Fig. 121. The pins in the chiming barrels.
Tuning the Bells.—Bells only very slightly out of tone offend the musical ear, and they may easily be corrected to the extent of half a tone. To sharpen the tone make the bell shorter by turning away the edge of it if it be a shell, or by cutting off if it be a rod or tube; to flatten the tone, thin the back basin-shaped part of the bell by turning some off the outside. Bells which are cracked give a poor sound because the edges of the crack interfere with each other when vibrating. They may be repaired by sawing through the crack to the end of it, so that the edges will not touch each other when vibrating. If there is danger of the crack extending further into the bell, first drill a round hole in the solid metal just beyond the end of the crack, and then saw through into the hole; this will generally prevent any further trouble.
Marking the Chime Barrel.—The chime barrel in small clocks is of brass and should be as large in diameter as can be conveniently got in. To mark off the positions of the pins for the Cambridge chimes, first put the barrel in the lathe and trace circles round the barrel at distances apart corresponding to the positions of the hammer tails. There are five chimes of four bells each for every rotation of the barrel, and a rest equal to two or three notes between each chime. Assuming the rest to be equal to three notes, divide the circumference of the barrel into thirty-five equal parts by means of an index plate, and draw lines at these points across the barrel with the point of the tool by moving it with the slide rest screw. Call the hammer for the highest note D, and that for the lowest note F. Then the first pin is to be inserted where one of the lines across the barrel crosses the first circle; the second pin where the next line crosses the second circle; the third pin where the third line crosses the third circle and the fourth pin where the fourth line crosses the four circle, because the notes of the first chime are in the order, D, C, Bb, F. Then miss three lines for the rest. The first note of the second chime is Bb and the pins for it will consequently be inserted where the first line after the rest crosses the third circle, and so on. Where two or more notes on the same bell come so close as to make it difficult to strike them properly, it is usual to put in another hammer, as it shown in [Fig. 121], where there are two Fs. In fine clocks the pins are of varying lengths so as to strike the hammers on the bells with varying force and thus give more expression to the music.
The following gives the Cambridge Chimes, which are used in the Westminster Great Clock. They are founded on a phrase in the opening symphony of Handel’s air, “I know that my Redeemer liveth,” and were arranged by Dr. Crotch for the clock of Great St. Mary’s, Cambridge, in 1793.
Fig. 122. Westminster chimes.