverse to the soundbow; measures 13 inches in length, and terminates at a distance of five inches from the edge of the bell. This crack occurs within 15 inches of a node, and the amplitude of the vibrations immediately adjacent to the crack was found by experiment to be less than that of the vibrations at the node itself.

3. Besides these two principal cracks there are three minor ones, varying from two and–a–quarter to two inches in length. Were the bell closely examined with a good light, additional small cracks might possibly be detected.

4. To the naked eye the sides of the cracks appear to be in mechanical contact; but when a wet cloth is dabbed against the larger ones, the water is forced into them, or soaks in by capillary attraction; and when the bell is struck, subsequently, bubbles issue from the cracks throughout their entire length.

SECTION 4. —ON THE CONNEXION OF THE CRACKS AND CAVITIES.

1. The pits above described, and other smaller ones which have been disclosed by the removal of a portion of one of the ornamental rings of the bell, must undoubtedly be regarded as defects; but how far such defects are unavoidable in the casting of large bells I am not in a position to state. My duty at present is to estimate the possible influence of the cavities in the production of the cracks.

2. The first principal crack, C_1, certainly originates (or terminates) among the lower members of the group of cavities denoted by G₁. Two of the lowest holes, both of them small ones, are about three–fourths of an inch apart, and occur nearly in the same vertical line. The crack commences midway between both, intersects the lower cavity, and afterwards passes through an impitted portion of the bell.

3. Hence, though the superior end of this crack is close to the cavities of the group G_1, the fact that it reaches less than half an inch higher than the lowest member of the group, and that the main body of the group is not intersected by the crack, tends, in my opinion, to invalidate the assumption that this crack has been caused by the cavities.

4. But setting this apparently doubtful case aside, a reference to the other cracks appears to me quite fatal to the notion that the cracks are due to the cavities. The second principal crack, denoted by the symbol C₂, is the most formidable of all, and it runs from beginning to end through an impitted portion of the metal. As for the minor cracks, no cavity occurs near to any one of them.

5. With regard to the manner in which the bell is suspended, I do not think that it had any influence in producing the cracks.

6. Thus, I think the elements of the question reduce themselves to two, viz., the quality of the metal of the bell, and the magnitude of the blows to which it has been subjected. Each of these elements must be considered in relation to the other, and when this is done, the conclusion appears to me unavoidable that the hammer and the bell are not suited to each other. If the bell be of the proper composition and right tenacity, the blows of the hammer must have been too heavy; if, on the contrary, the blows have been no heavier than they ought to have been, it necessarily follows that the metal of the great bell of Westminster is too brittle.

CONCLUDING REMARKS.

1. In the foregoing report I have confined myself to the physical portion of the question. With the chemical and metallurgical portion I have nothing to do; nevertheless I may be permitted to suggest, as possible, that, owing to the imperfect mixture of the constituents, or through a defect of cooling, the external portions of the soundbow may vary in composition, or tenacity from the average composition, or tenacity, of the bell; thus forming a brittle casing round a less brittle mass. The truth or fallacy of this conjecture experiment alone can decide; but if it should prove true, the cracks might, with proper care, be confined to the brittle envelope, and thus the bell might be saved.

2. The depth of the cracks is unknown; but they appear to penetrate the soundbow to only a small fraction of its thickness. If they were sufficiently deep the grating of their surfaces would injure the tone of the bell, but, as yet, the tone appears to be unaffected.

3. It seems to me worthy of consideration whether the cracks at present existing ought not to be cut out of the soundbow, the striking permitted to proceed as formerly, and the bell subjected to a daily examination. The result might be, that the metal, being partially relieved from strain at the places now intersected by the cracks, would yield no further.

4. It might also be desirable to shift the position of the hammer so as to produce a different system of vibrating segments from that now existing. If, under these circumstances, new cracks should appear, or the old ones show a tendency to deepen, the recasting of the bell would seem to be the only resource.

(signed) John Tyndall.

To the First Commissioner | of Works, &c.

John Tyndall, ‘REPORT on the Great Bell of Westminster’, Bell (New Palace, Westminster): Copy of reports on the condition of the great bell of the new Palace of Westminster, (1860) p. 553.